Osteoporosis

C

Comorbidities

?

Discussions and Knowledge Gaps

6

Almost everything known about the cellular mechanism of age-related bone loss comes from mice and rats, whose skeletons have no osteonal remodeling of cortical bone and continue growing slowly after puberty. Human cortical bone loss proceeds largely through intracortical Haversian remodeling, a compartment that does not exist in the rodent. When an aged mouse gains cortical porosity, is that the same process that produces cortical porosity in an ageing human, or a structurally different route to a phenotype that merely looks alike on microCT?

HUMAN MODEL MISMATCH OPEN mismatch_op_rodent_cortical_remodeling

Attached to

pathophysiology#Microarchitectural Deterioration animal_models#Aged mouse skeleton as a model of human age-related bone loss animal_models#Ovariectomized rat

This is not a general caveat about rodents; it is specific and consequential. The cortical compartment carries most of the fracture burden after age 65 - predominantly cortical non-vertebral sites account for 80% of all fractures in old age, 68% of radial bone loss between 50 and 80 is cortical, and 84% of it occurs after 65 - and cortical porosity is the parameter that intervention studies in mice report. In humans that porosity arises specifically from intracortical remodeling adjacent to the marrow outrunning refill, which fragments the inner cortex into a trabecula-like structure. If the murine gain in porosity arises without Haversian remodeling, then the mechanistic step being modelled is absent from the model, and a treatment effect on murine cortical bone cannot be assumed to transfer. The same limitation constrains the OVX rat, which is the FDA-designated preclinical model, and it is why the same guidance requires a second non-rodent species alongside it - large-animal cortical bone does undergo Haversian remodeling. Note the mismatch is partial rather than total: the cancellous arm and the refilling defect are shared, and this entry records those links at moderate fidelity for that reason.

Show evidence (5 references)

PMID:23689830 SUPPORT Model Organism

"Unlike the human skeleton, however, the murine skeleton continues to grow slowly after puberty and lacks osteonal remodeling of cortical bone."

States the structural mismatch precisely and is the reason this is recorded as HUMAN_MODEL_MISMATCH rather than a knowledge gap.

PMID:23689830 SUPPORT Model Organism

"Knowledge of the causes of skeletal fragility in mice should therefore be applicable to humans if inherent limitations are recognized."

The author's own conditional endorsement - the model is usable but only with the mismatch made explicit, which is what this record does.

PMID:20472174 SUPPORT Human Clinical

"non-vertebral fractures at predominantly cortical sites account for 80% of all fractures and most fracture-related morbidity and mortality in old age"

Establishes that the compartment the rodent models least well is the one carrying the fracture burden, which is what makes this mismatch consequential rather than academic.

+ 2 more references

Clearing senescent cells causally reverses age-related bone loss in old mice, yet a phase 2 trial of intermittent dasatinib plus quercetin in postmenopausal women missed its primary endpoint: CTx did not differ from control at 20 weeks. Formation rose transiently, and in an exploratory subgroup with the highest T cell p16 burden both markers moved and radius BMD increased. Is senescent cell burden the effect modifier that decides who responds, and is there a measurable threshold below which senolytic therapy should not be expected to work?

KNOWLEDGE GAP OPEN gap_op_senescent_burden_stratification

Attached to

pathophysiology#Senescent Cell Accumulation in the Bone Microenvironment pathophysiology#Osteocyte SASP Acquisition

The mouse work is unusually strong - genetic and pharmacological clearance agree, the effect is absent in young animals, and the in vitro SASP experiment supplies the mechanism - so a negative human primary endpoint is more likely to reflect patient selection, dose, or timing than a false preclinical result. Postmenopausal women shortly after menopause are oestrogen-deficient rather than necessarily senescence-burdened, so the trial may have been run in the population least likely to respond. The subgroup finding is exploratory and underpowered and must not be read as a positive result; it is a hypothesis about stratification. Resolving this decides whether senolytics are an osteoporosis therapy at all, or a therapy for a definable high-burden subset.

Show evidence (3 references)

PMID:28825716 SUPPORT Model Organism

"In aged (20- to 22-month-old) mice with established bone loss, activation of the INK-ATTAC caspase 8 in senescent cells or treatment with senolytics or the JAKi for 2-4 months resulted in higher bone mass and strength and better bone microarchitecture than in vehicle-treated mice."

The preclinical result whose failure to translate defines this gap.

PMID:38956196 REFUTE Human Clinical

"intermittent D + Q treatment did not reduce bone resorption in the overall group of postmenopausal women"

The negative primary endpoint in humans, which contradicts a direct read-across from the mouse result.

PMID:38956196 SUPPORT Human Clinical

"our exploratory analyses indicate that further studies are needed testing the hypothesis that the underlying senescent cell burden may dictate the clinical response to senolytics"

The trialists' own framing of the stratification hypothesis this gap is asking about.

Marrow adipose volume rises with age, is higher again in osteoporosis at matched age, and is inversely correlated with trabecular bone volume. The shared osteoblast-adipocyte precursor makes lineage diversion the natural reading. But the human evidence is cross-sectional biopsy data, which cannot separate diversion of the precursor from passive filling of space vacated by lost bone. Which is it, and does marrow fat contribute causally to bone loss or merely mark it?

KNOWLEDGE GAP OPEN gap_op_marrow_adiposity_direction

Attached to

pathophysiology#Marrow Stromal Lineage Shift Toward Adipogenesis

The distinction changes what the marrow lineage node is for. If adipogenic diversion is causal, the precursor allocation step is a therapeutic target and marrow fat is a candidate biomarker of formation failure; if marrow fat is a space-filling consequence, it is neither. This entry currently asserts the lineage-shift reading in a pathophysiology node, so the uncertainty should be visible next to it rather than left implicit.

Show evidence (1 reference)

PMID:11708718 SUPPORT Human Clinical

"The cellular and molecular mechanisms mediating these changes remain to be determined."

The authors close on exactly this open question, four decades into the observation.

Every non-animal model recorded in this entry is either a murine immortalized cell line or a weeks-long human organoid built around a non-aging driver. No available NAM sustains a coupled remodeling unit long enough, or under enough mechanical loading, to express an aging phenotype. What would a human-cell-based system need in order to model age-related rather than tumour- or cytokine-driven bone loss?

KNOWLEDGE GAP OPEN gap_op_no_aging_capable_nam

Attached to

experimental_models# pathophysiology#Osteocyte SASP Acquisition pathophysiology#Marrow Stromal Lineage Shift Toward Adipogenesis

This is a tooling gap that propagates into every mechanistic claim in the aging arm of this entry. The senescence nodes rest on murine in vivo work plus conditioned-medium experiments; the marrow-lineage node rests on human biopsies with no functional system behind it. Because the rodent cortical mismatch recorded above limits what animal work can settle, the absence of an aging-capable human system is not a minor inconvenience - it is why two of this entry's three aging nodes cannot currently be tested in human cells at all. Candidate requirements: human osteocytes embedded in mineralized matrix, an osteoclast compartment, a marrow adipocyte compartment, physiological loading, and culture durations long enough for senescence to accumulate.

Show evidence (1 reference)

PMID:34499959 SUPPORT Other

"Recently, pre-clinical studies have linked the onset of age-related bone loss with an accumulation of senescent cells in the bone microenvironment."

Confirms that the evidence base for the senescence arm is preclinical, which is the substance of this gap.

EGFR signaling regulates the bone-remodeling unit, but its net effect is cell-type-specific and incompletely mapped: EGFR is anabolic via osteoprogenitor/osteoblast lineage support yet can also promote osteoclastogenesis. Which skeletal cell populations (mesenchymal progenitors, osteoblasts, osteocytes, osteoclasts) drive physiological versus pathological EGFR output during bone loss, when does activation occur across the remodeling cycle, and how could allosteric or bone-targeted EGFR modulation be tuned to preserve formation while limiting resorption?

KNOWLEDGE GAP OPEN gap_op_egfr_celltype_specificity

Attached to

pathophysiology#Bone Remodeling Imbalance pathophysiology#RANKL/OPG Dysregulation pathophysiology#Microarchitectural Deterioration

EGFR has cell-type-dependent and partly opposing roles in bone. In the osteoblast lineage, EGFR signaling is predominantly anabolic: it maintains the bone-marrow mesenchymal progenitor pool, supports osteoblast number and mineralization, and constrains adipogenic skewing, so loss of osteoblastic EGFR reduces trabecular bone mass. At the same time, EGFR activity can increase osteoclast number and influence the RANKL/OPG axis, meaning the receptor's effect on net remodeling depends on which compartment is engaged. Because EGFR feeds multiple downstream branches (MAPK/ERK, PI3K/Akt, NF-kB) that diverge between formation and resorption programs, a nonselective EGFR inhibitor risks suppressing the anabolic osteoblastic arm while incompletely controlling resorption. The Wang et al. (2026) review explicitly identifies the cell-type-specific and spatiotemporal mapping of EGFR activation and the mechanisms of its allosteric modulation as open questions for skeletal disease. Resolving where and when EGFR is anabolic versus pro-resorptive in osteoporotic bone is a prerequisite for designing bone-targeted, pathway-selective EGFR modulators that protect bone mass and microarchitecture.

Proposed experiments

Cell-type-resolved mapping of EGFR activation in the osteoporotic bone-remodeling unit

spatial single-cell multi-omics with conditional perturbation

exp_op_egfr_celltype_activation_mapping

In ovariectomized and aged mouse models of bone loss (and in human iliac-crest or hip specimens), use single-cell/spatial transcriptomics with phospho-EGFR immunostaining to localize EGFR pathway activation (MAPK/ERK, PI3K/Akt, NF-kB) across mesenchymal progenitors, osteoblasts, osteocytes, and osteoclasts. Pair this with lineage-specific EGFR gain- and loss-of-function and an allosteric vs. ATP-competitive inhibitor comparison to determine whether EGFR modulation can preserve osteoblastic bone formation while limiting osteoclast-mediated resorption, with readouts on trabecular microarchitecture.

Readouts

Lineage-resolved EGFR activation and remodeling balance

pathophysiology#Bone Remodeling Imbalance

Quantify phospho-EGFR and downstream pathway activity separately in osteoblast-lineage vs. osteoclast-lineage cells alongside formation (osteoblast number, mineral apposition) and resorption indices to map where EGFR drives anabolic vs. catabolic output.

phospho-EGFR immunohistochemistry single-cell RNA sequencing

Direction: NEGATIVE

EGFR effect on the RANKL/OPG axis

pathophysiology#RANKL/OPG Dysregulation

Measure RANKL and OPG expression and osteoclast differentiation under cell-type-targeted EGFR perturbation to test whether EGFR activity in the osteoblast/stromal compartment shifts the RANKL/OPG ratio toward osteoclastogenesis.

RANKL and OPG quantification osteoclast differentiation assay

Direction: POSITIVE

Trabecular microarchitecture response to EGFR modulation

pathophysiology#Microarchitectural Deterioration

Assess trabecular bone volume, number, thickness, and connectivity by micro-CT after lineage-specific EGFR loss/gain and after allosteric vs. pan-EGFR inhibition to detect protection or worsening of bone microarchitecture.

micro-computed tomography

Direction: NEGATIVE

Decision criterion

The cell-type-specific dual-role model is supported if (1) phospho-EGFR mapping shows distinct activation between osteoblast- and osteoclast-lineage cells, (2) osteoblastic EGFR loss reduces bone formation and trabecular mass while (3) EGFR perturbation measurably shifts the RANKL/OPG axis, and (4) an allosteric/bone-targeted modulator preserves microarchitecture better than a pan-EGFR inhibitor.

Would support

pathophysiology#Bone Remodeling Imbalance pathophysiology#RANKL/OPG Dysregulation pathophysiology#Microarchitectural Deterioration

Show evidence (3 references)

PMID:21542005 SUPPORT Model Organism

"Histologic analyses revealed decreases in osteoblast number and mineralization activity and an increase in osteoclast number."

Reduced osteoblastic EGFR activity simultaneously lowers osteoblast number/mineralization and raises osteoclast number, illustrating the cell-type-specific, opposing effects of EGFR on the remodeling unit.

PMID:21542005 SUPPORT Model Organism

"these data demonstrate that the EGFR signaling pathway is an important bone regulator and that it primarily plays an anabolic role in bone metabolism."

Establishes EGFR as a net anabolic bone regulator, anchoring the formation-supporting arm of the cell-type-specific EGFR gap in osteoporosis.

PMID:42218983 SUPPORT Other

"this review highlights several unresolved questions: the cell-type-specific and spatiotemporal mapping of EGFR activation, the precise mechanisms of its allosteric modulation, and the development of tissue-targeted, non-oncological EGFR delivery systems."

Recent comprehensive review explicitly identifies the three unresolved mechanistic gaps in EGFR's role in osteoporosis: cell-type-specific and spatiotemporal mapping of EGFR activation, allosteric modulation mechanisms, and tissue-targeted delivery strategies. Directly validates the scope and importance of this knowledge gap.

EGFR (an ErbB-family receptor tyrosine kinase) regulates the bone-remodeling unit through MAPK/ERK, PI3K/Akt, and NF-kB branches, but it remains unresolved how allosteric modulation of EGFR governs which pathway is selected in osteoblasts, osteoclasts, and osteocytes, and how pathological EGFR output could be inhibited in bone tissue-selectively without the off-target toxicity of pan-EGFR oncology inhibitors.

KNOWLEDGE GAP OPEN gap_op_egfr_allosteric_pathway_targeting

Attached to

pathophysiology#Bone Remodeling Imbalance

EGFR signaling has dual physiological versus pathological roles in skeletal homeostasis, and its effects on bone remodeling are transduced through divergent MAPK/ERK, PI3K/Akt, and NF-kB branches that differ between formation and resorption programs across osteoblasts, osteoclasts, and osteocytes. The precise mechanisms by which EGFR allosteric sites govern which of these pathways is engaged in bone remain undefined, so it is unclear whether a modulator could be tuned to preserve anabolic osteoblastic output while limiting pro-resorptive signaling. In parallel, the pan-EGFR inhibitors developed in oncology are nonselective and carry substantial off-target toxicity, leaving open how to achieve tissue-targeted delivery that selectively inhibits pathological EGFR in bone without disturbing systemic EGFR function. Resolving allosteric pathway selection and bone-targeted delivery is a prerequisite for translating EGFR biology into a safe bone-protective therapy that maintains remodeling balance.

Proposed experiments

Cell-type-resolved EGFR pathway-selectivity mapping in bone-remodeling cells

allosteric EGFR pathway-selectivity assay

exp_op_egfr_pathway_selectivity_bone_cells

In primary human osteoblasts, osteoclasts, and osteocyte-like cells (separately and in co-culture), apply allosteric EGFR modulators alongside orthosteric inhibitors and quantify which downstream branch (MAPK/ERK, PI3K/Akt, NF-kB) is engaged in each cell type and how formation readouts (ALP, mineralization) versus resorption readouts (TRAP, resorption-pit area) respond. The aim is to test whether allosteric modulation can preserve anabolic osteoblastic output while limiting pro-resorptive signaling, resolving the pathway-selection gap.

Model systems

Primary human osteoblast, osteoclast, and osteocyte cultures

Primary bone-remodeling-unit cell types cultured separately and in co-culture, enabling cell-type-resolved EGFR pathway and remodeling readouts.

PRIMARY CELL CULTURE

Bone-targeted EGFR inhibitor delivery and systemic off-target assessment

tissue-targeted drug-delivery assay

exp_op_egfr_bone_targeted_delivery

Evaluate bone-homing delivery vehicles (e.g., bisphosphonate-conjugated or mineral-binding carriers) for EGFR inhibitors in osteoblast-osteoclast co-culture and bone-on-chip systems, measuring on-target remodeling-balance effects versus off-target systemic EGFR blockade, to test whether tissue-selective delivery can protect bone without the systemic toxicity of pan-EGFR oncology inhibitors.

Model systems

Bone-on-chip and osteoblast-osteoclast co-culture delivery model

Microfluidic bone model and bone-cell co-cultures used to assess bone-targeted EGFR-inhibitor delivery and tissue selectivity.

ORGAN ON CHIP

Seeded from PMID:42218983 — The dual roles of EGFR signaling in skeletal homeostasis: from bone remodeling to cartilage metabolism and differentiated targeting strategies

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Pathophysiology

9

Osteocyte SASP Acquisition

With advancing age, osteocytes - the long-lived, terminally differentiated, non-dividing cells that orchestrate remodeling - upregulate p16INK4a and acquire a senescence-associated secretory phenotype. Osteocytes and myeloid cells are the two bone-resident populations that predominantly develop the SASP, and the secretome impairs osteoblast mineralization while enhancing osteoclast-progenitor survival. This is the aging arm of the entry: it is distinct from estrogen deficiency and operates in both sexes.

Osteocyte CL:0000137

SASP cytokine production by senescent osteocytes GO:0001816 ↑ INCREASED Inflammatory response in the bone microenvironment GO:0006954 ↑ INCREASED

Downstream

Senescent Cell Accumulation in the Bone Microenvironment Senescent cells are apoptosis-resistant, so once the rate at which they arise outpaces immune clearance they accumulate in the bone microenvironment.

RANKL/OPG Dysregulation Senescent-cell conditioned medium enhances osteoclast-progenitor survival and osteoclastogenesis, converging on the same resorption-favouring state that estrogen loss produces by a different route.

Show evidence (3 references)

PMID:27341653 SUPPORT Model Organism

"senescent myeloid cells and senescent osteocytes predominantly develop the SASP"

Identifies the osteocyte as one of the two SASP-developing populations in bone, which is why this node is scoped to osteocytes.

PMID:27341653 SUPPORT Model Organism

"revealed significantly more senescent osteocytes in old compared with young bone cortices (11% versus 2%, p < 0.001)"

Quantifies the age-associated rise in senescent osteocytes in vivo using SADS, without relying on cultured cells.

PMID:27341653 SUPPORT Model Organism

"Corresponding with the age-associated accumulation of senescent osteocytes was significantly higher expression of multiple SASP markers in osteocytes from old versus young mice"

Supports the cytokine-production and inflammatory-response annotations on this node - the SASP markers measured are the secreted products those processes name.

Senescent Cell Accumulation in the Bone Microenvironment

Senescent cells accumulate across multiple bone-resident lineages with age. The burden is causal rather than a bystander marker: clearing roughly 30% of p16-positive cells in aged mice, genetically or pharmacologically, yields higher bone mass, better microarchitecture and greater strength, and does so only in old animals.

Osteocyte CL:0000137

Cellular Senescence GO:0090398 ↑ INCREASED

Animal models

Aged INK-ATTAC senescent-cell clearance mouse Mouse rescues Clearing roughly 30% of senescent cells in old mice raises bone mass, microarchitecture and strength; the same treatment in 12-month-old mice changes nothing, which is the specificity control.

Downstream

Bone Remodeling Imbalance Clearing senescent cells lowers resorption while maintaining trabecular and raising cortical formation, which is the remodeling imbalance running in reverse.

Show evidence (1 reference)

PMID:28825716 SUPPORT Model Organism

"these data establish a causal role for senescent cells in bone loss with aging, and demonstrate that targeting these cells has both anti-resorptive and anabolic effects on bone"

The authors' own causal claim, supported by concordant genetic and pharmacological clearance arms in the same study.

Marrow Stromal Lineage Shift Toward Adipogenesis

Osteoblasts and marrow adipocytes derive from a shared mesenchymal precursor, and with age that precursor is allocated increasingly to the adipogenic rather than the osteogenic fate. Marrow adipose volume rises with age and is higher again in osteoporosis at matched age, and it is inversely correlated with trabecular bone volume - a decline in the regenerative output of the stromal compartment rather than a change in resorption.

Bone marrow mesenchymal stromal cell CL:0000134 Marrow adipocyte CL:0000136

Fat Cell Differentiation GO:0045444 ↑ INCREASED Osteoblast Differentiation GO:0001649 ↓ DECREASED

Downstream

Bone Remodeling Imbalance Diverting the shared precursor away from the osteoblast lineage reduces the formation side of the remodeling unit, so the imbalance is reached without any increase in osteoclast activity.

Show evidence (2 references)

PMID:11708718 SUPPORT Human Clinical

"There was an age-related inverse correlation between BV/TV and AV/TV (r = -0.58, P < 0.001)."

Iliac crest biopsies from 53 healthy individuals aged 30-100 show marrow fat rising as trabecular bone volume falls.

PMID:11708718 SUPPORT Human Clinical

"Compared with age-matched controls, patients with osteoporosis exhibited an increased AV/TV (P < 0.05) and decreased BV/TV (P < 0.05)"

The shift is greater in osteoporosis than in age-matched controls, so it is not purely a function of chronological age.

Bone Remodeling Imbalance

Increased osteoclast-mediated bone resorption relative to osteoblast- mediated bone formation leads to net bone loss. This imbalance accelerates with aging and estrogen deficiency.

Osteoclast CL:0000092 Osteoblast CL:0000062

Bone Remodeling GO:0046849

Experimental models

IDG-SW3 osteoblast-to-osteocyte differentiation line CELL LINE Provides a readout of sclerostin production and its hormonal regulation, the formation-side brake in the remodeling unit.

3D bone organoid with coupled osteoblast-osteoclast remodeling ORGANOID Reconstitutes both arms of the remodeling unit in human cells, with osteoblastic mineralization and osteoclastic resorption in the same construct.

Computational models

Bone cell population dynamics model of the remodeling unit KINETIC Reproduces a single remodeling cycle, self-sustaining cycling, and unstable runaway remodeling from one parameter set, and identifies autocrine regulation of osteoclasts as the parameter that decides which regime the system occupies.

Show evidence (2 references)

PMID:21874760 SUPPORT

"aromatase activity and estrogen production are necessary for longitudinal bone growth, attainment of peak bone mass, the pubertal growth spurt, epiphyseal closure, and normal bone remodeling in young individuals."

Demonstrates that estrogen, produced by aromatase, is essential for normal bone remodeling and that deficiency leads to bone loss

PMID:23748068 SUPPORT

"Estrogen deficiency can induce delayed epiphyseal closure, eunuchoid body habitus, osteopenia, and osteoporosis in both genders."

Clinical evidence showing that estrogen deficiency directly causes osteoporosis through impaired bone remodeling

RANKL/OPG Dysregulation

Increased RANKL and decreased osteoprotegerin (OPG) ratio promotes osteoclast differentiation and activation. Estrogen normally suppresses RANKL and increases OPG. Microgravity further exacerbates this imbalance by directly increasing the osteoblast RANKL/OPG ratio.

ESR1 hgnc:3467

Osteoclast Differentiation GO:0030316

nuclear estrogen receptor activity in bone lining cells GO:0030284 ↓ DECREASED

Animal models

Ovariectomized rat Rat recapitulates Reproduces the estrogen-deficiency route into accelerated remodeling, and the effect is estrogen-reversible, which establishes the mechanism rather than merely the phenotype.

Show evidence (5 references)

PMID:28744019 SUPPORT INDIRECT PRIMARY RESULT Model Organism

"Our data indicate that the increase in bone resorption observed in states of estrogen deficiency in mice is mainly caused by lack of ERα-mediated suppression of RANKL expression in bone lining cells."

Grounds the ESR1 descriptor on this node and its direction: selective deletion of estrogen receptor alpha in murine reconstitution models shows that the RANKL rise of estrogen deficiency is a loss of ERalpha-mediated suppression in bone lining cells. Graded INDIRECT because the finding is in ovariectomized mice rather than postmenopausal bone.

PMID:33692806 SUPPORT

"The differentiation of osteoclasts is induced significantly in the presence of IL-17 either directly (207), or indirectly, through upregulation of RANKL."

Demonstrates that RANKL upregulation is a key mechanism for osteoclast differentiation and bone resorption

PMID:21874760 SUPPORT

"with aging, individual differences in aromatase activity and thus in estrogen levels may significantly affect bone loss and fracture risk in both genders."

Shows that age-related decline in estrogen production affects bone metabolism, linking to RANKL/OPG dysregulation

+ 2 more references

Microarchitectural Deterioration

Loss of trabecular connectivity and cortical thinning compromises bone strength beyond what BMD alone indicates. Leads to fragility fractures with minimal trauma. Spaceflight accelerates this process, with incomplete microarchitectural recovery even one year post-flight.

Animal models

Ovariectomized rat Rat partially recapitulates Trabecular bone volume falls and stays low, so the structural endpoint is reproduced, but the driving turnover signal is not sustained.

Aged mouse skeleton as a model of human age-related bone loss Mouse partially recapitulates Aged mice lose cancellous bone, thin their cortices and gain cortical porosity as humans do, and the histologic cause is shared - resorption cavities that osteoblasts fail to refill.

Computational models

QCT-based finite element model of the proximal femur PHYSIOLOGICAL Turns the structural deterioration this node describes into a mechanical quantity that prospectively predicts hip fracture, and the load-to-strength ratio stays predictive after adjustment for areal BMD.

Show evidence (4 references)

PMID:23748068 SUPPORT

"recurrent bone fractures associated with minimal trauma starting in puberty"

Clinical case demonstrating that estrogen deficiency leads to fragility fractures due to compromised bone microarchitecture

PMID:35773442 SUPPORT Human Clinical

"nine of 17 astronauts did not fully recover tibia total BMD after 12-months"

HR-pQCT imaging of astronauts shows incomplete recovery of trabecular microarchitecture at weight-bearing tibia one year after long-duration spaceflight, equivalent to a decade of terrestrial bone aging

PMID:31897866 SUPPORT Human Clinical

"bone structural quality (e.g., trabecular microarchitecture) is diminished and the quality of bone material is reduced via impaired tissue mineralization, maturation, and maintenance (e.g., mediated by osteocytes)"

Review of spaceflight-induced bone quality changes beyond BMD, showing both structural and material quality are compromised in microgravity

+ 1 more reference

Osteocyte Mechanosensing Disruption in Microgravity

Microgravity removes gravitational mechanical loading, impairing osteocyte mechanotransduction. Osteocytes sense mechanical strain via the lacuno-canalicular network and regulate bone remodeling accordingly. In microgravity, loss of fluid shear stress and mechanical stimulation leads to downregulation of key osteocytic genes and impaired osteocyte differentiation.

Osteocyte CL:0000137

Cellular Response to Mechanical Stimulus GO:0071260

Experimental models

MLO-Y4 osteocyte-like cell line CELL LINE The line was established specifically to make osteocyte mechanotransduction and osteocyte-osteoblast-osteoclast communication testable in vitro.

Show evidence (2 references)

PMID:33835498 SUPPORT In Vitro

"µG impairs the differentiation of osteocytes, consistent with prior osteoblast spaceflight experiments, which resulted in the downregulation of key osteocytic genes"

Osteocyte cell line (Ocy454) flown on ISS SpaceX Dragon-6 showed impaired differentiation and altered glycolysis, effects not replicated by ground-based simulators

PMID:29559713 SUPPORT Human Clinical

"clear parallels exist between the effects of spaceflight, periods of immobilization and ageing, with possibly irreversible features"

Review establishes that spaceflight-induced skeletal changes parallel accelerated aging via disrupted mechanosensing

Microgravity-Accelerated Bone Resorption

Spaceflight induces a rapid and sustained increase in osteoclast-mediated bone resorption. Astronauts lose approximately 0.8% of bone mass per month in weight-bearing lower limb sites, with bone resorption markers plateauing at 113% above pre-flight levels within days of entering microgravity.

Bone Resorption GO:0045453

Show evidence (2 references)

PMID:32411816 SUPPORT Human Clinical

"Bone loss in space travelers is a major challenge for long-duration space exploration."

Meta-analysis of 148 space travelers quantifies lower limb bone loss rate of -0.8% per month during microgravity exposure

PMID:32411816 SUPPORT Human Clinical

"microgravity-induced bone loss is a significant and unresolved health risk for space travelers"

Bone resorption markers plateau at 113% above pre-flight levels with rapid onset (half-max at 11 days)

Spaceflight-Induced Mitochondrial Stress in Bone

Multi-omics analyses of spaceflight tissues reveal mitochondrial dysfunction as a central biological hub for spaceflight impact. Mitochondrial stress pathways are consistently enriched across multiple tissue types and missions, contributing to impaired cellular function in bone and other organ systems.

Mitochondrion Organization GO:0007005

Show evidence (1 reference)

PMID:33242417 SUPPORT Computational

"Overall pathway analyses on the multi-omics datasets showed significant enrichment for mitochondrial processes, as well as innate immunity, chronic inflammation, cell cycle, circadian rhythm, and olfactory functions"

GeneLab multi-omics integration across NASA missions identifies mitochondrial stress as a consistent spaceflight phenotype

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

5

Limbs 1

Femur Fractures OCCASIONAL HP:0031846

Major cause of morbidity and mortality

Show evidence (1 reference)

PMID:16983459 SUPPORT Human Clinical

"In the year 2000 there were an estimated 9.0 million osteoporotic fractures of which 1.6 million were at the hip, 1.7 million at the forearm and 1.4 million were clinical vertebral fractures."

Quantifies proximal femur (hip) fracture as one of the three dominant osteoporotic fracture sites worldwide.

Musculoskeletal 3

Decreased Bone Mineral Density VERY_FREQUENT Reduced bone mineral density HP:0004349

T-score <= -2.5

Show evidence (2 references)

PMID:23748068 SUPPORT

"Lumbar osteoporosis was detected in bone densitometry."

Clinical documentation of reduced bone mineral density in aromatase deficiency-induced osteoporosis

PMID:21874760 SUPPORT

"aromatase activity and estrogen production are necessary for longitudinal bone growth, attainment of peak bone mass, the pubertal growth spurt, epiphyseal closure, and normal bone remodeling in young individuals."

Demonstrates that estrogen deficiency impairs achievement and maintenance of peak bone mass, leading to decreased BMD

Vertebral Fractures FREQUENT Vertebral compression fracture HP:0002953

Show evidence (1 reference)

PMID:28168409 SUPPORT

"The prevalence of morphometric vertebral fractures in European women is highest in Scandinavia (26%) and lowest in Eastern Europe (18%)."

Systematic review documents high prevalence of vertebral fractures in osteoporosis, confirming this as a key phenotype.

Kyphosis FREQUENT HP:0002808

From vertebral compression fractures

Show evidence (1 reference)

PMID:9180660 SUPPORT Human Clinical

"a 15 degrees increase in kyphosis was associated with losing more than 4 cm of height (OR, 1.88; 95% CI, 1.79-2.03) and having a vertebral fracture (OR, 1.57; 95% CI, 1.46-1.69)"

In 6439 osteoporotic women, kyphosis tracks with vertebral fracture and height loss, supporting it as a downstream consequence of vertebral compression rather than an independent finding.

Growth 1

Height Loss FREQUENT Abnormality of body height HP:0000002

Progressive loss of adult stature from vertebral compression, not congenital short stature. HPO has no term for acquired height loss, so this is bound to the accurate parent HP:0000002 rather than to HP:0004322 (Short stature), which asserts height below -2 SD for age and sex and is a different claim.

Show evidence (1 reference)

PMID:14742889 SUPPORT Human Clinical

"women with at least 3 in. of height loss had odds of osteoporosis of the hip that were 9.6 times greater (95% confidence interval, 4.8-19.2) than women with less than an inch of height loss"

Measured height loss against self-reported maximum adult height is strongly associated with densitometric osteoporosis in 2108 women.

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Genetic Associations

4

LRP5 (Risk Factor)

Gene: LRP5 hgnc:6697 gene page relationship_type: SUSCEPTIBILITY

Show evidence (1 reference)

PMID:22504420 SUPPORT Human Clinical

"including at 18p11.21 (FAM210A), 7q21.3 (SLC25A13), 11q13.2 (LRP5), 4q22.1 (MEPE), 2p16.2 (SPTBN1) and 10q21.1 (DKK1)"

LRP5 is among the six loci associated with low-trauma fracture at genome-wide significance in 31,016 cases and 102,444 controls.

SOST (Risk Factor)

Gene: SOST hgnc:13771 gene page relationship_type: SUSCEPTIBILITY

Show evidence (1 reference)

PMID:22504420 SUPPORT Human Clinical

"In trans-iliac bone biopsies, expression of five genes correlated with LS-BMD and/or FN-BMD of the donors with P < 0.001 including PSME4 (2p16.2), DKK1 (10q21.1), C17orf91 (17p13.3), SOST (17q21.31_1) and DUSP3 (17q21.31_1)"

SOST expression in human bone biopsies correlates with the donor's bone mineral density, linking the GWAS locus to bone tissue rather than to a neighbouring gene.

ESR1 (Risk Factor)

Gene: ESR1 hgnc:3467 gene page relationship_type: SUSCEPTIBILITY

Show evidence (1 reference)

PMID:22504420 SUPPORT Human Clinical

"Three loci included variants localized less than 40 Kb from the initial main signal suggesting allelic heterogeneity and included the 1p31.3 (represented by rs17482952 near WLS), 6q25.1 (rs7751941 near ESR1) and the 16q12.1 (rs1564981 near CYLD) loci."

Conditional analysis resolves a second independent BMD signal at the ESR1 locus, establishing the association rather than a single tag SNP.

VDR (Disputed)

Gene: VDR hgnc:12679 gene page relationship_type: DISPUTED

Show evidence (1 reference)

PMID:16908916 REFUTE Human Clinical

"The FokI, BsmI, ApaI, and TaqI VDR polymorphisms are not associated with BMD or with fractures, but the Cdx2 polymorphism may be associated with risk for vertebral fractures."

Participant-level meta-analysis across 9 European cohorts finds no association between the commonly cited VDR polymorphisms and either BMD or fracture, contradicting the risk-factor claim.

💊

Medical Actions

7

Bisphosphonates

Action: bisphosphonate agent therapy Ontology label: Bisphosphonate Therapy NCIT:C198585

First-line antiresorptives (alendronate, risedronate, zoledronic acid).

Show evidence (4 references)

PMID:34807231 SUPPORT

"The pooled meta-analysis found that 12.4 months (95% CI, 6.3-18.4 months) were needed to avoid 1 nonvertebral fracture per 100 postmenopausal women receiving bisphosphonate therapy."

Meta-analysis of 10 RCTs with 23,384 women demonstrates bisphosphonate efficacy in preventing fractures.

PMID:31400472 SUPPORT Human Clinical

"combined Bis+ARED prevented declines in all DXA and QCT hip densitometry and in estimates of FE hip strengths"

Bisphosphonate alendronate combined with ARED resistive exercise prevented bone loss during ISS missions, while exercise alone only partially attenuated loss and did not suppress bone resorption

PMID:37723136 SUPPORT Model Organism

"BP-NELL-PEG significantly increased bone formation in flight and ground control mice without obvious adverse health effects"

Bisphosphonate-conjugated NELL-1 therapy showed bone-specific efficacy during 9-week ISS Rodent Research-5 mission

+ 1 more reference

Denosumab

Action: Pharmacotherapy NCIT:C15986

Agent: denosumab NCIT:C61313

Platform: Monoclonal antibody

RANKL inhibitor, potent antiresorptive.

Mechanism Target:

INHIBITS RANKL/OPG Dysregulation — Denosumab is a human monoclonal antibody against RANKL that blocks RANKL-RANK binding, acting on the same node the entry models as the driver of excess osteoclastogenesis.

Show evidence (2 references)

PMID:19671655 SUPPORT Human Clinical

"Denosumab is a fully human monoclonal antibody to the receptor activator of nuclear factor-kappaB ligand (RANKL) that blocks its binding to RANK, inhibiting the development and activity of osteoclasts, decreasing bone resorption, and increasing bone density."

States the molecular target and the resulting suppression of osteoclast development, which is the mechanism node this link points at.

PMID:33103722 SUPPORT Other

"Denosumab discontinuation is characterized by an increase in bone turnover overriding pre-treatment status, a rapid bone loss in the majority and multiple vertebral fractures (VFx) in some patients."

Confirms the inhibition is suppressive rather than corrective - withdrawing it returns the RANKL/OPG node past its pre-treatment state, which is why the link is INHIBITS and not RESTORES.

Show evidence (2 references)

PMID:19671655 SUPPORT Human Clinical

"denosumab reduced the risk of new radiographic vertebral fracture, with a cumulative incidence of 2.3% in the denosumab group, versus 7.2% in the placebo group (risk ratio, 0.32; 95% confidence interval [CI], 0.26 to 0.41; P<0.001)"

FREEDOM, a 7868-woman placebo-controlled trial, is the pivotal fracture-endpoint evidence for denosumab.

PMID:29105841 REFUTE Human Clinical

"the vertebral fracture rate increased from 1.2 per 100 participant-years during the on-treatment period to 7.1, similar to participants who received and then discontinued placebo"

Contradicts reading denosumab as a durable intervention. Unlike bisphosphonates, its effect is fully reversible on withdrawal, so the benefit above is conditional on uninterrupted dosing.

Teriparatide

Action: Pharmacotherapy NCIT:C15986

Agent: teriparatide NCIT:C61966

Platform: Peptide

PTH analog, anabolic agent for severe osteoporosis.

Show evidence (1 reference)

PMID:11346808 SUPPORT Human Clinical

"New vertebral fractures occurred in 14 percent of the women in the placebo group and in 5 percent and 4 percent, respectively, of the women in the 20-microg and 40-microg parathyroid hormone groups"

The Fracture Prevention Trial in 1637 postmenopausal women with prior vertebral fractures is the pivotal evidence for teriparatide.

Romosozumab

Action: Pharmacotherapy NCIT:C15986

Agent: romosozumab NCIT:C152246

Platform: Monoclonal antibody

Sclerostin inhibitor with dual anabolic/antiresorptive effect.

Show evidence (3 references)

PMID:27641143 SUPPORT Human Clinical

"At 12 months, new vertebral fractures had occurred in 16 of 3321 patients (0.5%) in the romosozumab group, as compared with 59 of 3322 (1.8%) in the placebo group (representing a 73% lower risk with romosozumab; P<0.001)."

FRAME, a 7180-woman placebo-controlled trial, establishes vertebral fracture reduction.

PMID:28892457 SUPPORT Human Clinical

"Over a period of 24 months, a 48% lower risk of new vertebral fractures was observed in the romosozumab-to-alendronate group"

ARCH shows superiority over active bisphosphonate comparison, not only over placebo.

PMID:28892457 REFUTE Human Clinical

"During year 1, positively adjudicated serious cardiovascular adverse events were observed more often with romosozumab than with alendronate (50 of 2040 patients [2.5%] vs. 38 of 2014 patients [1.9%])"

Contradicts an unqualified reading of romosozumab as a preferred agent; the cardiovascular imbalance in the same trial constrains where it is used.

Calcium Supplementation

Action: calcium supplementation Ontology label: Nutritional Support NCIT:C15433

Ensure adequate intake (1000-1200 mg/day).

Show evidence (2 references)

PMID:17720017 SUPPORT Human Clinical

"In trials that reported fracture as an outcome (17 trials, n=52 625), treatment was associated with a 12% risk reduction in fractures of all types (risk ratio 0.88, 95% CI 0.83-0.95; p=0.0004)."

Meta-analysis of 29 randomised trials (n=63,897) supporting calcium, alone or with vitamin D, as fracture-preventive.

PMID:17720017 SUPPORT Human Clinical

"The treatment effect was better with calcium doses of 1200 mg or more than with doses less than 1200 mg (0.80 vs 0.94; p=0.006)"

Sources the dose in this treatment's description; the benefit is dose-dependent, with 1200 mg the threshold the authors recommend.

Vitamin D Supplementation

Action: vitamin D supplementation Ontology label: Nutritional Supplementation NCIT:C15425

Maintain 25(OH)D >30 ng/mL.

Show evidence (1 reference)

PMID:17720017 SUPPORT Human Clinical

"with vitamin D doses of 800 IU or more than with doses less than 800 IU (0.84 vs 0.87; p=0.03)"

Fracture-risk reduction is greater at vitamin D doses of at least 800 IU, the dose the meta-analysis recommends alongside calcium.

Weight-Bearing Exercise

Action: physical therapy Ontology label: Physical Therapy NCIT:C15302

Platform: Behavioral / lifestyle

Helps maintain bone density.

Show evidence (2 references)

PMID:21735380 SUPPORT Human Clinical

"Our results suggest a relatively small statistically significant, but possibly important, effect of exercise on bone density compared with control groups."

Cochrane review of 43 RCTs (4320 participants) supports the bone density claim this treatment makes, and calibrates its size as small.

PMID:21735380 REFUTE Human Clinical

"The most effective type of exercise intervention on bone mineral density (BMD) for the neck of femur appears to be non-weight bearing high force exercise such as progressive resistance strength training for the lower limbs"

Contradicts the weight-bearing framing specifically - at the femoral neck the largest effect came from non-weight-bearing high-force training.

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Environmental Factors

7

Estrogen Deficiency

Menopause is major risk factor

Show evidence (2 references)

PMID:21874760 SUPPORT

"extraglandular aromatization of circulating androgen precursors is the major source of estrogen not only in men (since only 15% of circulating estradiol is released directly by the testis) but also in women after the menopause."

Post-menopausal women rely on aromatase activity for estrogen production, making aromatase activity critical for bone health after menopause

PMID:18567553 SUPPORT

"studies in these unusual patients have stimulated research on defining the role of estrogen in regulating bone metabolism in normal adult and aging men, providing further insights into estrogen regulation of bone metabolism not only in men, but also in women."

Demonstrates that estrogen is essential for bone metabolism in both sexes, with deficiency leading to osteoporosis

Mechanism Target:

TRIGGERS RANKL/OPG Dysregulation — Estrogen restrains osteoclastogenesis largely by holding down RANKL relative to osteoprotegerin, so its withdrawal releases that brake directly. This is why bone loss accelerates sharply at menopause rather than gradually with age.

Show evidence (1 reference)

PMID:12697741 SUPPORT Human Clinical

"The data suggest that upregulation of RANKL on bone marrow cells is an important determinant of increased bone resorption induced by estrogen deficiency."

Measures RANKL directly on bone marrow cells in early postmenopausal women, finding it two- to threefold higher than in premenopausal or estrogen-treated women and inversely correlated with serum estradiol. That reaches the specific ratio this node is named for, which the previous citation did not.

Calcium Deficiency

Inadequate dietary intake

Show evidence (1 reference)

PMID:35247225 SUPPORT Human Clinical

"When calcium intake is low or when calcium is poorly absorbed, bone resorption occurs since the stored calcium in bone is used to maintain normal biological functions."

States the mechanism by which low or poorly absorbed dietary calcium drives bone resorption, the basis for calcium deficiency as an osteoporosis risk factor.

Vitamin D Deficiency

Impairs calcium absorption

Show evidence (1 reference)

PMID:21872800 SUPPORT Human Clinical

"The consequences of vitamin D deficiency are secondary hyperparathyroidism and bone loss, leading to osteoporosis and fractures"

Directly states that vitamin D deficiency causes secondary hyperparathyroidism and bone loss leading to osteoporosis.

Mechanism Target:

PREDISPOSES Bone Remodeling Imbalance — Vitamin D deficiency causes secondary hyperparathyroidism, shifting remodeling toward net bone loss.

Show evidence (1 reference)

PMID:21872800 SUPPORT DIRECT Human Clinical

"The consequences of vitamin D deficiency are secondary hyperparathyroidism and bone loss, leading to osteoporosis and fractures"

States the deficiency-to-bone-loss route through secondary hyperparathyroidism, which is the remodeling imbalance this node represents.

Sedentary Lifestyle

Lack of weight-bearing exercise

Show evidence (1 reference)

PMID:30998435 SUPPORT Human Clinical

"Substitution of 30 min of sedentary time with an equal amount of light physical activity was associated with increased bone mineral density of about 3 mg/cm2 and a 12% reduced risk of osteoporosis in the spine."

Quantifies the bone-density benefit of replacing sedentary time with activity, the inverse relationship supporting sedentary lifestyle as an osteoporosis risk factor.

Smoking

Accelerates bone loss

Show evidence (1 reference)

PMID:30631414 SUPPORT

"tobacco smoking causes an imbalance in bone turnover, leading to lower bone mass and making bone vulnerable to osteoporosis and fracture"

Review confirms that smoking causes bone turnover imbalance that increases osteoporosis and fracture risk.

Mechanism Target:

EXACERBATES Bone Remodeling Imbalance — Tobacco shifts bone turnover toward loss on top of whatever else is driving remodelling, lowering attained bone mass. The responsible constituents and steps are not established in the cited source.

Show evidence (1 reference)

PMID:30631414 SUPPORT

"tobacco smoking causes an imbalance in bone turnover, leading to lower bone mass and making bone vulnerable to osteoporosis and fracture"

States that tobacco smoking causes an imbalance in bone turnover leading to lower bone mass, the remodelling imbalance this node represents.

Glucocorticoid Use

Major cause of secondary osteoporosis

Show evidence (1 reference)

PMID:29691807 SUPPORT

"Glucocorticoid-induced osteoporosis is the most common secondary cause of osteoporosis and the resulting fractures cause significant morbidity."

Review confirms glucocorticoid use is the leading cause of secondary osteoporosis.

Mechanism Target:

TRIGGERS Bone Remodeling Imbalance — Therapeutic glucocorticoids suppress osteoblast function and lifespan while prolonging osteoclast survival, tipping remodelling toward resorption. It is the commonest secondary cause of the disease.

Show evidence (1 reference)

PMID:29691807 SUPPORT

"Glucocorticoid-induced osteoporosis is the most common secondary cause of osteoporosis and the resulting fractures cause significant morbidity."

Identifies glucocorticoid-induced osteoporosis as the most common secondary cause of the disease, an iatrogenic exposure rather than a background risk factor.

Microgravity Exposure

Spaceflight-induced mechanical unloading causes 1-1.5% bone loss per month in weight-bearing sites. Recovery is slow and often incomplete, with some astronauts showing deficits equivalent to a decade of terrestrial aging after one year.

Show evidence (4 references)

PMID:32411816 SUPPORT Human Clinical

"Bone loss in space travelers is a major challenge for long-duration space exploration."

Systematic review and meta-analysis of bone loss in 148 space travelers documents site-dependent bone loss with lower limbs losing -0.8% per month

PMID:35773442 SUPPORT Human Clinical

"nine of 17 astronauts did not fully recover tibia total BMD after 12-months"

HR-pQCT study showing incomplete bone recovery one year after spaceflight, commensurate with a decade of terrestrial age-related bone loss

PMID:33597120 SUPPORT Human Clinical

"Increases in training volume predicted preservation of tibia bone strength and trabecular vBMD and thickness"

Pre-flight exercise and in-flight resistance training predict and partially mitigate spaceflight bone loss in astronauts

+ 1 more reference

Mechanism Target:

TRIGGERS Microgravity-Accelerated Bone Resorption — Unloading removes the mechanical signal that normally restrains resorption, and this entry carries a node specifically for it. Notably the loss is not fully reversible on return to gravity.

Show evidence (1 reference)

PMID:35773442 SUPPORT Human Clinical

"nine of 17 astronauts did not fully recover tibia total BMD after 12-months"

Reports that nine of seventeen astronauts had not fully recovered tibial bone mineral density twelve months after flight, the incomplete recovery this node's downstream consequences reflect.

🔬

Biochemical Markers

4

Bone Turnover Markers (Variable)

Context: CTX, P1NP indicate resorption/formation rates

Show evidence (1 reference)

PMID:21184054 SUPPORT Other

"IOF/IFCC recommend one bone formation marker (s-PINP) and one bone resorption marker (s-CTX) to be used as reference markers"

Joint IOF/IFCC position designating s-PINP and s-CTX as the reference formation and resorption markers, which is why this entry names that pair.

Calcium (Normal)

Context: Usually normal in primary osteoporosis; measured chiefly to exclude a secondary cause such as primary hyperparathyroidism.

Show evidence (1 reference)

PMID:12364413 SUPPORT INDIRECT Human Clinical

"A testing strategy involving measurement of 24-h urine calcium, serum calcium, and serum PTH for all women and serum TSH among women on thyroid replacement therapy would have been sufficient to diagnose 47 of these 55 women (85%)"

Supports the screening role of serum calcium by inference - it is informative because an abnormal value reclassifies the case as secondary, which presupposes that it is normal in primary osteoporosis.

Vitamin D (Variable)

Context: Deficiency common and contributes to bone loss

Show evidence (1 reference)

PMID:15928802 SUPPORT Human Clinical

"hip BMD was negatively and independently associated with PTH levels (p=0.04), and positively and independently associated with 25(OH)D levels (p=0.03)"

In 615 community-dwelling postmenopausal women, 25(OH)D tracks positively with hip BMD independently of PTH.

PTH (Variable)

Context: May be elevated with vitamin D deficiency

Show evidence (1 reference)

PMID:15928802 SUPPORT Human Clinical

"Low levels of serum calcium stimulate the secretion of parathyroid hormone (PTH), which maintains serum calcium levels at the expense of increased bone turnover, bone loss and increased risk of fractures"

States the secondary-hyperparathyroidism route by which a raised PTH translates into increased bone turnover and bone loss.

🔬

Diagnosis

1

Bone Densitometry (DXA)

Dual-energy X-ray absorptiometry of the proximal femur is the measurement on which the WHO operational definition of osteoporosis rests: bone mineral density is expressed as a T-score, the number of standard deviations from the young adult female mean, and osteoporosis is a T-score 2.5 SD or more below that mean. The femoral neck is the WHO reference site; ISCD practice also reads the lumbar spine and takes the lowest of the spine and hip T-scores.

dual-energy X-ray absorptiometry NCIT:C48789

DXA femur T-score LOINC:38263-0 DXA lumbar spine T-score LOINC:38267-1

Reference Ranges

DXA Femur [T-score] Bone density (WHO diagnostic criteria for osteoporosis (published 1994), femoral neck DXA referenced to the NHANES III database for women aged 20-29 years, as restated by Kanis 2002 and Kanis et al. 2008; applied to women and men)

Osteoporosis (< -2.5 {SD}) Low bone mass (osteopenia) (-2.5–-1.0 {SD})

Osteoporosis: Meets the WHO densitometric definition of osteoporosis; with one or more fragility fractures it is severe (established) osteoporosis.

Low bone mass (osteopenia): WHO low bone mass (osteopenia); below the young adult range but above the osteoporosis threshold.

The criterion is stated two ways in the cited abstracts. As a sentence it is inclusive: a BMD "2.5 SD or more below" the young adult mean (PMID:12057569, PMID:18180210), and Lombardi and Ross restate it as "at least 2.5 SD below" (PMID:11730255). The parenthetical forms are exclusive: "T score < -2.5 SD" (PMID:12057569) and "-2.5 < T-score < -1.0" for osteopenia (PMID:11730255). The half-open bands below put -2.5 itself in the low bone mass band, which agrees with the parentheticals and disagrees by one point with the sentence form of the WHO definition. Severe (established) osteoporosis is osteoporosis plus one or more fragility fractures (PMID:12057569), a fracture criterion rather than a further T-score band, so it is not recorded as a band. No normal band is recorded because none of the cached abstracts states the normal T-score range in quotable text. Kanis et al. 2008 name the femoral neck as the reference site; LOINC:38263-0 is the femur code, bound because searches of the Monarch KG for "Femur neck", "Femur.neck", "Femoral neck T-score" and "neck Bone density" returned no femoral-neck-specific DXA T-score code.

Show evidence (5 references)

PMID:12057569 SUPPORT REVIEW SYNTHESIS Other

"Osteoporosis is defined as a BMD 2.5 SD or more below the average value for premenopausal women (T score < -2.5 SD)."

States the WHO osteoporosis cut-off reproduced as the upper boundary of the osteoporosis band.

PMID:12057569 SUPPORT REVIEW SYNTHESIS Other

"Severe osteoporosis denotes osteoporosis in the presence of one or more fragility fractures."

Severe osteoporosis adds a fracture criterion to the T-score band, which is why it is described in the notes rather than as a further band.

PMID:18180210 SUPPORT Other

"In 1994, the World Health Organization published diagnostic criteria for osteoporosis."

Dates the WHO criteria named in the population field.

+ 2 more references

The lumbar spine T-score code is bound as a second measurement because ISCD practice reads the spine with the hip sites; the WHO bands are recorded on the femur code only, because the cited WHO sources define them at the femoral neck. The ISCD 2005 skeletal-site position (PMID:16731427) is abstract-only in the cache and does not restate the bands.

Show evidence (3 references)

PMID:18180210 SUPPORT Other

"this paper proposes that the reference standard should be based on bone mineral density (BMD) measurement made at the femoral neck with dual-energy X-ray absorptiometry (DXA)."

Names femoral-neck DXA as the reference measurement for describing osteoporosis, the first measurement bound on this row.

PMID:12057569 SUPPORT REVIEW SYNTHESIS Other

"The recommended site for diagnosis is the proximal femur with dual energy X-ray absorptiometry (DXA)."

Grounds the femur T-score as the diagnostic measurement.

PMID:16731427 SUPPORT Other

"the lowest T-score of the total proximal femur, femoral neck, trochanter, and spine should continue to be used for diagnosis"

One of the questions the 2005 ISCD positions were convened to answer; its wording ("continue to be used") records that the spine was already read alongside the hip sites for diagnosis, which is why the lumbar spine T-score is bound as a second measurement. The abstract does not state the answer the conference reached.

📊

Prevalence

4

Worldwide

Point Prevalence 18300.0 per 100,000 (16200.0–20700.0) >1 in 1,000

Pooled estimate from 86 studies (103,334,579 participants, ages 15-105). The pooled population mixes screening settings and age structures, so this is an order-of-magnitude figure rather than an age-standardised rate.

Show evidence (1 reference)

PMID:34657598 SUPPORT Human Clinical

"the prevalence of osteoporosis in the world was reported to be 18.3 (95% CI 16.2-20.7)"

Systematic review and meta-analysis giving the pooled worldwide point prevalence of osteoporosis.

Women worldwide

Point Prevalence 23100.0 per 100,000 (19800.0–26900.0) >1 in 1,000

Sex-stratified arm of the same meta-analysis (70 studies, 800,457 women).

Show evidence (1 reference)

PMID:34657598 SUPPORT Human Clinical

"the prevalence of osteoporosis in women of the world was reported to be 23.1 (95% CI 19.8-26.9)"

Quantifies the roughly two-fold female excess that motivates the estrogen-deficiency arm of the mechanism.

Men worldwide

Point Prevalence 11700.0 per 100,000 (9600.0–14100.0) >1 in 1,000

Sex-stratified arm of the same meta-analysis (40 studies, 453,964 men).

Show evidence (1 reference)

PMID:34657598 SUPPORT Human Clinical

"the prevalence of osteoporosis among men of the world was found to be 11.7"

Osteoporosis in men is common rather than exceptional, which is why the entry is not scoped to postmenopausal disease.

United States adults aged 50 years and older (NHANES 2005-2010)

Point Prevalence 10300.0 per 100,000 >1 in 1,000

Densitometric definition (T-score <= -2.5 at femoral neck or lumbar spine) in a nationally representative survey; a further 43.9% had low bone mass.

Show evidence (1 reference)

PMID:24771492 SUPPORT Human Clinical

"Based on an overall 10.3% prevalence of osteoporosis, we estimated that in 2010, 10.2 million older adults had osteoporosis."

Nationally representative DXA-based prevalence estimate for a single well-characterised population.

📊

Related Datasets

6

37-Day microgravity exposure in 16-Week female C57BL/6J mice during NASA RR-1 -- bone loss at weight-bearing sites osdr:OSD-804

MicroCT analysis of femur and vertebrae from mice on the RR-1 NASA Validation Flight (SpaceX-4). Demonstrates significant cancellous and cortical bone loss in femur but not L2 vertebrae after 37 days of microgravity.

mouse

Conditions: spaceflight microgravity ground control

Single cell transcriptional profiling of femur bone marrow from mice flown on RRRM-2 osdr:OSD-402

Single-cell RNA-seq of femur bone marrow from young and old mice after 55-58 days of microgravity on the ISS (SpaceX-18 / RR-17 RRRM-2 mission).

mouse SINGLE CELL RNA SEQ

Conditions: spaceflight microgravity ground control young mice old mice

Single cell transcriptional profiling of humerus bone marrow from mice flown on RRRM-2 osdr:OSD-403

Single-cell RNA-seq of humerus bone marrow from young and old mice after 55-58 days of microgravity on the ISS (SpaceX-18 / RR-17 RRRM-2 mission).

mouse SINGLE CELL RNA SEQ

Conditions: spaceflight microgravity ground control

Effects of Spaceflight on Bone Microarchitecture in Axial and Appendicular Skeleton in Growing Ovariectomized Rats (STS-62) osdr:OSD-351

MicroCT analysis of femur, humerus, lumbar vertebra, and calvarium from ovariectomized rats flown on STS-62. Spaceflight reduced cortical bone growth in appendicular skeleton.

rat

Conditions: spaceflight microgravity ovariectomized ground control

Fifteen days of microgravity causes growth in calvaria of mice (STS-131) osdr:OSD-486

MicroCT analysis of skull bone from mice flown on STS-131. Demonstrates site-specific bone adaptation with skull bone volume increasing in microgravity, contrasting with weight-bearing site losses.

mouse

Conditions: spaceflight microgravity ground control

RR-1 NASA Validation Flight - Mouse liver transcriptomic, proteomic, epigenomic and histology data osdr:OSD-47

Multi-omics dataset from the RR-1 mission (SpaceX-4) including RNA-seq, whole-genome bisulfite sequencing, mass spectrometry, and histology from C57BL/6J mice after 37 days of spaceflight.

mouse BULK RNA SEQ

Conditions: spaceflight microgravity ground control

PMID:33242417

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Experimental Models

3

MLO-Y4 osteocyte-like cell line CELL_LINE namo:CellLineModel

Murine long bone osteocyte line immortalized under an osteocalcin promoter. It made the osteocyte experimentally accessible, which matters here because the osteocyte is the cell this entry's senescence and mechanosensing nodes both run through.

Organism

mouse NCBITaxon:10090

Publication

PMID:9421234

Pathograph links

Osteocyte Mechanosensing Disruption in Microgravity measures fidelity: low The line was established specifically to make osteocyte mechanotransduction and osteocyte-osteoblast-osteoclast communication testable in vitro.

Limitations: Murine, immortalized by SV40 large T antigen, and cultured on collagen-coated plastic rather than embedded in mineralized matrix, so the lacuno-canalicular geometry that carries the mechanical signal in vivo is absent. It does not model aging.

Show evidence (1 reference)

PMID:9421234 SUPPORT In Vitro

"This newly established cell line should prove useful for studying the effects of mechanical stress on osteocyte function and for determining the means whereby osteocytes communicate with other bone cells such as osteoblasts and osteoclasts."

The authors' stated purpose matches the mechanism this link points at.

IDG-SW3 osteoblast-to-osteocyte differentiation line CELL_LINE namo:CellLineModel

A conditionally immortalized line that traverses the whole osteoblast-to-late-osteocyte transition in culture and expresses SOST/sclerostin under PTH and 1,25-dihydroxyvitamin D3 control. That makes it the accessible in vitro system for the sclerostin axis this entry targets pharmacologically with romosozumab.

Organism

mouse NCBITaxon:10090

Culture

3D matrix or 2D osteogenic culture at 37 C without interferon gamma

Publication

PMID:21735478

Pathograph links

Bone Remodeling Imbalance measures fidelity: low Provides a readout of sclerostin production and its hormonal regulation, the formation-side brake in the remodeling unit.

Limitations: Murine and conditionally immortalized. It reconstructs a differentiation trajectory, not a remodeling unit - there are no osteoclasts in it, so the balance this entry models cannot be observed directly.

Show evidence (1 reference)

PMID:21735478 SUPPORT In Vitro

"Like late osteocytes, they develop a dendritic morphology and express SOST/sclerostin and fibroblast growth factor 23 (FGF-23), regulated by parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D(3)."

Establishes that the regulated sclerostin readout is present, which is what this link claims the model measures.

3D bone organoid with coupled osteoblast-osteoclast remodeling ORGANOID namo:Organoid

A human bone organoid built by letting marrow stromal cell-derived osteoblasts mineralize an endosteal-like matrix for 21 days, then adding marrow macrophage-derived osteoclasts to restore resorption. It is the closest available non-animal system to the coupled remodeling unit this entry models, and it reports the same CTX-1 resorption marker used clinically.

Organism

human NCBITaxon:9606

Publication

PMID:34173283

Pathograph links

Bone Remodeling Imbalance partially recapitulates fidelity: moderate Reconstitutes both arms of the remodeling unit in human cells, with osteoblastic mineralization and osteoclastic resorption in the same construct.

Limitations: The imbalance is imposed by adding myeloma plasma cells, not by estrogen withdrawal or aging, so the disease-relevant driver is a different one. It is short-term (weeks), has no vasculature, no marrow adipocyte compartment, and no mechanical loading, so neither the senescence nor the marrow-lineage node in this entry can be studied in it.

Readouts

Type I collagen C-telopeptide release from the organoid

Direction: INCREASED

Interpretation: Uses the same resorption marker as the clinical CTX assay, so the in vitro and clinical readouts are directly comparable.

Show evidence (1 reference)

PMID:34173283 SUPPORT In Vitro

"increases levels of CTX-1, and decreased expression of osteoblastic genes"

Reports the coupled shift toward resorption that this link claims the organoid reproduces.

Show evidence (1 reference)

PMID:34173283 SUPPORT In Vitro

"On day 22, bone marrow macrophage (BMM)-derived osteoclasts (OCs) were introduced to enhance the resorptive capability of the model and recapitulate the balanced homeostatic nature of skeletal remodeling."

Documents that both arms of the remodeling unit are present, which is the basis for treating this as a model of remodeling balance.

Developed and validated for myeloma bone disease, not for osteoporosis. The normal-bone arm of the platform is what is relevant here; no published application to age-related or postmenopausal bone loss is recorded, and it carries no aging axis.

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Animal Models

3

Ovariectomized rat SURGICAL

Bilateral ovariectomy in the rat produces acute estrogen withdrawal, a surge in cancellous bone turnover, and net trabecular bone loss at the proximal tibia, distal femur and lumbar vertebrae. The 1994 FDA preclinical guidance names it as the rodent species for evaluating agents intended to treat or prevent postmenopausal osteoporosis, which makes it the regulatory reference model rather than merely a common one.

Species

Rat

Genotype

Wild type, surgically ovariectomized

Publication

PMID:8579908

Pathograph links

RANKL/OPG Dysregulation recapitulates fidelity: high Reproduces the estrogen-deficiency route into accelerated remodeling, and the effect is estrogen-reversible, which establishes the mechanism rather than merely the phenotype.

Limitations: Captures the estrogen-withdrawal arm only. It says nothing about the senescence and marrow-lineage arms of this entry, which are age-driven and present in both sexes.

Readouts

Cancellous bone turnover after ovariectomy

Direction: INCREASED

Interpretation: Activation of remodeling is the proximate consequence of estrogen withdrawal in this model.

Show evidence (1 reference)

PMID:8579908 SUPPORT Model Organism

"These data illustrate that cancellous bone turnover increases following OVX and this increased bone turnover produces bone loss."

Reports the turnover measurement and links it causally to the bone loss.

Show evidence (1 reference)

PMID:8579908 SUPPORT Model Organism

"Estrogen completely blocks the activation of bone turnover and bone loss."

The rescue arm shows the model's bone loss runs through estrogen deficiency, which is what makes it informative for this node.

Microarchitectural Deterioration partially recapitulates fidelity: moderate Trabecular bone volume falls and stays low, so the structural endpoint is reproduced, but the driving turnover signal is not sustained.

Limitations: Over a 12-month study the cancellous turnover indices return to sham control values while trabecular bone volume remains low, so the model decouples the mechanism from the deficit at exactly the durations the guidance recommends. It also lacks intracortical Haversian remodeling entirely, so cortical porosity cannot be studied in it.

Readouts

Cancellous bone turnover at 12 months

Direction: UNCHANGED

Interpretation: A genuine negative result - turnover normalizes even though the structural deficit persists.

Show evidence (1 reference)

PMID:8579908 SUPPORT Model Organism

"the indices of cancellous bone turnover return to the value of sham controls, although the trabecular bone volume remains lower than that of sham controls in OVX rats"

Documents the decoupling that limits long-duration studies in this model.

Show evidence (1 reference)

PMID:8579908 SUPPORT Model Organism

"it is difficult to determine the effects of potential therapeutic agents on the bone turnover in estrogen deficient conditions"

The authors' own statement of the limitation, which is why this link is PARTIALLY_RECAPITULATES rather than RECAPITULATES.

Aged INK-ATTAC senescent-cell clearance mouse TRANSGENIC

A suicide transgene that lets p16INK4a-expressing senescent cells be killed on demand with AP20187. Applied to 20-22-month-old mice with bone loss already established, it is an interventional test of whether senescent cells cause age-related bone loss rather than merely accompany it.

Species

Mouse

Genotype

INK-ATTAC transgenic (inducible caspase 8 under the p16INK4a promoter)

Publication

PMID:28825716

Pathograph links

Senescent Cell Accumulation in the Bone Microenvironment rescues fidelity: moderate Clearing roughly 30% of senescent cells in old mice raises bone mass, microarchitecture and strength; the same treatment in 12-month-old mice changes nothing, which is the specificity control.

Limitations: Murine, and the transgene is not a therapy. The mouse skeleton lacks osteonal cortical remodeling, so the cortical arm of the result does not transfer directly. The human phase 2 senolytic trial did not reproduce the resorption effect at its primary endpoint - see the discussions section.

Readouts

Spine trabecular bone volume fraction after senescent-cell clearance

Direction: RESTORED

Interpretation: Structural correlate of removing the senescent-cell burden in an already-osteopenic skeleton.

Show evidence (1 reference)

PMID:28825716 SUPPORT Model Organism

"AP20187-treated mice had better spine trabecular bone microarchitecture (e.g., 52% higher BV/TV"

The primary structural measurement behind the rescue claim.

Senescent osteocyte burden

Direction: DECREASED

Interpretation: Target engagement - confirms the transgene did what it was meant to before the bone effect is attributed to it.

Show evidence (1 reference)

PMID:28825716 SUPPORT Model Organism

"This was confirmed by fewer senescent osteocytes in AP20187- versus vehicle-treated mice"

Independent confirmation of clearance by SADS, alongside p16INK4a and EGFP mRNA.

Show evidence (1 reference)

PMID:28825716 SUPPORT Model Organism

"treatment of young adult (12-month-old) INK-ATTAC mice with AP20187 did not alter bone parameters"

The age-specificity control is what makes this model informative for an aging node rather than for bone mass generally.

🧮

Computational Models

2

QCT-based finite element model of the proximal femur PHYSIOLOGICAL

A patient-specific structural model built from a quantitative CT scan of the hip, solving for femoral strength and for the ratio of the load a fall would impose to the strength available to resist it. It is the computational counterpart of this entry's claim that microarchitecture and geometry carry fracture risk that areal BMD alone does not.

PMID:19049327

Pathograph links

Microarchitectural Deterioration measures fidelity: moderate Turns the structural deterioration this node describes into a mechanical quantity that prospectively predicts hip fracture, and the load-to-strength ratio stays predictive after adjustment for areal BMD.

Limitations: Resolves geometry and density at CT scale, so it does not represent trabecular connectivity or cortical porosity directly, and it says nothing about the cellular mechanisms upstream. Validated here in older men; the cohort is not the population most affected.

Readouts

Load-to-strength ratio adjusted for areal BMD

Direction: INCREASED

Interpretation: Retained prediction after BMD adjustment is what shows the model captures structural information beyond density.

Show evidence (1 reference)

PMID:19049327 SUPPORT Human Clinical

"When adjusted additionally for aBMD, the load-to-strength ratio remained significantly associated with fracture (HR = 3.1, 95% CI: 1.6-6.1)."

The BMD-adjusted hazard ratio is the specific result behind this readout.

Show evidence (1 reference)

PMID:19049327 SUPPORT Human Clinical

"These results provide insight into hip fracture etiology and demonstrate the ability of FE-based biomechanical analysis of QCT scans to prospectively predict hip fractures in men."

Prospective case-cohort validation in 3549 community-dwelling men establishes the model as predictive rather than merely descriptive.

Bone cell population dynamics model of the remodeling unit KINETIC

A differential-equation model of autocrine and paracrine signalling between osteoblast and osteoclast lineages at a single remodeling site, solving for cell population dynamics and the resulting change in bone mass. It is the formal counterpart of this entry's Bone Remodeling Imbalance node.

PMID:14499354

Pathograph links

Bone Remodeling Imbalance measures fidelity: low Reproduces a single remodeling cycle, self-sustaining cycling, and unstable runaway remodeling from one parameter set, and identifies autocrine regulation of osteoclasts as the parameter that decides which regime the system occupies.

Limitations: A single-site abstraction with lumped parameters and no spatial structure, no osteocyte compartment, and no aging or estrogen axis, so it cannot represent the senescence or marrow-lineage nodes in this entry. Its pathological regime was matched to Paget disease rather than to osteoporosis.

Show evidence (1 reference)

PMID:14499354 SUPPORT Computational

"the mathematical model revealed that interactions among osteoblasts and osteoclasts result in complex, nonlinear system behavior, which cannot be deduced from studies of each cell type alone"

States what the model contributes over single-cell-type reasoning, which is the reason to record it against the imbalance node.

No committed dismech-perturb run exists for this model; it is recorded as published external work, not as an executable model in this repository.

Show evidence (1 reference)

PMID:14499354 SUPPORT Computational

"Parametric analysis demonstrated that the mode of dynamic behavior in the system depends strongly on the regulation of osteoclasts by autocrine factors, such as transforming growth factor beta."

Identifies the parameter that determines system behaviour, the model's substantive prediction.

{ }

Source YAML

click to show

name: Osteoporosis
creation_date: '2025-12-18T17:01:35Z'
description: >-
  Osteoporosis results from an imbalance in bone remodeling, with osteoclast-mediated resorption outpacing osteoblast-mediated formation.
  An increased RANKL-to-osteoprotegerin ratio, accelerated by estrogen deficiency and aging, promotes osteoclastogenesis and net bone loss.
  Progressive loss of bone mass and microarchitectural deterioration reduce bone strength and predispose to fragility fractures.
category: Complex
parents:
- Musculoskeletal Disease
- Metabolic Disease
disease_term:
  preferred_term: osteoporosis
  term:
    id: MONDO:0005298
    label: osteoporosis
prevalence:
- population: Worldwide
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 18300.0
  rate_low: 16200.0
  rate_high: 20700.0
  notes: >-
    Pooled estimate from 86 studies (103,334,579 participants, ages 15-105). The
    pooled population mixes screening settings and age structures, so this is an
    order-of-magnitude figure rather than an age-standardised rate.
  evidence:
  - reference: PMID:34657598
    reference_title: "The global prevalence of osteoporosis in the world: a comprehensive systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the prevalence of osteoporosis in the world was reported to be 18.3
      (95% CI 16.2-20.7)
    explanation: Systematic review and meta-analysis giving the pooled worldwide
      point prevalence of osteoporosis.
- population: Women worldwide
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 23100.0
  rate_low: 19800.0
  rate_high: 26900.0
  notes: Sex-stratified arm of the same meta-analysis (70 studies, 800,457 women).
  evidence:
  - reference: PMID:34657598
    reference_title: "The global prevalence of osteoporosis in the world: a comprehensive systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the prevalence of osteoporosis in women of the world was reported to
      be 23.1 (95% CI 19.8-26.9)
    explanation: Quantifies the roughly two-fold female excess that motivates the
      estrogen-deficiency arm of the mechanism.
- population: Men worldwide
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 11700.0
  rate_low: 9600.0
  rate_high: 14100.0
  notes: Sex-stratified arm of the same meta-analysis (40 studies, 453,964 men).
  evidence:
  - reference: PMID:34657598
    reference_title: "The global prevalence of osteoporosis in the world: a comprehensive systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the prevalence of osteoporosis among men of the world was found to be
      11.7
    explanation: Osteoporosis in men is common rather than exceptional, which is
      why the entry is not scoped to postmenopausal disease.
- population: United States adults aged 50 years and older (NHANES 2005-2010)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 10300.0
  notes: >-
    Densitometric definition (T-score <= -2.5 at femoral neck or lumbar spine) in a
    nationally representative survey; a further 43.9% had low bone mass.
  evidence:
  - reference: PMID:24771492
    reference_title: "The recent prevalence of osteoporosis and low bone mass in the United States based on bone mineral density at the femoral neck or lumbar spine."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Based on an overall 10.3% prevalence of osteoporosis, we estimated
      that in 2010, 10.2 million older adults had osteoporosis.
    explanation: Nationally representative DXA-based prevalence estimate for a
      single well-characterised population.
pathophysiology:
- name: Osteocyte SASP Acquisition
  conforms_to: "cellular_senescence#Senescence-Associated Secretory Phenotype"
  biological_scale: CELLULAR
  description: >-
    With advancing age, osteocytes - the long-lived, terminally differentiated,
    non-dividing cells that orchestrate remodeling - upregulate p16INK4a and
    acquire a senescence-associated secretory phenotype. Osteocytes and myeloid
    cells are the two bone-resident populations that predominantly develop the
    SASP, and the secretome impairs osteoblast mineralization while enhancing
    osteoclast-progenitor survival. This is the aging arm of the entry: it is
    distinct from estrogen deficiency and operates in both sexes.
  cell_types:
  - preferred_term: Osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  biological_processes:
  - preferred_term: SASP cytokine production by senescent osteocytes
    modifier: INCREASED
    term:
      id: GO:0001816
      label: cytokine production
  - preferred_term: Inflammatory response in the bone microenvironment
    modifier: INCREASED
    term:
      id: GO:0006954
      label: inflammatory response
  downstream:
  - target: Senescent Cell Accumulation in the Bone Microenvironment
    causal_link_type: DIRECT
    description: >-
      Senescent cells are apoptosis-resistant, so once the rate at which they
      arise outpaces immune clearance they accumulate in the bone
      microenvironment.
  - target: RANKL/OPG Dysregulation
    causal_link_type: DIRECT
    description: >-
      Senescent-cell conditioned medium enhances osteoclast-progenitor survival
      and osteoclastogenesis, converging on the same resorption-favouring state
      that estrogen loss produces by a different route.
    evidence:
    - reference: PMID:28825716
      reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: senescent-cell conditioned medium impaired osteoblast
        mineralization and enhanced osteoclast-progenitor survival, leading to
        increased osteoclastogenesis
      explanation: Establishes that the secreted SASP, rather than the senescent
        cell itself, is what acts on the osteoblast and osteoclast compartments.
  evidence:
  - reference: PMID:27341653
    reference_title: "Identification of Senescent Cells in the Bone Microenvironment."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: senescent myeloid cells and senescent osteocytes predominantly
      develop the SASP
    explanation: Identifies the osteocyte as one of the two SASP-developing
      populations in bone, which is why this node is scoped to osteocytes.
  - reference: PMID:27341653
    reference_title: "Identification of Senescent Cells in the Bone Microenvironment."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "revealed significantly more senescent osteocytes in old compared
      with young bone cortices (11% versus 2%, p < 0.001)"
    explanation: Quantifies the age-associated rise in senescent osteocytes in
      vivo using SADS, without relying on cultured cells.
  - reference: PMID:27341653
    reference_title: "Identification of Senescent Cells in the Bone Microenvironment."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Corresponding with the age-associated accumulation of senescent
      osteocytes was significantly higher expression of multiple SASP markers in
      osteocytes from old versus young mice
    explanation: Supports the cytokine-production and inflammatory-response
      annotations on this node - the SASP markers measured are the secreted
      products those processes name.
- name: Senescent Cell Accumulation in the Bone Microenvironment
  conforms_to: "cellular_senescence#Senescent Cell Accumulation"
  biological_scale: TISSUE
  description: >-
    Senescent cells accumulate across multiple bone-resident lineages with age.
    The burden is causal rather than a bystander marker: clearing roughly 30% of
    p16-positive cells in aged mice, genetically or pharmacologically, yields
    higher bone mass, better microarchitecture and greater strength, and does so
    only in old animals.
  cell_types:
  - preferred_term: Osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  biological_processes:
  - preferred_term: Cellular Senescence
    modifier: INCREASED
    term:
      id: GO:0090398
      label: cellular senescence
  downstream:
  - target: Bone Remodeling Imbalance
    causal_link_type: DIRECT
    description: >-
      Clearing senescent cells lowers resorption while maintaining trabecular and
      raising cortical formation, which is the remodeling imbalance running in
      reverse.
    evidence:
    - reference: PMID:28825716
      reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: The beneficial effects of targeting senescent cells were due to
        lower bone resorption with either maintained (trabecular) or higher
        (cortical) bone formation as compared to vehicle-treated mice
      explanation: Shows the intervention acts on both arms of the remodeling
        balance, which is what makes this an upstream node rather than a
        resorption-only one.
  evidence:
  - reference: PMID:28825716
    reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: these data establish a causal role for senescent cells in bone loss
      with aging, and demonstrate that targeting these cells has both
      anti-resorptive and anabolic effects on bone
    explanation: The authors' own causal claim, supported by concordant genetic
      and pharmacological clearance arms in the same study.
- name: Marrow Stromal Lineage Shift Toward Adipogenesis
  conforms_to: "stem_cell_exhaustion#Decline in Stem Cell Self-Renewal and Function"
  biological_scale: CELLULAR
  description: >-
    Osteoblasts and marrow adipocytes derive from a shared mesenchymal precursor,
    and with age that precursor is allocated increasingly to the adipogenic
    rather than the osteogenic fate. Marrow adipose volume rises with age and is
    higher again in osteoporosis at matched age, and it is inversely correlated
    with trabecular bone volume - a decline in the regenerative output of the
    stromal compartment rather than a change in resorption.
  cell_types:
  - preferred_term: Bone marrow mesenchymal stromal cell
    term:
      id: CL:0000134
      label: mesenchymal stem cell
  - preferred_term: Marrow adipocyte
    term:
      id: CL:0000136
      label: adipocyte
  biological_processes:
  - preferred_term: Fat Cell Differentiation
    modifier: INCREASED
    term:
      id: GO:0045444
      label: fat cell differentiation
  - preferred_term: Osteoblast Differentiation
    modifier: DECREASED
    term:
      id: GO:0001649
      label: osteoblast differentiation
  downstream:
  - target: Bone Remodeling Imbalance
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Diverting the shared precursor away from the osteoblast lineage reduces the
      formation side of the remodeling unit, so the imbalance is reached without
      any increase in osteoclast activity.
  notes: >-
    The direction of causation is not settled by the cited human data, which are
    cross-sectional biopsies; the correlation is consistent with lineage
    diversion but does not exclude marrow adiposity being a consequence of bone
    loss. See the discussions section. On the conformance: this node models the
    differentiation-potential aspect of stem cell exhaustion, not the
    pool-maintenance aspect, so the module's GO:0019827 (stem cell population
    maintenance) is deliberately absent rather than overlooked - the claim is
    that the precursor is allocated differently, not that fewer precursors
    remain. GO:0045444 and GO:0001649 carry that allocation claim directly.
  evidence:
  - reference: PMID:11708718
    reference_title: "Adipocyte tissue volume in bone marrow is increased with aging and in patients with osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was an age-related inverse correlation between BV/TV and
      AV/TV (r = -0.58, P < 0.001)."
    explanation: Iliac crest biopsies from 53 healthy individuals aged 30-100
      show marrow fat rising as trabecular bone volume falls.
  - reference: PMID:11708718
    reference_title: "Adipocyte tissue volume in bone marrow is increased with aging and in patients with osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Compared with age-matched controls, patients with osteoporosis
      exhibited an increased AV/TV (P < 0.05) and decreased BV/TV (P < 0.05)"
    explanation: The shift is greater in osteoporosis than in age-matched
      controls, so it is not purely a function of chronological age.
- name: Bone Remodeling Imbalance
  description: >
    Increased osteoclast-mediated bone resorption relative to osteoblast- mediated
    bone formation leads to net bone loss. This imbalance accelerates with aging and
    estrogen deficiency.
  cell_types:
  - preferred_term: Osteoclast
    term:
      id: CL:0000092
      label: osteoclast
  - preferred_term: Osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  biological_processes:
  - preferred_term: Bone Remodeling
    term:
      id: GO:0046849
      label: bone remodeling
  evidence:
  - reference: PMID:21874760
    reference_title: "Aromatase activity and bone loss."
    supports: SUPPORT
    snippet: aromatase activity and estrogen production are necessary for longitudinal
      bone growth, attainment of peak bone mass, the pubertal growth spurt, epiphyseal
      closure, and normal bone remodeling in young individuals.
    explanation: Demonstrates that estrogen, produced by aromatase, is essential for
      normal bone remodeling and that deficiency leads to bone loss
  - reference: PMID:23748068
    reference_title: "Aromatase deficiency, a rare syndrome: case report."
    supports: SUPPORT
    snippet: Estrogen deficiency can induce delayed epiphyseal closure, eunuchoid
      body habitus, osteopenia, and osteoporosis in both genders.
    explanation: Clinical evidence showing that estrogen deficiency directly causes
      osteoporosis through impaired bone remodeling
- name: RANKL/OPG Dysregulation
  description: >
    Increased RANKL and decreased osteoprotegerin (OPG) ratio promotes osteoclast
    differentiation and activation. Estrogen normally suppresses RANKL and increases
    OPG. Microgravity further exacerbates this imbalance by directly increasing the
    osteoblast RANKL/OPG ratio.
  genes:
  - preferred_term: ESR1
    description: >-
      The gene encoding estrogen receptor alpha, the receptor through which
      estrogen suppresses RANKL expression in bone lining cells; it is the loss
      of that receptor-mediated suppression after estrogen withdrawal that this
      node models, not a variant in the gene. The entry's ESR1 susceptibility
      record is the common-variant counterpart of this mechanism.
    term:
      id: hgnc:3467
      label: ESR1
  molecular_functions:
  - preferred_term: nuclear estrogen receptor activity in bone lining cells
    modifier: DECREASED
    description: >-
      Receptor occupancy falls with circulating estrogen; the receptor itself
      is intact, so the activity is reduced rather than absent or altered.
    term:
      id: GO:0030284
      label: nuclear estrogen receptor activity
  biological_processes:
  - preferred_term: Osteoclast Differentiation
    term:
      id: GO:0030316
      label: osteoclast differentiation
  evidence:
  - reference: PMID:28744019
    reference_title: "Estrogen Regulates Bone Turnover by Targeting RANKL Expression in Bone Lining Cells."
    supports: SUPPORT
    directness: INDIRECT
    quote_role: PRIMARY_RESULT
    evidence_source: MODEL_ORGANISM
    snippet: "Our data indicate that the increase in bone resorption observed in states of estrogen deficiency in mice is mainly caused by lack of ERα-mediated suppression of RANKL expression in bone lining cells."
    explanation: >-
      Grounds the ESR1 descriptor on this node and its direction: selective
      deletion of estrogen receptor alpha in murine reconstitution models shows
      that the RANKL rise of estrogen deficiency is a loss of ERalpha-mediated
      suppression in bone lining cells. Graded INDIRECT because the finding is
      in ovariectomized mice rather than postmenopausal bone.
  - reference: PMID:33692806
    reference_title: "Role of the IL-23/IL-17 Pathway in Rheumatic Diseases: An Overview."
    supports: SUPPORT
    snippet: The differentiation of osteoclasts is induced significantly in the presence
      of IL-17 either directly (207), or indirectly, through upregulation of RANKL.
    explanation: Demonstrates that RANKL upregulation is a key mechanism for osteoclast
      differentiation and bone resorption
  - reference: PMID:21874760
    reference_title: "Aromatase activity and bone loss."
    supports: SUPPORT
    snippet: with aging, individual differences in aromatase activity and thus in
      estrogen levels may significantly affect bone loss and fracture risk in both
      genders.
    explanation: Shows that age-related decline in estrogen production affects bone
      metabolism, linking to RANKL/OPG dysregulation
  - reference: PMID:16927271
    reference_title: "Modeled microgravity stimulates osteoclastogenesis and bone resorption by increasing osteoblast RANKL/OPG ratio."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: microgravity is capable to indirectly stimulate osteoclast formation
      and activity by regulating osteoblast secretion of crucial regulatory factors
      such as RANKL and OPG
    explanation: Simulated microgravity using NASA Rotating Wall Vessel increases
      osteoblast RANKL/OPG ratio, stimulating osteoclastogenesis and bone resorption
  - reference: PMID:26318907
    reference_title: "Osteoprotegerin is an effective countermeasure for spaceflight-induced bone loss in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: SF/VEH mice had lower BV/TV vs. GC/VEH mice, while SF/OPG-Fc mice had
      greater BV/TV than SF/VEH or GC/VEH
    explanation: OPG-Fc (RANKL inhibitor) prevented spaceflight-induced bone loss
      in mice during 12-day shuttle mission, confirming RANKL pathway centrality
  notes: >-
    ESR1 is bound here because this is the node the entry's Estrogen Deficiency
    exposure triggers and the one whose description places estrogen's restraint
    of RANKL. Estrogen receptor alpha also acts on bone through a second route
    not modelled on this node: its deletion from differentiated osteoclasts
    causes trabecular bone loss, and estrogen induces osteoclast apoptosis
    through Fas ligand only when osteoclast ERalpha is present (PMID:17803905).
    That is an osteoclast-lifespan mechanism rather than a RANKL/OPG one, and
    it belongs on the Bone Remodeling Imbalance node if a curator wires it.
    ESR2 is not bound: the cited work attributes the RANKL effect to the alpha
    receptor.

    No cell type is bound on this node, although the molecular-function
    descriptor names bone lining cells, because CL has no term for that cell.
    An exact OLS search of cl for "bone lining cell" returns nothing; a
    non-exact search for "bone lining" returns CL:0000092 osteoclast,
    CL:0020016 osteochondral skeletal stem cell and CL:0002157 endosteal cell
    alongside unrelated haematopoietic terms; and a broader search for
    "lining" returns only lineage and lateral-line matches. Endosteal cell is
    the closest and is still wrong: CL defines it as the cell making up the
    vascular membrane lining the marrow cavity, and its extended description
    has it differentiating into osteoblasts, so it is an active osteogenic
    cell of the endosteum rather than the quiescent flattened cell that covers
    a quiet bone surface, which is what the descriptor means. No CL term
    request has been filed; if one is, link it here.
- name: Microarchitectural Deterioration
  description: >
    Loss of trabecular connectivity and cortical thinning compromises bone strength
    beyond what BMD alone indicates. Leads to fragility fractures with minimal trauma.
    Spaceflight accelerates this process, with incomplete microarchitectural recovery
    even one year post-flight.
  evidence:
  - reference: PMID:23748068
    reference_title: "Aromatase deficiency, a rare syndrome: case report."
    supports: SUPPORT
    snippet: recurrent bone fractures associated with minimal trauma starting in puberty
    explanation: Clinical case demonstrating that estrogen deficiency leads to fragility
      fractures due to compromised bone microarchitecture
  - reference: PMID:35773442
    reference_title: "Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: nine of 17 astronauts did not fully recover tibia total BMD after 12-months
    explanation: HR-pQCT imaging of astronauts shows incomplete recovery of trabecular
      microarchitecture at weight-bearing tibia one year after long-duration spaceflight,
      equivalent to a decade of terrestrial bone aging
  - reference: PMID:31897866
    reference_title: "Spaceflight-Induced Bone Tissue Changes that Affect Bone Quality and Increase Fracture Risk."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: bone structural quality (e.g., trabecular microarchitecture) is diminished
      and the quality of bone material is reduced via impaired tissue mineralization,
      maturation, and maintenance (e.g., mediated by osteocytes)
    explanation: Review of spaceflight-induced bone quality changes beyond BMD, showing
      both structural and material quality are compromised in microgravity
  - reference: PMID:28572612
    reference_title: "One-month spaceflight compromises the bone microstructure, tissue-level mechanical properties, osteocyte survival and lacunae volume in mature mice skeletons."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: osteocyte death was evidenced along with a smaller and more spherical
      shape of the osteocyte lacunae because of lacunar mineralization
    explanation: Bion-M1 one-month spaceflight caused trabecular disconnection, cortical
      thinning, and osteocyte death in mice, with no recovery at 8 days
- name: Osteocyte Mechanosensing Disruption in Microgravity
  description: Microgravity removes gravitational mechanical loading, impairing osteocyte
    mechanotransduction. Osteocytes sense mechanical strain via the lacuno-canalicular
    network and regulate bone remodeling accordingly. In microgravity, loss of fluid
    shear stress and mechanical stimulation leads to downregulation of key osteocytic
    genes and impaired osteocyte differentiation.
  cell_types:
  - preferred_term: Osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  biological_processes:
  - preferred_term: Cellular Response to Mechanical Stimulus
    term:
      id: GO:0071260
      label: cellular response to mechanical stimulus
  evidence:
  - reference: PMID:33835498
    reference_title: "Global transcriptomic analysis of a murine osteocytic cell line subjected to spaceflight."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: µG impairs the differentiation of osteocytes, consistent with prior osteoblast
      spaceflight experiments, which resulted in the downregulation of key osteocytic
      genes
    explanation: Osteocyte cell line (Ocy454) flown on ISS SpaceX Dragon-6 showed
      impaired differentiation and altered glycolysis, effects not replicated by ground-based
      simulators
  - reference: PMID:29559713
    reference_title: "Skeletal changes during and after spaceflight."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: clear parallels exist between the effects of spaceflight, periods of
      immobilization and ageing, with possibly irreversible features
    explanation: Review establishes that spaceflight-induced skeletal changes parallel
      accelerated aging via disrupted mechanosensing
- name: Microgravity-Accelerated Bone Resorption
  conforms_to: "osteoporosis_bone_resorption#Increased Osteoclastic Bone Resorption"
  description: Spaceflight induces a rapid and sustained increase in osteoclast-mediated
    bone resorption. Astronauts lose approximately 0.8% of bone mass per month in
    weight-bearing lower limb sites, with bone resorption markers plateauing at 113%
    above pre-flight levels within days of entering microgravity.
  biological_processes:
  - preferred_term: Bone Resorption
    term:
      id: GO:0045453
      label: bone resorption
  evidence:
  - reference: PMID:32411816
    reference_title: "A systematic review and meta-analysis of bone loss in space travelers."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Bone loss in space travelers is a major challenge for long-duration space
      exploration.
    explanation: Meta-analysis of 148 space travelers quantifies lower limb bone loss
      rate of -0.8% per month during microgravity exposure
  - reference: PMID:32411816
    reference_title: "A systematic review and meta-analysis of bone loss in space travelers."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: microgravity-induced bone loss is a significant and unresolved health
      risk for space travelers
    explanation: Bone resorption markers plateau at 113% above pre-flight levels with
      rapid onset (half-max at 11 days)
- name: Spaceflight-Induced Mitochondrial Stress in Bone
  description: Multi-omics analyses of spaceflight tissues reveal mitochondrial dysfunction
    as a central biological hub for spaceflight impact. Mitochondrial stress pathways
    are consistently enriched across multiple tissue types and missions, contributing
    to impaired cellular function in bone and other organ systems.
  biological_processes:
  - preferred_term: Mitochondrion Organization
    term:
      id: GO:0007005
      label: mitochondrion organization
  evidence:
  - reference: PMID:33242417
    reference_title: "Comprehensive Multi-omics Analysis Reveals Mitochondrial Stress as a Central Biological Hub for Spaceflight Impact."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: Overall pathway analyses on the multi-omics datasets showed significant
      enrichment for mitochondrial processes, as well as innate immunity, chronic
      inflammation, cell cycle, circadian rhythm, and olfactory functions
    explanation: GeneLab multi-omics integration across NASA missions identifies mitochondrial
      stress as a consistent spaceflight phenotype
phenotypes:
- name: Decreased Bone Mineral Density
  category: Musculoskeletal
  frequency: VERY_FREQUENT
  diagnostic: true
  notes: T-score <= -2.5
  phenotype_term:
    preferred_term: Decreased Bone Density
    term:
      id: HP:0004349
      label: Reduced bone mineral density
  evidence:
  - reference: PMID:23748068
    reference_title: "Aromatase deficiency, a rare syndrome: case report."
    supports: SUPPORT
    snippet: Lumbar osteoporosis was detected in bone densitometry.
    explanation: Clinical documentation of reduced bone mineral density in aromatase
      deficiency-induced osteoporosis
  - reference: PMID:21874760
    reference_title: "Aromatase activity and bone loss."
    supports: SUPPORT
    snippet: aromatase activity and estrogen production are necessary for longitudinal
      bone growth, attainment of peak bone mass, the pubertal growth spurt, epiphyseal
      closure, and normal bone remodeling in young individuals.
    explanation: Demonstrates that estrogen deficiency impairs achievement and maintenance
      of peak bone mass, leading to decreased BMD
- name: Vertebral Fractures
  category: Musculoskeletal
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Vertebral Compression Fractures
    term:
      id: HP:0002953
      label: Vertebral compression fracture
  evidence:
  - reference: PMID:28168409
    reference_title: "Worldwide prevalence and incidence of osteoporotic vertebral fractures."
    supports: SUPPORT
    snippet: The prevalence of morphometric vertebral fractures in European women
      is highest in Scandinavia (26%) and lowest in Eastern Europe (18%).
    explanation: Systematic review documents high prevalence of vertebral fractures
      in osteoporosis, confirming this as a key phenotype.
- name: Femur Fractures
  category: Musculoskeletal
  frequency: OCCASIONAL
  notes: Major cause of morbidity and mortality
  phenotype_term:
    preferred_term: Femur Fractures
    term:
      id: HP:0031846
      label: Femur fracture
  evidence:
  - reference: PMID:16983459
    reference_title: "An estimate of the worldwide prevalence and disability associated with osteoporotic fractures."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: In the year 2000 there were an estimated 9.0 million osteoporotic
      fractures of which 1.6 million were at the hip, 1.7 million at the forearm
      and 1.4 million were clinical vertebral fractures.
    explanation: Quantifies proximal femur (hip) fracture as one of the three
      dominant osteoporotic fracture sites worldwide.
- name: Kyphosis
  category: Musculoskeletal
  frequency: FREQUENT
  notes: From vertebral compression fractures
  phenotype_term:
    preferred_term: Kyphosis
    term:
      id: HP:0002808
      label: Kyphosis
  evidence:
  - reference: PMID:9180660
    reference_title: "Correlates of kyphosis in older women. The Fracture Intervention Trial Research Group."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: a 15 degrees increase in kyphosis was associated with losing more
      than 4 cm of height (OR, 1.88; 95% CI, 1.79-2.03) and having a vertebral
      fracture (OR, 1.57; 95% CI, 1.46-1.69)
    explanation: In 6439 osteoporotic women, kyphosis tracks with vertebral
      fracture and height loss, supporting it as a downstream consequence of
      vertebral compression rather than an independent finding.
- name: Height Loss
  category: Musculoskeletal
  frequency: FREQUENT
  notes: >-
    Progressive loss of adult stature from vertebral compression, not congenital
    short stature. HPO has no term for acquired height loss, so this is bound to
    the accurate parent HP:0000002 rather than to HP:0004322 (Short stature),
    which asserts height below -2 SD for age and sex and is a different claim.
  phenotype_term:
    preferred_term: Height loss
    term:
      id: HP:0000002
      label: Abnormality of body height
  evidence:
  - reference: PMID:14742889
    reference_title: "Height loss and osteoporosis of the hip."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: women with at least 3 in. of height loss had odds of osteoporosis of
      the hip that were 9.6 times greater (95% confidence interval, 4.8-19.2) than
      women with less than an inch of height loss
    explanation: Measured height loss against self-reported maximum adult height
      is strongly associated with densitometric osteoporosis in 2108 women.
biochemical:
- name: Bone Turnover Markers
  presence: Variable
  context: CTX, P1NP indicate resorption/formation rates
  evidence:
  - reference: PMID:21184054
    reference_title: "Markers of bone turnover for the prediction of fracture risk and monitoring of osteoporosis treatment: a need for international reference standards."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: IOF/IFCC recommend one bone formation marker (s-PINP) and one bone
      resorption marker (s-CTX) to be used as reference markers
    explanation: Joint IOF/IFCC position designating s-PINP and s-CTX as the
      reference formation and resorption markers, which is why this entry names
      that pair.
- name: Calcium
  presence: Normal
  context: >-
    Usually normal in primary osteoporosis; measured chiefly to exclude a
    secondary cause such as primary hyperparathyroidism.
  evidence:
  - reference: PMID:12364413
    reference_title: "Yield of laboratory testing to identify secondary contributors to osteoporosis in otherwise healthy women."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: A testing strategy involving measurement of 24-h urine calcium, serum
      calcium, and serum PTH for all women and serum TSH among women on thyroid
      replacement therapy would have been sufficient to diagnose 47 of these 55
      women (85%)
    explanation: Supports the screening role of serum calcium by inference - it is
      informative because an abnormal value reclassifies the case as secondary,
      which presupposes that it is normal in primary osteoporosis.
- name: Vitamin D
  presence: Variable
  context: Deficiency common and contributes to bone loss
  evidence:
  - reference: PMID:15928802
    reference_title: "Vitamin D, parathyroid hormone levels and bone mineral density in community-dwelling older women: the Rancho Bernardo Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: hip BMD was negatively and independently associated with PTH levels
      (p=0.04), and positively and independently associated with 25(OH)D levels
      (p=0.03)
    explanation: In 615 community-dwelling postmenopausal women, 25(OH)D tracks
      positively with hip BMD independently of PTH.
- name: PTH
  presence: Variable
  context: May be elevated with vitamin D deficiency
  evidence:
  - reference: PMID:15928802
    reference_title: "Vitamin D, parathyroid hormone levels and bone mineral density in community-dwelling older women: the Rancho Bernardo Study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Low levels of serum calcium stimulate the secretion of parathyroid
      hormone (PTH), which maintains serum calcium levels at the expense of
      increased bone turnover, bone loss and increased risk of fractures
    explanation: States the secondary-hyperparathyroidism route by which a raised
      PTH translates into increased bone turnover and bone loss.
diagnosis:
- name: Bone Densitometry (DXA)
  description: >-
    Dual-energy X-ray absorptiometry of the proximal femur is the measurement on
    which the WHO operational definition of osteoporosis rests: bone mineral
    density is expressed as a T-score, the number of standard deviations from the
    young adult female mean, and osteoporosis is a T-score 2.5 SD or more below
    that mean. The femoral neck is the WHO reference site; ISCD practice also
    reads the lumbar spine and takes the lowest of the spine and hip T-scores.
  diagnosis_term:
    preferred_term: dual-energy X-ray absorptiometry
    term:
      id: NCIT:C48789
      label: Dual X-ray Absorptiometry
  measurements:
  - preferred_term: DXA femur T-score
    term:
      id: LOINC:38263-0
      label: DXA Femur [T-score] Bone density
  - preferred_term: DXA lumbar spine T-score
    term:
      id: LOINC:38267-1
      label: DXA Lumbar spine [T-score] Bone density
  reference_ranges:
  - loinc_term:
      id: LOINC:38263-0
      label: DXA Femur [T-score] Bone density
    unit: '{SD}'
    population: >-
      WHO diagnostic criteria for osteoporosis (published 1994), femoral neck
      DXA referenced to the NHANES III database for women aged 20-29 years, as
      restated by Kanis 2002 and Kanis et al. 2008; applied to women and men
    notes: >-
      The criterion is stated two ways in the cited abstracts. As a sentence it is
      inclusive: a BMD "2.5 SD or more below" the young adult mean (PMID:12057569,
      PMID:18180210), and Lombardi and Ross restate it as "at least 2.5 SD below"
      (PMID:11730255). The parenthetical forms are exclusive: "T score < -2.5 SD"
      (PMID:12057569) and "-2.5 < T-score < -1.0" for osteopenia (PMID:11730255).
      The half-open bands below put -2.5 itself in the low bone mass band, which
      agrees with the parentheticals and disagrees by one point with the
      sentence form of the WHO definition. Severe (established) osteoporosis is
      osteoporosis plus one or more fragility fractures (PMID:12057569), a
      fracture criterion rather than a further T-score band, so it is not
      recorded as a band. No normal band is recorded because none of the cached
      abstracts states the normal T-score range in quotable text. Kanis et al.
      2008 name the femoral neck as the reference site; LOINC:38263-0 is the
      femur code, bound because searches of the Monarch KG for "Femur neck",
      "Femur.neck", "Femoral neck T-score" and "neck Bone density" returned no
      femoral-neck-specific DXA T-score code.
    evidence:
    - reference: PMID:12057569
      reference_title: "Diagnosis of osteoporosis and assessment of fracture risk."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Osteoporosis is defined as a BMD 2.5 SD or more below the average value for premenopausal women (T score < -2.5 SD)."
      explanation: States the WHO osteoporosis cut-off reproduced as the upper boundary of the osteoporosis band.
    - reference: PMID:12057569
      reference_title: "Diagnosis of osteoporosis and assessment of fracture risk."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Severe osteoporosis denotes osteoporosis in the presence of one or more fragility fractures."
      explanation: Severe osteoporosis adds a fracture criterion to the T-score band, which is why it is described in the notes rather than as a further band.
    - reference: PMID:18180210
      reference_title: "A reference standard for the description of osteoporosis."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "In 1994, the World Health Organization published diagnostic criteria for osteoporosis."
      explanation: Dates the WHO criteria named in the population field.
    - reference: PMID:18180210
      reference_title: "A reference standard for the description of osteoporosis."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "The recommended reference range is the NHANES III reference database for femoral neck measurements in women aged 20-29 years. A similar cut-off value for femoral neck BMD that is used to define osteoporosis in women can be used for the diagnosis of osteoporosis in men - namely, a value for BMD 2.5 SD or more below the average for young adult women."
      explanation: Fixes the reference population the T-score is computed against and extends the same cut-off to men.
    - reference: PMID:11730255
      reference_title: "The assessment of bone mass in men."
      supports: SUPPORT
      evidence_source: OTHER
      quote_role: REVIEW_SYNTHESIS
      snippet: "Patients with BMD at least 2.5 SD below the young adult mean (T-score < -2.5) have osteoporosis, and those with BMD between 1 and -2.5 SD below the young adult mean (-2.5 < T-score < -1.0) have osteopenia."
      explanation: Restates the WHO criteria including the osteopenia band between -2.5 and -1.0.
    interpretation_bands:
    - name: Osteoporosis
      upper_bound: -2.5
      unit: '{SD}'
      abnormal_flag: LOW
      interpretation: >-
        Meets the WHO densitometric definition of osteoporosis; with one or more
        fragility fractures it is severe (established) osteoporosis.
    - name: Low bone mass (osteopenia)
      lower_bound: -2.5
      upper_bound: -1.0
      unit: '{SD}'
      abnormal_flag: LOW
      interpretation: >-
        WHO low bone mass (osteopenia); below the young adult range but above the
        osteoporosis threshold.
  notes: >-
    The lumbar spine T-score code is bound as a second measurement because ISCD
    practice reads the spine with the hip sites; the WHO bands are recorded on
    the femur code only, because the cited WHO sources define them at the
    femoral neck. The ISCD 2005 skeletal-site position (PMID:16731427) is
    abstract-only in the cache and does not restate the bands.
  evidence:
  - reference: PMID:18180210
    reference_title: "A reference standard for the description of osteoporosis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "this paper proposes that the reference standard should be based on bone mineral density (BMD) measurement made at the femoral neck with dual-energy X-ray absorptiometry (DXA)."
    explanation: Names femoral-neck DXA as the reference measurement for describing osteoporosis, the first measurement bound on this row.
  - reference: PMID:12057569
    reference_title: "Diagnosis of osteoporosis and assessment of fracture risk."
    supports: SUPPORT
    evidence_source: OTHER
    quote_role: REVIEW_SYNTHESIS
    snippet: "The recommended site for diagnosis is the proximal femur with dual energy X-ray absorptiometry (DXA)."
    explanation: Grounds the femur T-score as the diagnostic measurement.
  - reference: PMID:16731427
    reference_title: "Skeletal sites for osteoporosis diagnosis: the 2005 ISCD Official Positions."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "the lowest T-score of the total proximal femur, femoral neck, trochanter, and spine should continue to be used for diagnosis"
    explanation: >-
      One of the questions the 2005 ISCD positions were convened to answer; its
      wording ("continue to be used") records that the spine was already read
      alongside the hip sites for diagnosis, which is why the lumbar spine
      T-score is bound as a second measurement. The abstract does not state the
      answer the conference reached.
genetic:
- name: LRP5
  gene_term:
    preferred_term: LRP5
    term:
      id: hgnc:6697
      label: LRP5
  association: Risk Factor
  relationship_type: SUSCEPTIBILITY
  notes: >-
    Wnt co-receptor. One of only six BMD loci in the GEFOS meta-analysis whose
    association with fracture itself reached genome-wide significance, so the
    effect is not purely a BMD surrogate.
  evidence:
  - reference: PMID:22504420
    reference_title: "Genome-wide meta-analysis identifies 56 bone mineral density loci and reveals 14 loci associated with risk of fracture."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: including at 18p11.21 (FAM210A), 7q21.3 (SLC25A13), 11q13.2 (LRP5),
      4q22.1 (MEPE), 2p16.2 (SPTBN1) and 10q21.1 (DKK1)
    explanation: LRP5 is among the six loci associated with low-trauma fracture at
      genome-wide significance in 31,016 cases and 102,444 controls.
- name: SOST
  gene_term:
    preferred_term: SOST
    term:
      id: hgnc:13771
      label: SOST
  association: Risk Factor
  relationship_type: SUSCEPTIBILITY
  notes: >-
    Encodes sclerostin, the osteocyte-derived Wnt inhibitor that romosozumab
    blocks; the common-variant association and the drug target are the same
    node of the pathway.
  evidence:
  - reference: PMID:22504420
    reference_title: "Genome-wide meta-analysis identifies 56 bone mineral density loci and reveals 14 loci associated with risk of fracture."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In trans-iliac bone biopsies, expression of five genes correlated
      with LS-BMD and/or FN-BMD of the donors with P < 0.001 including PSME4
      (2p16.2), DKK1 (10q21.1), C17orf91 (17p13.3), SOST (17q21.31_1) and DUSP3
      (17q21.31_1)"
    explanation: SOST expression in human bone biopsies correlates with the donor's
      bone mineral density, linking the GWAS locus to bone tissue rather than to a
      neighbouring gene.
- name: ESR1
  gene_term:
    preferred_term: ESR1
    term:
      id: hgnc:3467
      label: ESR1
  association: Risk Factor
  relationship_type: SUSCEPTIBILITY
  notes: >-
    Estrogen receptor alpha. Carries two independent BMD association signals at
    6q25.1, the common-variant counterpart of the estrogen-deficiency mechanism.
  evidence:
  - reference: PMID:22504420
    reference_title: "Genome-wide meta-analysis identifies 56 bone mineral density loci and reveals 14 loci associated with risk of fracture."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Three loci included variants localized less than 40 Kb from the
      initial main signal suggesting allelic heterogeneity and included the 1p31.3
      (represented by rs17482952 near WLS), 6q25.1 (rs7751941 near ESR1) and the
      16q12.1 (rs1564981 near CYLD) loci.
    explanation: Conditional analysis resolves a second independent BMD signal at
      the ESR1 locus, establishing the association rather than a single tag SNP.
- name: VDR
  gene_term:
    preferred_term: VDR
    term:
      id: hgnc:12679
      label: VDR
  association: Disputed
  relationship_type: DISPUTED
  notes: >-
    Vitamin D receptor. Retained as a historically prominent candidate gene, not
    as an established risk locus: the classic restriction-fragment polymorphisms
    failed in a 26,242-participant meta-analysis, and VDR is not among the 56
    genome-wide significant BMD loci. Only the Cdx2 promoter variant retains a
    marginal vertebral-fracture signal.
  evidence:
  - reference: PMID:16908916
    reference_title: "The association between common vitamin D receptor gene variations and osteoporosis: a participant-level meta-analysis."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: The FokI, BsmI, ApaI, and TaqI VDR polymorphisms are not associated
      with BMD or with fractures, but the Cdx2 polymorphism may be associated with
      risk for vertebral fractures.
    explanation: Participant-level meta-analysis across 9 European cohorts finds no
      association between the commonly cited VDR polymorphisms and either BMD or
      fracture, contradicting the risk-factor claim.
environmental:
- name: Estrogen Deficiency
  exposure_term:
    preferred_term: low endogenous estrogen exposure
    modifier: DECREASED
    term:
      id: ECTO:9000010
      label: exposure to estrogens
  influences_mechanisms:
  - target: RANKL/OPG Dysregulation
    environmental_effect: TRIGGERS
    causal_link_type: DIRECT
    description: >-
      Estrogen restrains osteoclastogenesis largely by holding down RANKL
      relative to osteoprotegerin, so its withdrawal releases that brake
      directly. This is why bone loss accelerates sharply at menopause rather
      than gradually with age.
    evidence:
    - reference: PMID:12697741
      reference_title: "Role of RANK ligand in mediating increased bone resorption in early postmenopausal women."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The data suggest that upregulation of RANKL on bone marrow cells is an important determinant of increased bone resorption induced by estrogen deficiency."
      explanation: >-
        Measures RANKL directly on bone marrow cells in early postmenopausal
        women, finding it two- to threefold higher than in premenopausal or
        estrogen-treated women and inversely correlated with serum estradiol.
        That reaches the specific ratio this node is named for, which the
        previous citation did not.
  notes: Menopause is major risk factor
  evidence:
  - reference: PMID:21874760
    reference_title: "Aromatase activity and bone loss."
    supports: SUPPORT
    snippet: extraglandular aromatization of circulating androgen precursors is the
      major source of estrogen not only in men (since only 15% of circulating estradiol
      is released directly by the testis) but also in women after the menopause.
    explanation: Post-menopausal women rely on aromatase activity for estrogen production,
      making aromatase activity critical for bone health after menopause
  - reference: PMID:18567553
    reference_title: "Estrogen and bone: insights from estrogen-resistant, aromatase-deficient, and normal men."
    supports: SUPPORT
    snippet: studies in these unusual patients have stimulated research on defining
      the role of estrogen in regulating bone metabolism in normal adult and aging
      men, providing further insights into estrogen regulation of bone metabolism
      not only in men, but also in women.
    explanation: Demonstrates that estrogen is essential for bone metabolism in both
      sexes, with deficiency leading to osteoporosis
- name: Calcium Deficiency
  notes: Inadequate dietary intake
  evidence:
  - reference: PMID:35247225
    reference_title: "Calcium deficiency worldwide: prevalence of inadequate intakes and associated health outcomes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "When calcium intake is low or when calcium is poorly absorbed, bone resorption occurs since the stored calcium in bone is used to maintain normal biological functions."
    explanation: States the mechanism by which low or poorly absorbed dietary calcium drives bone resorption, the basis for calcium deficiency as an osteoporosis risk factor.
- name: Vitamin D Deficiency
  exposure_term:
    preferred_term: low vitamin D exposure
    modifier: DECREASED
    term:
      id: ECTO:9000133
      label: exposure to vitamin D
  notes: Impairs calcium absorption
  evidence:
  - reference: PMID:21872800
    reference_title: "The effect of vitamin D on bone and osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The consequences of vitamin D deficiency are secondary hyperparathyroidism and bone loss, leading to osteoporosis and fractures"
    explanation: Directly states that vitamin D deficiency causes secondary hyperparathyroidism and bone loss leading to osteoporosis.
  influences_mechanisms:
  - target: Bone Remodeling Imbalance
    environmental_effect: PREDISPOSES
    causal_link_type: DIRECT
    description: Vitamin D deficiency causes secondary hyperparathyroidism, shifting remodeling toward net bone loss.
    evidence:
    - reference: PMID:21872800
      reference_title: "The effect of vitamin D on bone and osteoporosis."
      supports: SUPPORT
      directness: DIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: The consequences of vitamin D deficiency are secondary hyperparathyroidism and bone loss, leading to osteoporosis and fractures
      explanation: States the deficiency-to-bone-loss route through secondary hyperparathyroidism, which is the remodeling imbalance this node represents.
- name: Sedentary Lifestyle
  exposure_term:
    preferred_term: sedentary lifestyle
    term:
      id: ECTO:6000004
      label: exposure to sedentary lifestyle
  evidence:
  - reference: PMID:30998435
    reference_title: "Substitution of sedentary time with light physical activity is related to increased bone density in U.S. women over 50 years old. An iso-temporal substitution analysis based on the National health and Nutrition Examination Survey."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Substitution of 30 min of sedentary time with an equal amount of light physical activity was associated with increased bone mineral density of about 3 mg/cm2 and a 12% reduced risk of osteoporosis in the spine."
    explanation: Quantifies the bone-density benefit of replacing sedentary time with activity, the inverse relationship supporting sedentary lifestyle as an osteoporosis risk factor.
  notes: Lack of weight-bearing exercise
- name: Smoking
  exposure_term:
    preferred_term: exposure to tobacco smoking
    term:
      id: ECTO:6000029
      label: exposure to tobacco smoking
  influences_mechanisms:
  - target: Bone Remodeling Imbalance
    environmental_effect: EXACERBATES
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Tobacco shifts bone turnover toward loss on top of whatever else is
      driving remodelling, lowering attained bone mass. The responsible
      constituents and steps are not established in the cited source.
    evidence:
    - reference: PMID:30631414
      reference_title: "The Effect of Tobacco Smoking on Bone Mass: An Overview of Pathophysiologic Mechanisms."
      supports: SUPPORT
      snippet: "tobacco smoking causes an imbalance in bone turnover, leading to lower bone mass and making bone vulnerable to osteoporosis and fracture"
      explanation: >-
        States that tobacco smoking causes an imbalance in bone turnover
        leading to lower bone mass, the remodelling imbalance this node
        represents.
  notes: Accelerates bone loss
  evidence:
  - reference: PMID:30631414
    reference_title: "The Effect of Tobacco Smoking on Bone Mass: An Overview of Pathophysiologic Mechanisms."
    supports: SUPPORT
    snippet: tobacco smoking causes an imbalance in bone turnover, leading to lower
      bone mass and making bone vulnerable to osteoporosis and fracture
    explanation: Review confirms that smoking causes bone turnover imbalance that
      increases osteoporosis and fracture risk.
- name: Glucocorticoid Use
  exposure_term:
    preferred_term: exposure to glucocorticoid
    term:
      id: ECTO:0000259
      label: exposure to glucocorticoid
  influences_mechanisms:
  - target: Bone Remodeling Imbalance
    environmental_effect: TRIGGERS
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Therapeutic glucocorticoids suppress osteoblast function and lifespan
      while prolonging osteoclast survival, tipping remodelling toward
      resorption. It is the commonest secondary cause of the disease.
    evidence:
    - reference: PMID:29691807
      reference_title: "Glucocorticoid-induced osteoporosis: an update."
      supports: SUPPORT
      snippet: "Glucocorticoid-induced osteoporosis is the most common secondary cause of osteoporosis and the resulting fractures cause significant morbidity."
      explanation: >-
        Identifies glucocorticoid-induced osteoporosis as the most common
        secondary cause of the disease, an iatrogenic exposure rather than a
        background risk factor.
  notes: Major cause of secondary osteoporosis
  evidence:
  - reference: PMID:29691807
    reference_title: "Glucocorticoid-induced osteoporosis: an update."
    supports: SUPPORT
    snippet: Glucocorticoid-induced osteoporosis is the most common secondary cause
      of osteoporosis and the resulting fractures cause significant morbidity.
    explanation: Review confirms glucocorticoid use is the leading cause of secondary
      osteoporosis.
- name: Microgravity Exposure
  influences_mechanisms:
  - target: Microgravity-Accelerated Bone Resorption
    environmental_effect: TRIGGERS
    causal_link_type: DIRECT
    description: >-
      Unloading removes the mechanical signal that normally restrains
      resorption, and this entry carries a node specifically for it. Notably
      the loss is not fully reversible on return to gravity.
    evidence:
    - reference: PMID:35773442
      reference_title: "Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "nine of 17 astronauts did not fully recover tibia total BMD after 12-months"
      explanation: >-
        Reports that nine of seventeen astronauts had not fully recovered
        tibial bone mineral density twelve months after flight, the incomplete
        recovery this node's downstream consequences reflect.
  notes: >
    Spaceflight-induced mechanical unloading causes 1-1.5% bone loss per month in
    weight-bearing sites. Recovery is slow and often incomplete, with some astronauts
    showing deficits equivalent to a decade of terrestrial aging after one year.
  evidence:
  - reference: PMID:32411816
    reference_title: "A systematic review and meta-analysis of bone loss in space travelers."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Bone loss in space travelers is a major challenge for long-duration space
      exploration.
    explanation: Systematic review and meta-analysis of bone loss in 148 space travelers
      documents site-dependent bone loss with lower limbs losing -0.8% per month
  - reference: PMID:35773442
    reference_title: "Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: nine of 17 astronauts did not fully recover tibia total BMD after 12-months
    explanation: HR-pQCT study showing incomplete bone recovery one year after spaceflight,
      commensurate with a decade of terrestrial age-related bone loss
  - reference: PMID:33597120
    reference_title: "Pre-flight exercise and bone metabolism predict unloading-induced bone loss due to spaceflight."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Increases in training volume predicted preservation of tibia bone strength
      and trabecular vBMD and thickness
    explanation: Pre-flight exercise and in-flight resistance training predict and
      partially mitigate spaceflight bone loss in astronauts
  - reference: PMID:32900939
    reference_title: "Targeting myostatin/activin A protects against skeletal muscle and bone loss during spaceflight."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Wild type mice lost significant muscle and bone mass during the 33 d
      spent in microgravity
    explanation: ISS Rodent Research-19 confirmed that microgravity causes significant
      bone and muscle loss in wild-type mice
treatments:
- name: Bisphosphonates
  description: First-line antiresorptives (alendronate, risedronate, zoledronic acid).
  treatment_term:
    preferred_term: bisphosphonate agent therapy
    term:
      id: NCIT:C198585
      label: Bisphosphonate Therapy
  evidence:
  - reference: PMID:34807231
    reference_title: "Time to Benefit of Bisphosphonate Therapy for the Prevention of Fractures Among Postmenopausal Women With Osteoporosis: A Meta-analysis of Randomized Clinical Trials."
    supports: SUPPORT
    snippet: The pooled meta-analysis found that 12.4 months (95% CI, 6.3-18.4 months)
      were needed to avoid 1 nonvertebral fracture per 100 postmenopausal women receiving
      bisphosphonate therapy.
    explanation: Meta-analysis of 10 RCTs with 23,384 women demonstrates bisphosphonate
      efficacy in preventing fractures.
  - reference: PMID:31400472
    reference_title: "Resistive exercise in astronauts on prolonged spaceflights provides partial protection against spaceflight-induced bone loss."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: combined Bis+ARED prevented declines in all DXA and QCT hip densitometry
      and in estimates of FE hip strengths
    explanation: Bisphosphonate alendronate combined with ARED resistive exercise
      prevented bone loss during ISS missions, while exercise alone only partially
      attenuated loss and did not suppress bone resorption
  - reference: PMID:37723136
    reference_title: "Bisphosphonate conjugation enhances the bone-specificity of NELL-1-based systemic therapy for spaceflight-induced bone loss in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: BP-NELL-PEG significantly increased bone formation in flight and ground
      control mice without obvious adverse health effects
    explanation: Bisphosphonate-conjugated NELL-1 therapy showed bone-specific efficacy
      during 9-week ISS Rodent Research-5 mission
  - reference: PMID:36592456
    reference_title: "Pharmacologic Treatment of Primary Osteoporosis or Low Bone Mass to Prevent Fractures in Adults: A Living Clinical Guideline From the American College of Physicians."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "ACP suggests that clinicians use bisphosphonates for initial pharmacologic treatment to reduce the risk of fractures in males diagnosed with primary osteoporosis (conditional recommendation; low-certainty evidence)."
    explanation: The ACP living clinical guideline recommends bisphosphonates as initial pharmacologic treatment to reduce fracture risk in primary osteoporosis.
- name: Denosumab
  description: RANKL inhibitor, potent antiresorptive.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: denosumab
      term:
        id: NCIT:C61313
        label: Denosumab
  target_mechanisms:
  - target: RANKL/OPG Dysregulation
    treatment_effect: INHIBITS
    description: >-
      Denosumab is a human monoclonal antibody against RANKL that blocks RANKL-RANK
      binding, acting on the same node the entry models as the driver of excess
      osteoclastogenesis.
    evidence:
    - reference: PMID:19671655
      reference_title: "Denosumab for prevention of fractures in postmenopausal women with osteoporosis."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Denosumab is a fully human monoclonal antibody to the receptor
        activator of nuclear factor-kappaB ligand (RANKL) that blocks its binding
        to RANK, inhibiting the development and activity of osteoclasts, decreasing
        bone resorption, and increasing bone density.
      explanation: States the molecular target and the resulting suppression of
        osteoclast development, which is the mechanism node this link points at.
    - reference: PMID:33103722
      reference_title: "Fracture risk and management of discontinuation of denosumab therapy: a systematic review and position statement by ECTS."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: Denosumab discontinuation is characterized by an increase in bone
        turnover overriding pre-treatment status, a rapid bone loss in the
        majority and multiple vertebral fractures (VFx) in some patients.
      explanation: Confirms the inhibition is suppressive rather than corrective -
        withdrawing it returns the RANKL/OPG node past its pre-treatment state,
        which is why the link is INHIBITS and not RESTORES.
  evidence:
  - reference: PMID:19671655
    reference_title: "Denosumab for prevention of fractures in postmenopausal women with osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: denosumab reduced the risk of new radiographic vertebral fracture,
      with a cumulative incidence of 2.3% in the denosumab group, versus 7.2% in
      the placebo group (risk ratio, 0.32; 95% confidence interval [CI], 0.26 to
      0.41; P<0.001)
    explanation: FREEDOM, a 7868-woman placebo-controlled trial, is the pivotal
      fracture-endpoint evidence for denosumab.
  - reference: PMID:29105841
    reference_title: "Vertebral Fractures After Discontinuation of Denosumab: A Post Hoc Analysis of the Randomized Placebo-Controlled FREEDOM Trial and Its Extension."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: the vertebral fracture rate increased from 1.2 per 100
      participant-years during the on-treatment period to 7.1, similar to
      participants who received and then discontinued placebo
    explanation: Contradicts reading denosumab as a durable intervention. Unlike
      bisphosphonates, its effect is fully reversible on withdrawal, so the
      benefit above is conditional on uninterrupted dosing.
  notes: >-
    Effect is not retained after withdrawal. Discontinuation releases the
    suppressed resorption as a rebound that overshoots pre-treatment bone
    turnover, returning vertebral fracture rates to untreated levels and
    concentrating risk in multiple rather than single fractures, so stopping
    requires transition to another antiresorptive rather than simple cessation.
    This is a property of the RANKL/OPG node the drug acts on - it inhibits the
    pathway without altering the state that drives it - and is the mechanistic
    reason denosumab is not interchangeable with a bisphosphonate.
- name: Teriparatide
  description: PTH analog, anabolic agent for severe osteoporosis.
  therapeutic_modality: PEPTIDE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: teriparatide
      term:
        id: NCIT:C61966
        label: Teriparatide
  notes: >-
    Anabolic rather than antiresorptive: intermittent daily dosing raises bone
    formation, which is the opposite of the sustained PTH elevation that drives
    bone loss in secondary hyperparathyroidism.
  evidence:
  - reference: PMID:11346808
    reference_title: "Effect of parathyroid hormone (1-34) on fractures and bone mineral density in postmenopausal women with osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: New vertebral fractures occurred in 14 percent of the women in the
      placebo group and in 5 percent and 4 percent, respectively, of the women in
      the 20-microg and 40-microg parathyroid hormone groups
    explanation: The Fracture Prevention Trial in 1637 postmenopausal women with
      prior vertebral fractures is the pivotal evidence for teriparatide.
- name: Romosozumab
  description: Sclerostin inhibitor with dual anabolic/antiresorptive effect.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: romosozumab
      term:
        id: NCIT:C152246
        label: Romosozumab
  notes: >-
    Targets the product of SOST, which also carries a common-variant BMD
    association in this entry's genetic section. Carries an imbalance in
    adjudicated serious cardiovascular events against alendronate in ARCH, which
    is why it is positioned for high fracture risk rather than first line.
  evidence:
  - reference: PMID:27641143
    reference_title: "Romosozumab Treatment in Postmenopausal Women with Osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: At 12 months, new vertebral fractures had occurred in 16 of 3321
      patients (0.5%) in the romosozumab group, as compared with 59 of 3322 (1.8%)
      in the placebo group (representing a 73% lower risk with romosozumab;
      P<0.001).
    explanation: FRAME, a 7180-woman placebo-controlled trial, establishes vertebral
      fracture reduction.
  - reference: PMID:28892457
    reference_title: "Romosozumab or Alendronate for Fracture Prevention in Women with Osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Over a period of 24 months, a 48% lower risk of new vertebral
      fractures was observed in the romosozumab-to-alendronate group
    explanation: ARCH shows superiority over active bisphosphonate comparison, not
      only over placebo.
  - reference: PMID:28892457
    reference_title: "Romosozumab or Alendronate for Fracture Prevention in Women with Osteoporosis."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: During year 1, positively adjudicated serious cardiovascular adverse
      events were observed more often with romosozumab than with alendronate (50 of
      2040 patients [2.5%] vs. 38 of 2014 patients [1.9%])
    explanation: Contradicts an unqualified reading of romosozumab as a preferred
      agent; the cardiovascular imbalance in the same trial constrains where it is
      used.
- name: Calcium Supplementation
  description: Ensure adequate intake (1000-1200 mg/day).
  treatment_term:
    preferred_term: calcium supplementation
    term:
      id: NCIT:C15433
      label: Nutritional Support
  evidence:
  - reference: PMID:17720017
    reference_title: "Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In trials that reported fracture as an outcome (17 trials, n=52 625),
      treatment was associated with a 12% risk reduction in fractures of all types
      (risk ratio 0.88, 95% CI 0.83-0.95; p=0.0004)."
    explanation: Meta-analysis of 29 randomised trials (n=63,897) supporting calcium,
      alone or with vitamin D, as fracture-preventive.
  - reference: PMID:17720017
    reference_title: "Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The treatment effect was better with calcium doses of 1200 mg or more
      than with doses less than 1200 mg (0.80 vs 0.94; p=0.006)
    explanation: Sources the dose in this treatment's description; the benefit is
      dose-dependent, with 1200 mg the threshold the authors recommend.
- name: Vitamin D Supplementation
  description: Maintain 25(OH)D >30 ng/mL.
  treatment_term:
    preferred_term: vitamin D supplementation
    term:
      id: NCIT:C15425
      label: Nutritional Supplementation
  evidence:
  - reference: PMID:17720017
    reference_title: "Use of calcium or calcium in combination with vitamin D supplementation to prevent fractures and bone loss in people aged 50 years and older: a meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: with vitamin D doses of 800 IU or more than with doses less than 800
      IU (0.84 vs 0.87; p=0.03)
    explanation: Fracture-risk reduction is greater at vitamin D doses of at least
      800 IU, the dose the meta-analysis recommends alongside calcium.
- name: Weight-Bearing Exercise
  description: Helps maintain bone density.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  notes: >-
    The Cochrane review supports a small BMD effect but found no reduction in
    fracture numbers, and the largest femoral-neck effect came from
    non-weight-bearing progressive resistance training rather than from
    weight-bearing exercise, so the name of this treatment is narrower than the
    evidence behind it.
  evidence:
  - reference: PMID:21735380
    reference_title: "Exercise for preventing and treating osteoporosis in postmenopausal women."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Our results suggest a relatively small statistically significant, but
      possibly important, effect of exercise on bone density compared with control
      groups.
    explanation: Cochrane review of 43 RCTs (4320 participants) supports the bone
      density claim this treatment makes, and calibrates its size as small.
  - reference: PMID:21735380
    reference_title: "Exercise for preventing and treating osteoporosis in postmenopausal women."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: The most effective type of exercise intervention on bone mineral
      density (BMD) for the neck of femur appears to be non-weight bearing high
      force exercise such as progressive resistance strength training for the lower
      limbs
    explanation: Contradicts the weight-bearing framing specifically - at the
      femoral neck the largest effect came from non-weight-bearing high-force
      training.
animal_models:
- name: Ovariectomized rat
  species: Rat
  genotype: Wild type, surgically ovariectomized
  category: SURGICAL
  description: >-
    Bilateral ovariectomy in the rat produces acute estrogen withdrawal, a surge
    in cancellous bone turnover, and net trabecular bone loss at the proximal
    tibia, distal femur and lumbar vertebrae. The 1994 FDA preclinical guidance
    names it as the rodent species for evaluating agents intended to treat or
    prevent postmenopausal osteoporosis, which makes it the regulatory reference
    model rather than merely a common one.
  publication: PMID:8579908
  modeled_mechanisms:
  - target: RANKL/OPG Dysregulation
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Reproduces the estrogen-deficiency route into accelerated remodeling, and
      the effect is estrogen-reversible, which establishes the mechanism rather
      than merely the phenotype.
    limitations: >-
      Captures the estrogen-withdrawal arm only. It says nothing about the
      senescence and marrow-lineage arms of this entry, which are age-driven and
      present in both sexes.
    readouts:
    - name: Cancellous bone turnover after ovariectomy
      target: RANKL/OPG Dysregulation
      direction: INCREASED
      interpretation: Activation of remodeling is the proximate consequence of
        estrogen withdrawal in this model.
      evidence:
      - reference: PMID:8579908
        reference_title: "FDA Guidelines and animal models for osteoporosis."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: These data illustrate that cancellous bone turnover increases
          following OVX and this increased bone turnover produces bone loss.
        explanation: Reports the turnover measurement and links it causally to the
          bone loss.
    evidence:
    - reference: PMID:8579908
      reference_title: "FDA Guidelines and animal models for osteoporosis."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: Estrogen completely blocks the activation of bone turnover and
        bone loss.
      explanation: The rescue arm shows the model's bone loss runs through
        estrogen deficiency, which is what makes it informative for this node.
  - target: Microarchitectural Deterioration
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Trabecular bone volume falls and stays low, so the structural endpoint is
      reproduced, but the driving turnover signal is not sustained.
    limitations: >-
      Over a 12-month study the cancellous turnover indices return to sham
      control values while trabecular bone volume remains low, so the model
      decouples the mechanism from the deficit at exactly the durations the
      guidance recommends. It also lacks intracortical Haversian remodeling
      entirely, so cortical porosity cannot be studied in it.
    readouts:
    - name: Cancellous bone turnover at 12 months
      target: Microarchitectural Deterioration
      direction: UNCHANGED
      interpretation: A genuine negative result - turnover normalizes even though
        the structural deficit persists.
      evidence:
      - reference: PMID:8579908
        reference_title: "FDA Guidelines and animal models for osteoporosis."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: the indices of cancellous bone turnover return to the value of
          sham controls, although the trabecular bone volume remains lower than
          that of sham controls in OVX rats
        explanation: Documents the decoupling that limits long-duration studies in
          this model.
    evidence:
    - reference: PMID:8579908
      reference_title: "FDA Guidelines and animal models for osteoporosis."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: it is difficult to determine the effects of potential therapeutic
        agents on the bone turnover in estrogen deficient conditions
      explanation: The authors' own statement of the limitation, which is why this
        link is PARTIALLY_RECAPITULATES rather than RECAPITULATES.
- name: Aged INK-ATTAC senescent-cell clearance mouse
  species: Mouse
  genotype: INK-ATTAC transgenic (inducible caspase 8 under the p16INK4a promoter)
  category: TRANSGENIC
  description: >-
    A suicide transgene that lets p16INK4a-expressing senescent cells be killed
    on demand with AP20187. Applied to 20-22-month-old mice with bone loss
    already established, it is an interventional test of whether senescent cells
    cause age-related bone loss rather than merely accompany it.
  publication: PMID:28825716
  modeled_mechanisms:
  - target: Senescent Cell Accumulation in the Bone Microenvironment
    relationship: RESCUES
    fidelity: MODERATE
    description: >-
      Clearing roughly 30% of senescent cells in old mice raises bone mass,
      microarchitecture and strength; the same treatment in 12-month-old mice
      changes nothing, which is the specificity control.
    limitations: >-
      Murine, and the transgene is not a therapy. The mouse skeleton lacks
      osteonal cortical remodeling, so the cortical arm of the result does not
      transfer directly. The human phase 2 senolytic trial did not reproduce the
      resorption effect at its primary endpoint - see the discussions section.
    readouts:
    - name: Spine trabecular bone volume fraction after senescent-cell clearance
      target: Senescent Cell Accumulation in the Bone Microenvironment
      direction: RESTORED
      interpretation: Structural correlate of removing the senescent-cell burden
        in an already-osteopenic skeleton.
      evidence:
      - reference: PMID:28825716
        reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "AP20187-treated mice had better spine trabecular bone
          microarchitecture (e.g., 52% higher BV/TV"
        explanation: The primary structural measurement behind the rescue claim.
    - name: Senescent osteocyte burden
      target: Senescent Cell Accumulation in the Bone Microenvironment
      direction: DECREASED
      interpretation: Target engagement - confirms the transgene did what it was
        meant to before the bone effect is attributed to it.
      evidence:
      - reference: PMID:28825716
        reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: This was confirmed by fewer senescent osteocytes in AP20187-
          versus vehicle-treated mice
        explanation: Independent confirmation of clearance by SADS, alongside
          p16INK4a and EGFP mRNA.
    evidence:
    - reference: PMID:28825716
      reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: treatment of young adult (12-month-old) INK-ATTAC mice with
        AP20187 did not alter bone parameters
      explanation: The age-specificity control is what makes this model
        informative for an aging node rather than for bone mass generally.
- name: Aged mouse skeleton as a model of human age-related bone loss
  species: Mouse
  genotype: Wild type, aged
  category: NATURAL
  description: >-
    The background model underneath most murine bone aging work. Recorded
    explicitly because its divergences from the human skeleton are the reason
    several results in this entry are held at moderate rather than high fidelity.
  publication: PMID:23689830
  modeled_mechanisms:
  - target: Microarchitectural Deterioration
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Aged mice lose cancellous bone, thin their cortices and gain cortical
      porosity as humans do, and the histologic cause is shared - resorption
      cavities that osteoblasts fail to refill.
    limitations: >-
      The murine skeleton keeps growing slowly after puberty and has no osteonal
      remodeling of cortical bone, so the human intracortical remodeling
      compartment has no counterpart in it. Cortical findings therefore arise
      through a different structural process than the one they are taken to
      model.
    evidence:
    - reference: PMID:23689830
      reference_title: "The relevance of mouse models for investigating age-related bone loss in humans."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: like humans, mice exhibit loss of cancellous bone, thinning of
        cortical bone, and increased cortical porosity with advancing age
      explanation: States the shared phenotype that makes the aged mouse
        informative for this node.
    - reference: PMID:23689830
      reference_title: "The relevance of mouse models for investigating age-related bone loss in humans."
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: Unlike the human skeleton, however, the murine skeleton continues
        to grow slowly after puberty and lacks osteonal remodeling of cortical
        bone.
      explanation: Contradicts treating murine cortical bone loss as a direct
        model of human intracortical remodeling, and is the basis of the
        HUMAN_MODEL_MISMATCH discussion.
experimental_models:
- name: MLO-Y4 osteocyte-like cell line
  experimental_model_type: CELL_LINE
  namo_type: namo:CellLineModel
  description: >-
    Murine long bone osteocyte line immortalized under an osteocalcin promoter.
    It made the osteocyte experimentally accessible, which matters here because
    the osteocyte is the cell this entry's senescence and mechanosensing nodes
    both run through.
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  cell_types:
  - preferred_term: Osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  publication: PMID:9421234
  modeled_mechanisms:
  - target: Osteocyte Mechanosensing Disruption in Microgravity
    relationship: MEASURES
    fidelity: LOW
    description: >-
      The line was established specifically to make osteocyte mechanotransduction
      and osteocyte-osteoblast-osteoclast communication testable in vitro.
    limitations: >-
      Murine, immortalized by SV40 large T antigen, and cultured on
      collagen-coated plastic rather than embedded in mineralized matrix, so the
      lacuno-canalicular geometry that carries the mechanical signal in vivo is
      absent. It does not model aging.
    evidence:
    - reference: PMID:9421234
      reference_title: "Establishment of an osteocyte-like cell line, MLO-Y4."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: This newly established cell line should prove useful for studying
        the effects of mechanical stress on osteocyte function and for
        determining the means whereby osteocytes communicate with other bone
        cells such as osteoblasts and osteoclasts.
      explanation: The authors' stated purpose matches the mechanism this link
        points at.
- name: IDG-SW3 osteoblast-to-osteocyte differentiation line
  experimental_model_type: CELL_LINE
  namo_type: namo:CellLineModel
  description: >-
    A conditionally immortalized line that traverses the whole
    osteoblast-to-late-osteocyte transition in culture and expresses
    SOST/sclerostin under PTH and 1,25-dihydroxyvitamin D3 control. That makes it
    the accessible in vitro system for the sclerostin axis this entry targets
    pharmacologically with romosozumab.
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  cell_types:
  - preferred_term: Osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  culture_system: 3D matrix or 2D osteogenic culture at 37 C without interferon gamma
  publication: PMID:21735478
  modeled_mechanisms:
  - target: Bone Remodeling Imbalance
    relationship: MEASURES
    fidelity: LOW
    description: >-
      Provides a readout of sclerostin production and its hormonal regulation,
      the formation-side brake in the remodeling unit.
    limitations: >-
      Murine and conditionally immortalized. It reconstructs a differentiation
      trajectory, not a remodeling unit - there are no osteoclasts in it, so the
      balance this entry models cannot be observed directly.
    evidence:
    - reference: PMID:21735478
      reference_title: "Cell line IDG-SW3 replicates osteoblast-to-late-osteocyte differentiation in vitro and accelerates bone formation in vivo."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Like late osteocytes, they develop a dendritic morphology and
        express SOST/sclerostin and fibroblast growth factor 23 (FGF-23),
        regulated by parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D(3).
      explanation: Establishes that the regulated sclerostin readout is present,
        which is what this link claims the model measures.
- name: 3D bone organoid with coupled osteoblast-osteoclast remodeling
  experimental_model_type: ORGANOID
  namo_type: namo:Organoid
  description: >-
    A human bone organoid built by letting marrow stromal cell-derived
    osteoblasts mineralize an endosteal-like matrix for 21 days, then adding
    marrow macrophage-derived osteoclasts to restore resorption. It is the
    closest available non-animal system to the coupled remodeling unit this
    entry models, and it reports the same CTX-1 resorption marker used
    clinically.
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  cell_types:
  - preferred_term: Osteoclast
    term:
      id: CL:0000092
      label: osteoclast
  - preferred_term: Bone marrow mesenchymal stromal cell
    term:
      id: CL:0000134
      label: mesenchymal stem cell
  publication: PMID:34173283
  notes: >-
    Developed and validated for myeloma bone disease, not for osteoporosis. The
    normal-bone arm of the platform is what is relevant here; no published
    application to age-related or postmenopausal bone loss is recorded, and it
    carries no aging axis.
  modeled_mechanisms:
  - target: Bone Remodeling Imbalance
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reconstitutes both arms of the remodeling unit in human cells, with
      osteoblastic mineralization and osteoclastic resorption in the same
      construct.
    limitations: >-
      The imbalance is imposed by adding myeloma plasma cells, not by estrogen
      withdrawal or aging, so the disease-relevant driver is a different one. It
      is short-term (weeks), has no vasculature, no marrow adipocyte
      compartment, and no mechanical loading, so neither the senescence nor the
      marrow-lineage node in this entry can be studied in it.
    readouts:
    - name: Type I collagen C-telopeptide release from the organoid
      target: Bone Remodeling Imbalance
      direction: INCREASED
      interpretation: Uses the same resorption marker as the clinical CTX assay,
        so the in vitro and clinical readouts are directly comparable.
      evidence:
      - reference: PMID:34173283
        reference_title: "A functional three-dimensional microphysiological human model of myeloma bone disease."
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: increases levels of CTX-1, and decreased expression of
          osteoblastic genes
        explanation: Reports the coupled shift toward resorption that this link
          claims the organoid reproduces.
    evidence:
    - reference: PMID:34173283
      reference_title: "A functional three-dimensional microphysiological human model of myeloma bone disease."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: On day 22, bone marrow macrophage (BMM)-derived osteoclasts (OCs)
        were introduced to enhance the resorptive capability of the model and
        recapitulate the balanced homeostatic nature of skeletal remodeling.
      explanation: Documents that both arms of the remodeling unit are present,
        which is the basis for treating this as a model of remodeling balance.
datasets:
- title: 37-Day microgravity exposure in 16-Week female C57BL/6J mice during NASA
    RR-1 -- bone loss at weight-bearing sites
  description: MicroCT analysis of femur and vertebrae from mice on the RR-1 NASA
    Validation Flight (SpaceX-4). Demonstrates significant cancellous and cortical
    bone loss in femur but not L2 vertebrae after 37 days of microgravity.
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  conditions:
  - spaceflight microgravity
  - ground control
  accession: osdr:OSD-804
- title: Single cell transcriptional profiling of femur bone marrow from mice flown
    on RRRM-2
  description: Single-cell RNA-seq of femur bone marrow from young and old mice after
    55-58 days of microgravity on the ISS (SpaceX-18 / RR-17 RRRM-2 mission).
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  data_type: SINGLE_CELL_RNA_SEQ
  conditions:
  - spaceflight microgravity
  - ground control
  - young mice
  - old mice
  accession: osdr:OSD-402
- title: Single cell transcriptional profiling of humerus bone marrow from mice flown
    on RRRM-2
  description: Single-cell RNA-seq of humerus bone marrow from young and old mice
    after 55-58 days of microgravity on the ISS (SpaceX-18 / RR-17 RRRM-2 mission).
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  data_type: SINGLE_CELL_RNA_SEQ
  conditions:
  - spaceflight microgravity
  - ground control
  accession: osdr:OSD-403
- title: Effects of Spaceflight on Bone Microarchitecture in Axial and Appendicular
    Skeleton in Growing Ovariectomized Rats (STS-62)
  description: MicroCT analysis of femur, humerus, lumbar vertebra, and calvarium
    from ovariectomized rats flown on STS-62. Spaceflight reduced cortical bone growth
    in appendicular skeleton.
  organism:
    preferred_term: rat
    term:
      id: NCBITaxon:10116
      label: Rattus norvegicus
  conditions:
  - spaceflight microgravity
  - ovariectomized
  - ground control
  accession: osdr:OSD-351
- title: Fifteen days of microgravity causes growth in calvaria of mice (STS-131)
  description: MicroCT analysis of skull bone from mice flown on STS-131. Demonstrates
    site-specific bone adaptation with skull bone volume increasing in microgravity,
    contrasting with weight-bearing site losses.
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  conditions:
  - spaceflight microgravity
  - ground control
  accession: osdr:OSD-486
- title: RR-1 NASA Validation Flight - Mouse liver transcriptomic, proteomic, epigenomic
    and histology data
  description: Multi-omics dataset from the RR-1 mission (SpaceX-4) including RNA-seq,
    whole-genome bisulfite sequencing, mass spectrometry, and histology from C57BL/6J
    mice after 37 days of spaceflight.
  organism:
    preferred_term: mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  data_type: BULK_RNA_SEQ
  publication: PMID:33242417
  conditions:
  - spaceflight microgravity
  - ground control
  accession: osdr:OSD-47
computational_models:
- name: QCT-based finite element model of the proximal femur
  model_type: PHYSIOLOGICAL
  description: >-
    A patient-specific structural model built from a quantitative CT scan of the
    hip, solving for femoral strength and for the ratio of the load a fall would
    impose to the strength available to resist it. It is the computational
    counterpart of this entry's claim that microarchitecture and geometry carry
    fracture risk that areal BMD alone does not.
  modeled_mechanisms:
  - target: Microarchitectural Deterioration
    relationship: MEASURES
    fidelity: MODERATE
    description: >-
      Turns the structural deterioration this node describes into a mechanical
      quantity that prospectively predicts hip fracture, and the load-to-strength
      ratio stays predictive after adjustment for areal BMD.
    limitations: >-
      Resolves geometry and density at CT scale, so it does not represent
      trabecular connectivity or cortical porosity directly, and it says nothing
      about the cellular mechanisms upstream. Validated here in older men; the
      cohort is not the population most affected.
    readouts:
    - name: Load-to-strength ratio adjusted for areal BMD
      target: Microarchitectural Deterioration
      direction: INCREASED
      interpretation: Retained prediction after BMD adjustment is what shows the
        model captures structural information beyond density.
      evidence:
      - reference: PMID:19049327
        reference_title: "Finite element analysis of the proximal femur and hip fracture risk in older men."
        supports: SUPPORT
        evidence_source: HUMAN_CLINICAL
        snippet: "When adjusted additionally for aBMD, the load-to-strength ratio
          remained significantly associated with fracture (HR = 3.1, 95% CI:
          1.6-6.1)."
        explanation: The BMD-adjusted hazard ratio is the specific result behind
          this readout.
  publication: PMID:19049327
  evidence:
  - reference: PMID:19049327
    reference_title: "Finite element analysis of the proximal femur and hip fracture risk in older men."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: These results provide insight into hip fracture etiology and
      demonstrate the ability of FE-based biomechanical analysis of QCT scans to
      prospectively predict hip fractures in men.
    explanation: Prospective case-cohort validation in 3549 community-dwelling
      men establishes the model as predictive rather than merely descriptive.
- name: Bone cell population dynamics model of the remodeling unit
  model_type: KINETIC
  description: >-
    A differential-equation model of autocrine and paracrine signalling between
    osteoblast and osteoclast lineages at a single remodeling site, solving for
    cell population dynamics and the resulting change in bone mass. It is the
    formal counterpart of this entry's Bone Remodeling Imbalance node.
  modeled_mechanisms:
  - target: Bone Remodeling Imbalance
    relationship: MEASURES
    fidelity: LOW
    description: >-
      Reproduces a single remodeling cycle, self-sustaining cycling, and unstable
      runaway remodeling from one parameter set, and identifies autocrine
      regulation of osteoclasts as the parameter that decides which regime the
      system occupies.
    limitations: >-
      A single-site abstraction with lumped parameters and no spatial structure,
      no osteocyte compartment, and no aging or estrogen axis, so it cannot
      represent the senescence or marrow-lineage nodes in this entry. Its
      pathological regime was matched to Paget disease rather than to
      osteoporosis.
    evidence:
    - reference: PMID:14499354
      reference_title: "Mathematical model predicts a critical role for osteoclast autocrine regulation in the control of bone remodeling."
      supports: SUPPORT
      evidence_source: COMPUTATIONAL
      snippet: the mathematical model revealed that interactions among
        osteoblasts and osteoclasts result in complex, nonlinear system behavior,
        which cannot be deduced from studies of each cell type alone
      explanation: States what the model contributes over single-cell-type
        reasoning, which is the reason to record it against the imbalance node.
  publication: PMID:14499354
  notes: >-
    No committed dismech-perturb run exists for this model; it is recorded as
    published external work, not as an executable model in this repository.
  evidence:
  - reference: PMID:14499354
    reference_title: "Mathematical model predicts a critical role for osteoclast autocrine regulation in the control of bone remodeling."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: Parametric analysis demonstrated that the mode of dynamic behavior
      in the system depends strongly on the regulation of osteoclasts by
      autocrine factors, such as transforming growth factor beta.
    explanation: Identifies the parameter that determines system behaviour, the
      model's substantive prediction.
references:
- reference: DOI:10.1038/s41413-024-00362-4
  title: 'Mapping RANKL- and OPG-expressing cells in bone tissue: the bone surface
    cells as activators of osteoclastogenesis and promoters of the denosumab rebound
    effect'
  findings: []
- reference: DOI:10.1186/s13018-023-04179-5
  title: 'The association of osteoprotegerin and RANKL with osteoporosis: a systematic
    review with meta-analysis'
  findings: []
- reference: DOI:10.3389/fendo.2024.1487998
  title: 'Effects of probiotic supplementation on bone health in postmenopausal women:
    a systematic review and meta-analysis'
  findings: []
- reference: DOI:10.3390/biomedicines12081635
  title: 'Mechanistic Insights and Therapeutic Strategies in Osteoporosis: A Comprehensive
    Review'
  findings: []
- reference: DOI:10.3390/biomedicines13061443
  title: 'Beyond Bone Loss: A Biology Perspective on Osteoporosis Pathogenesis, Multi-Omics
    Approaches, and Interconnected Mechanisms'
  findings: []
discussions:
- discussion_id: mismatch_op_rodent_cortical_remodeling
  prompt: >-
    Almost everything known about the cellular mechanism of age-related bone loss
    comes from mice and rats, whose skeletons have no osteonal remodeling of
    cortical bone and continue growing slowly after puberty. Human cortical bone
    loss proceeds largely through intracortical Haversian remodeling, a
    compartment that does not exist in the rodent. When an aged mouse gains
    cortical porosity, is that the same process that produces cortical porosity
    in an ageing human, or a structurally different route to a phenotype that
    merely looks alike on microCT?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  rationale: >-
    This is not a general caveat about rodents; it is specific and consequential.
    The cortical compartment carries most of the fracture burden after age 65 -
    predominantly cortical non-vertebral sites account for 80% of all fractures
    in old age, 68% of radial bone loss between 50 and 80 is cortical, and 84% of
    it occurs after 65 - and cortical porosity is the parameter that intervention
    studies in mice report. In humans that porosity arises specifically from
    intracortical remodeling adjacent to the marrow outrunning refill, which
    fragments the inner cortex into a trabecula-like structure. If the murine
    gain in porosity arises without Haversian remodeling, then the mechanistic
    step being modelled is absent from the model, and a treatment effect on
    murine cortical bone cannot be assumed to transfer. The same limitation
    constrains the OVX rat, which is the FDA-designated preclinical model, and it
    is why the same guidance requires a second non-rodent species alongside it -
    large-animal cortical bone does undergo Haversian remodeling. Note the
    mismatch is partial rather than total: the cancellous arm and the refilling
    defect are shared, and this entry records those links at moderate fidelity
    for that reason.
  attaches_to:
  - pathophysiology#Microarchitectural Deterioration
  - animal_models#Aged mouse skeleton as a model of human age-related bone loss
  - animal_models#Ovariectomized rat
  evidence:
  - reference: PMID:23689830
    reference_title: "The relevance of mouse models for investigating age-related bone loss in humans."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Unlike the human skeleton, however, the murine skeleton continues to
      grow slowly after puberty and lacks osteonal remodeling of cortical bone.
    explanation: States the structural mismatch precisely and is the reason this
      is recorded as HUMAN_MODEL_MISMATCH rather than a knowledge gap.
  - reference: PMID:23689830
    reference_title: "The relevance of mouse models for investigating age-related bone loss in humans."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Knowledge of the causes of skeletal fragility in mice should
      therefore be applicable to humans if inherent limitations are recognized.
    explanation: The author's own conditional endorsement - the model is usable
      but only with the mismatch made explicit, which is what this record does.
  - reference: PMID:20472174
    reference_title: "Intracortical remodelling and porosity in the distal radius and post-mortem femurs of women: a cross-sectional study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: non-vertebral fractures at predominantly cortical sites account for
      80% of all fractures and most fracture-related morbidity and mortality in
      old age
    explanation: Establishes that the compartment the rodent models least well is
      the one carrying the fracture burden, which is what makes this mismatch
      consequential rather than academic.
  - reference: PMID:20472174
    reference_title: "Intracortical remodelling and porosity in the distal radius and post-mortem femurs of women: a cross-sectional study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Remodelling within cortex adjacent to the marrow accounted for 49.9
      mg (45.4-53.7) hydroxyapatite (47%) of bone loss.
    explanation: Names intracortical remodeling as the quantitative source of
      nearly half of human bone loss - the specific process the murine skeleton
      has no counterpart for.
  - reference: PMID:7626315
    reference_title: "The potential of sheep for the study of osteopenia: current status and comparison with other animal models."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Sheep have cortical bone that is plexiform in structure although
      haversian remodeling is seen in older animals.
    explanation: Supports the reading that large-animal models are used partly
      because their cortical bone remodels osteonally, which is the capability
      the required second non-rodent species supplies.
- discussion_id: gap_op_senescent_burden_stratification
  prompt: >-
    Clearing senescent cells causally reverses age-related bone loss in old mice,
    yet a phase 2 trial of intermittent dasatinib plus quercetin in
    postmenopausal women missed its primary endpoint: CTx did not differ from
    control at 20 weeks. Formation rose transiently, and in an exploratory
    subgroup with the highest T cell p16 burden both markers moved and radius BMD
    increased. Is senescent cell burden the effect modifier that decides who
    responds, and is there a measurable threshold below which senolytic therapy
    should not be expected to work?
  kind: KNOWLEDGE_GAP
  status: OPEN
  rationale: >-
    The mouse work is unusually strong - genetic and pharmacological clearance
    agree, the effect is absent in young animals, and the in vitro SASP
    experiment supplies the mechanism - so a negative human primary endpoint is
    more likely to reflect patient selection, dose, or timing than a false
    preclinical result. Postmenopausal women shortly after menopause are
    oestrogen-deficient rather than necessarily senescence-burdened, so the trial
    may have been run in the population least likely to respond. The subgroup
    finding is exploratory and underpowered and must not be read as a positive
    result; it is a hypothesis about stratification. Resolving this decides
    whether senolytics are an osteoporosis therapy at all, or a therapy for a
    definable high-burden subset.
  attaches_to:
  - pathophysiology#Senescent Cell Accumulation in the Bone Microenvironment
  - pathophysiology#Osteocyte SASP Acquisition
  evidence:
  - reference: PMID:28825716
    reference_title: "Targeting cellular senescence prevents age-related bone loss in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: In aged (20- to 22-month-old) mice with established bone loss,
      activation of the INK-ATTAC caspase 8 in senescent cells or treatment with
      senolytics or the JAKi for 2-4 months resulted in higher bone mass and
      strength and better bone microarchitecture than in vehicle-treated mice.
    explanation: The preclinical result whose failure to translate defines this
      gap.
  - reference: PMID:38956196
    reference_title: "Effects of intermittent senolytic therapy on bone metabolism in postmenopausal women: a phase 2 randomized controlled trial."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: intermittent D + Q treatment did not reduce bone resorption in the
      overall group of postmenopausal women
    explanation: The negative primary endpoint in humans, which contradicts a
      direct read-across from the mouse result.
  - reference: PMID:38956196
    reference_title: "Effects of intermittent senolytic therapy on bone metabolism in postmenopausal women: a phase 2 randomized controlled trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: our exploratory analyses indicate that further studies are needed
      testing the hypothesis that the underlying senescent cell burden may
      dictate the clinical response to senolytics
    explanation: The trialists' own framing of the stratification hypothesis this
      gap is asking about.
- discussion_id: gap_op_marrow_adiposity_direction
  prompt: >-
    Marrow adipose volume rises with age, is higher again in osteoporosis at
    matched age, and is inversely correlated with trabecular bone volume. The
    shared osteoblast-adipocyte precursor makes lineage diversion the natural
    reading. But the human evidence is cross-sectional biopsy data, which cannot
    separate diversion of the precursor from passive filling of space vacated by
    lost bone. Which is it, and does marrow fat contribute causally to bone loss
    or merely mark it?
  kind: KNOWLEDGE_GAP
  status: OPEN
  rationale: >-
    The distinction changes what the marrow lineage node is for. If adipogenic
    diversion is causal, the precursor allocation step is a therapeutic target
    and marrow fat is a candidate biomarker of formation failure; if marrow fat
    is a space-filling consequence, it is neither. This entry currently asserts
    the lineage-shift reading in a pathophysiology node, so the uncertainty
    should be visible next to it rather than left implicit.
  attaches_to:
  - pathophysiology#Marrow Stromal Lineage Shift Toward Adipogenesis
  evidence:
  - reference: PMID:11708718
    reference_title: "Adipocyte tissue volume in bone marrow is increased with aging and in patients with osteoporosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The cellular and molecular mechanisms mediating these changes remain
      to be determined.
    explanation: The authors close on exactly this open question, four decades
      into the observation.
- discussion_id: gap_op_no_aging_capable_nam
  prompt: >-
    Every non-animal model recorded in this entry is either a murine immortalized
    cell line or a weeks-long human organoid built around a non-aging driver. No
    available NAM sustains a coupled remodeling unit long enough, or under enough
    mechanical loading, to express an aging phenotype. What would a
    human-cell-based system need in order to model age-related rather than
    tumour- or cytokine-driven bone loss?
  kind: KNOWLEDGE_GAP
  status: OPEN
  rationale: >-
    This is a tooling gap that propagates into every mechanistic claim in the
    aging arm of this entry. The senescence nodes rest on murine in vivo work
    plus conditioned-medium experiments; the marrow-lineage node rests on human
    biopsies with no functional system behind it. Because the rodent cortical
    mismatch recorded above limits what animal work can settle, the absence of an
    aging-capable human system is not a minor inconvenience - it is why two of
    this entry's three aging nodes cannot currently be tested in human cells at
    all. Candidate requirements: human osteocytes embedded in mineralized matrix,
    an osteoclast compartment, a marrow adipocyte compartment, physiological
    loading, and culture durations long enough for senescence to accumulate.
  attaches_to:
  - experimental_models#
  - pathophysiology#Osteocyte SASP Acquisition
  - pathophysiology#Marrow Stromal Lineage Shift Toward Adipogenesis
  evidence:
  - reference: PMID:34499959
    reference_title: "The role of senolytics in osteoporosis and other skeletal pathologies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: Recently, pre-clinical studies have linked the onset of age-related
      bone loss with an accumulation of senescent cells in the bone
      microenvironment.
    explanation: Confirms that the evidence base for the senescence arm is
      preclinical, which is the substance of this gap.
- discussion_id: gap_op_egfr_celltype_specificity
  prompt: >-
    EGFR signaling regulates the bone-remodeling unit, but its net effect is
    cell-type-specific and incompletely mapped: EGFR is anabolic via
    osteoprogenitor/osteoblast lineage support yet can also promote
    osteoclastogenesis. Which skeletal cell populations (mesenchymal
    progenitors, osteoblasts, osteocytes, osteoclasts) drive physiological
    versus pathological EGFR output during bone loss, when does activation
    occur across the remodeling cycle, and how could allosteric or
    bone-targeted EGFR modulation be tuned to preserve formation while limiting
    resorption?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Bone Remodeling Imbalance
  - pathophysiology#RANKL/OPG Dysregulation
  - pathophysiology#Microarchitectural Deterioration
  rationale: >-
    EGFR has cell-type-dependent and partly opposing roles in bone. In the
    osteoblast lineage, EGFR signaling is predominantly anabolic: it maintains
    the bone-marrow mesenchymal progenitor pool, supports osteoblast number and
    mineralization, and constrains adipogenic skewing, so loss of osteoblastic
    EGFR reduces trabecular bone mass. At the same time, EGFR activity can
    increase osteoclast number and influence the RANKL/OPG axis, meaning the
    receptor's effect on net remodeling depends on which compartment is
    engaged. Because EGFR feeds multiple downstream branches (MAPK/ERK,
    PI3K/Akt, NF-kB) that diverge between formation and resorption programs, a
    nonselective EGFR inhibitor risks suppressing the anabolic osteoblastic arm
    while incompletely controlling resorption. The Wang et al. (2026) review
    explicitly identifies the cell-type-specific and spatiotemporal mapping of
    EGFR activation and the mechanisms of its allosteric modulation as open
    questions for skeletal disease. Resolving where and when EGFR is anabolic
    versus pro-resorptive in osteoporotic bone is a prerequisite for designing
    bone-targeted, pathway-selective EGFR modulators that protect bone mass and
    microarchitecture.
  proposed_experiments:
  - experiment_id: exp_op_egfr_celltype_activation_mapping
    name: Cell-type-resolved mapping of EGFR activation in the osteoporotic bone-remodeling unit
    description: >-
      In ovariectomized and aged mouse models of bone loss (and in human
      iliac-crest or hip specimens), use single-cell/spatial transcriptomics
      with phospho-EGFR immunostaining to localize EGFR pathway activation
      (MAPK/ERK, PI3K/Akt, NF-kB) across mesenchymal progenitors, osteoblasts,
      osteocytes, and osteoclasts. Pair this with lineage-specific EGFR
      gain- and loss-of-function and an allosteric vs. ATP-competitive
      inhibitor comparison to determine whether EGFR modulation can preserve
      osteoblastic bone formation while limiting osteoclast-mediated
      resorption, with readouts on trabecular microarchitecture.
    experiment_type:
      preferred_term: spatial single-cell multi-omics with conditional perturbation
    readouts:
    - name: Lineage-resolved EGFR activation and remodeling balance
      target: pathophysiology#Bone Remodeling Imbalance
      description: >
        Quantify phospho-EGFR and downstream pathway activity separately in
        osteoblast-lineage vs. osteoclast-lineage cells alongside formation
        (osteoblast number, mineral apposition) and resorption indices to map
        where EGFR drives anabolic vs. catabolic output.
      assays:
      - preferred_term: phospho-EGFR immunohistochemistry
      - preferred_term: single-cell RNA sequencing
      direction: NEGATIVE
    - name: EGFR effect on the RANKL/OPG axis
      target: pathophysiology#RANKL/OPG Dysregulation
      description: >
        Measure RANKL and OPG expression and osteoclast differentiation under
        cell-type-targeted EGFR perturbation to test whether EGFR activity in
        the osteoblast/stromal compartment shifts the RANKL/OPG ratio toward
        osteoclastogenesis.
      assays:
      - preferred_term: RANKL and OPG quantification
      - preferred_term: osteoclast differentiation assay
      direction: POSITIVE
    - name: Trabecular microarchitecture response to EGFR modulation
      target: pathophysiology#Microarchitectural Deterioration
      description: >
        Assess trabecular bone volume, number, thickness, and connectivity by
        micro-CT after lineage-specific EGFR loss/gain and after allosteric vs.
        pan-EGFR inhibition to detect protection or worsening of bone
        microarchitecture.
      assays:
      - preferred_term: micro-computed tomography
      direction: NEGATIVE
    decision_criterion: >-
      The cell-type-specific dual-role model is supported if (1) phospho-EGFR
      mapping shows distinct activation between osteoblast- and
      osteoclast-lineage cells, (2) osteoblastic EGFR loss reduces bone
      formation and trabecular mass while (3) EGFR perturbation measurably
      shifts the RANKL/OPG axis, and (4) an allosteric/bone-targeted modulator
      preserves microarchitecture better than a pan-EGFR inhibitor.
    would_support:
    - pathophysiology#Bone Remodeling Imbalance
    - pathophysiology#RANKL/OPG Dysregulation
    - pathophysiology#Microarchitectural Deterioration
  evidence:
  - reference: PMID:21542005
    reference_title: "Epidermal growth factor receptor plays an anabolic role in bone metabolism in vivo."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Histologic analyses revealed decreases in osteoblast number and mineralization activity and an increase in osteoclast number."
    explanation: >-
      Reduced osteoblastic EGFR activity simultaneously lowers osteoblast
      number/mineralization and raises osteoclast number, illustrating the
      cell-type-specific, opposing effects of EGFR on the remodeling unit.
  - reference: PMID:21542005
    reference_title: "Epidermal growth factor receptor plays an anabolic role in bone metabolism in vivo."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "these data demonstrate that the EGFR signaling pathway is an important bone regulator and that it primarily plays an anabolic role in bone metabolism."
    explanation: >-
      Establishes EGFR as a net anabolic bone regulator, anchoring the
      formation-supporting arm of the cell-type-specific EGFR gap in
      osteoporosis.
  - reference: PMID:42218983
    reference_title: "The dual roles of EGFR signaling in skeletal homeostasis: from bone remodeling to cartilage metabolism and differentiated targeting strategies."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "this review highlights several unresolved questions: the cell-type-specific and spatiotemporal mapping of EGFR activation, the precise mechanisms of its allosteric modulation, and the development of tissue-targeted, non-oncological EGFR delivery systems."
    explanation: >-
      Recent comprehensive review explicitly identifies the three unresolved
      mechanistic gaps in EGFR's role in osteoporosis: cell-type-specific and
      spatiotemporal mapping of EGFR activation, allosteric modulation
      mechanisms, and tissue-targeted delivery strategies. Directly validates the
      scope and importance of this knowledge gap.
- discussion_id: gap_op_egfr_allosteric_pathway_targeting
  prompt: >-
    EGFR (an ErbB-family receptor tyrosine kinase) regulates the bone-remodeling
    unit through MAPK/ERK, PI3K/Akt, and NF-kB branches, but it remains
    unresolved how allosteric modulation of EGFR governs which pathway is
    selected in osteoblasts, osteoclasts, and osteocytes, and how pathological
    EGFR output could be inhibited in bone tissue-selectively without the
    off-target toxicity of pan-EGFR oncology inhibitors.
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#Bone Remodeling Imbalance
  rationale: >-
    EGFR signaling has dual physiological versus pathological roles in skeletal
    homeostasis, and its effects on bone remodeling are transduced through
    divergent MAPK/ERK, PI3K/Akt, and NF-kB branches that differ between
    formation and resorption programs across osteoblasts, osteoclasts, and
    osteocytes. The precise mechanisms by which EGFR allosteric sites govern
    which of these pathways is engaged in bone remain undefined, so it is unclear
    whether a modulator could be tuned to preserve anabolic osteoblastic output
    while limiting pro-resorptive signaling. In parallel, the pan-EGFR inhibitors
    developed in oncology are nonselective and carry substantial off-target
    toxicity, leaving open how to achieve tissue-targeted delivery that
    selectively inhibits pathological EGFR in bone without disturbing systemic
    EGFR function. Resolving allosteric pathway selection and bone-targeted
    delivery is a prerequisite for translating EGFR biology into a safe
    bone-protective therapy that maintains remodeling balance.
  proposed_experiments:
  - experiment_id: exp_op_egfr_pathway_selectivity_bone_cells
    name: Cell-type-resolved EGFR pathway-selectivity mapping in bone-remodeling cells
    description: >-
      In primary human osteoblasts, osteoclasts, and osteocyte-like cells
      (separately and in co-culture), apply allosteric EGFR modulators alongside
      orthosteric inhibitors and quantify which downstream branch (MAPK/ERK,
      PI3K/Akt, NF-kB) is engaged in each cell type and how formation readouts
      (ALP, mineralization) versus resorption readouts (TRAP, resorption-pit
      area) respond. The aim is to test whether allosteric modulation can
      preserve anabolic osteoblastic output while limiting pro-resorptive
      signaling, resolving the pathway-selection gap.
    experiment_type:
      preferred_term: allosteric EGFR pathway-selectivity assay
    model_systems:
    - name: Primary human osteoblast, osteoclast, and osteocyte cultures
      description: >-
        Primary bone-remodeling-unit cell types cultured separately and in
        co-culture, enabling cell-type-resolved EGFR pathway and remodeling
        readouts.
      experimental_model_type: PRIMARY_CELL_CULTURE
  - experiment_id: exp_op_egfr_bone_targeted_delivery
    name: Bone-targeted EGFR inhibitor delivery and systemic off-target assessment
    description: >-
      Evaluate bone-homing delivery vehicles (e.g., bisphosphonate-conjugated or
      mineral-binding carriers) for EGFR inhibitors in osteoblast-osteoclast
      co-culture and bone-on-chip systems, measuring on-target remodeling-balance
      effects versus off-target systemic EGFR blockade, to test whether
      tissue-selective delivery can protect bone without the systemic toxicity of
      pan-EGFR oncology inhibitors.
    experiment_type:
      preferred_term: tissue-targeted drug-delivery assay
    model_systems:
    - name: Bone-on-chip and osteoblast-osteoclast co-culture delivery model
      description: >-
        Microfluidic bone model and bone-cell co-cultures used to assess
        bone-targeted EGFR-inhibitor delivery and tissue selectivity.
      experimental_model_type: ORGAN_ON_CHIP
  notes: "Seeded from PMID:42218983 — The dual roles of EGFR signaling in skeletal homeostasis: from bone remodeling to cartilage metabolism and differentiated targeting strategies"

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References & Deep Research

References

5

DOI:10.1038/s41413-024-00362-4

Mapping RANKL- and OPG-expressing cells in bone tissue: the bone surface cells as activators of osteoclastogenesis and promoters of the denosumab rebound effect