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Event: 2091

Key Event Title

A descriptive phrase which defines a discrete biological change that can be measured. More help

Occurrence, Bone Loss

Short name
The KE short name should be a reasonable abbreviation of the KE title and is used in labelling this object throughout the AOP-Wiki. More help
Bone Loss
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Biological Context

Structured terms, selected from a drop-down menu, are used to identify the level of biological organization for each KE. More help
Level of Biological Organization

Organ term

The location/biological environment in which the event takes place.The biological context describes the location/biological environment in which the event takes place.  For molecular/cellular events this would include the cellular context (if known), organ context, and species/life stage/sex for which the event is relevant. For tissue/organ events cellular context is not applicable.  For individual/population events, the organ context is not applicable.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help

Key Event Components

The KE, as defined by a set structured ontology terms consisting of a biological process, object, and action with each term originating from one of 14 biological ontologies (Ives, et al., 2017; Biological process describes dynamics of the underlying biological system (e.g., receptor signalling).Biological process describes dynamics of the underlying biological system (e.g., receptor signaling).  The biological object is the subject of the perturbation (e.g., a specific biological receptor that is activated or inhibited). Action represents the direction of perturbation of this system (generally increased or decreased; e.g., ‘decreased’ in the case of a receptor that is inhibited to indicate a decrease in the signaling by that receptor).  Note that when editing Event Components, clicking an existing Event Component from the Suggestions menu will autopopulate these fields, along with their source ID and description.  To clear any fields before submitting the event component, use the 'Clear process,' 'Clear object,' or 'Clear action' buttons.  If a desired term does not exist, a new term request may be made via Term Requests.  Event components may not be edited; to edit an event component, remove the existing event component and create a new one using the terms that you wish to add.  Further information on Event Components and Biological Context may be viewed on the attached pdf. More help

Key Event Overview

AOPs Including This Key Event

All of the AOPs that are linked to this KE will automatically be listed in this subsection. This table can be particularly useful for derivation of AOP networks including the KE. Clicking on the name of the AOP will bring you to the individual page for that AOP. More help
AOP Name Role of event in AOP Point of Contact Author Status OECD Status
Deposition of energy leading to bone loss AdverseOutcome Cataia Ives (send email) Open for citation & comment

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KE.In many cases, individual species identified in these structured fields will be those for which the strongest evidence used in constructing the AOP was available in relation to this KE. More help
Term Scientific Term Evidence Link
human Homo sapiens Moderate NCBI
rat Rattus norvegicus Moderate NCBI
mouse Mus musculus Moderate NCBI

Life Stages

An indication of the the relevant life stage(s) for this KE. More help
Life stage Evidence
All life stages Moderate

Sex Applicability

An indication of the the relevant sex for this KE. More help
Term Evidence
Unspecific Moderate

Key Event Description

A description of the biological state being observed or measured, the biological compartment in which it is measured, and its general role in the biology should be provided. More help

Bone loss describes the reduction in bone mass or density, which can be caused by various processes and is a characteristic of osteopenia, and osteoporosis, and can lead to bone fracture. An imbalance between bone resorption and formation towards higher bone abrasion contributes to bone loss (Bikle and Halloran, 1999). A decline of bone mineralization and bone density over time or a significant deviation from established reference ranges are direct indicators of bone loss (Cummings, Bates, and Black, 2002). In addition, bone loss can lead to increased risk of bone fractures as bone loss interferes with overall bone integrity and its capacity to withstand mechanical load (Cummings, Bates, and Black, 2002).

How It Is Measured or Detected

A description of the type(s) of measurements that can be employed to evaluate the KE and the relative level of scientific confidence in those measurements.These can range from citation of specific validated test guidelines, citation of specific methods published in the peer reviewed literature, or outlines of a general protocol or approach (e.g., a protein may be measured by ELISA). Do not provide detailed protocols. More help

Listed below are common methods for detecting the KE; however, there may be other comparable methods that are not listed 

Measurement method  



OECD Approved Assay  

X-ray and imaging options:  

  • Single energy x-ray absorptiometry (S[E]XA)  

  • Dual-energy x-ray absorptiometry (D[E]XA)  

  • Single-photon absorptiometry (SPA)  

  • Dual-photon absorptiometry (DPA)  

  • Quantitative computed tomography (QCT)  

  • Radiographic absorptiometry  

  • Ultrasound (quantitative bone ultrasonography)  

  • Magnetic resonance imaging (MRI)  

Carter, Bouxsein, and Marcus, 1992  


Cummings, Bates, and Black, 2002  


Russo, 2009  


Rho, Ashman, and Turner, 1993  

Bone mineral density (BMD) is a direct measurement of bone matrix composition. Less mineral dense bones indicate bone loss.   


Measurement of bone minerals via staining methods:  

  • Xylenol orange (bone formation marker)  

  • Calcein green (bone formation marker)  

  • Tetracycline (bone formation marker)  

  • Von Kossa (calcium salt stain, non-specific)  

  • Alizarin red (calcium cation stain)  

All listed chemicals stain calcium. 

Kulak and Dempster, 2010  


Wang et al., 2006  

Less bone deposition and/or reduced mineral dense bones indicate bone loss.   

Comment: xylenol orange, calcein green, and tetracycline are calcium binding fluorescent dyes that are used to label new bone deposition. 

Von Kossa method is based on the binding of silver ions to anions (phosphates, sulfates, or carbonates) of calcium salts and the reduction of silver salts to form dark brown or black metallic silver staining. Unlike the non-specificity of von Kossa for calcium, alizarin red reacts with calcium cation to form a chelate.  


Static bone histomorphometry of an intact iliac crest bone biopsy (2D and 3D Structural measurements):  

  • Marrow diameter 

  • Marrow area 

  • Marrow volume 

  • Trabecular number, spacing, width, diameter, thickness  

  • Cortical thickness, area, and porosity (bone-specific surface) 

  • Cancellous bone volume  

  • Mineralized volume, thickness  

  • Osteoid surface, volume, thickness  

  • Interstitial thickness  

  • Bone volume fraction (BV/TV) 

  • Wall width, thickness  

  • Percent eroded surface 

  • Serial block imaging (aka serial block-face scanning electron microscopy)  

Dempster et al., 2013  

Static bone histomorphometry with structural measurements is the quantitative measure of bone structure at a fixed time point.  Bone histomorphometry is most useful when interpreted in the context with other data such as structural analysis (CT, DEXA), serum markers of bone turnover etc. 


Measurements of bone mechanical resistance:   

  • Energy-absorbing bone capacity. Bones that cannot absorb as much energy after trauma are more likely to fracture. 

  • Stress-strain curve. Measures the strain exhibited on a bone according to increasing applied stress until fracture. 

  • Three-point bending test. Is a structural mechanical test where the entire bone is hold in a fixture attached to a material testing machine and the mid-diaphysis is loaded until broken. 3PB measures applied load and corresponding bone displacement indicating bone mechanical properties. Combination of 3PB and microCT data of the mid-diaphysis allows to calculate bone material properties. 

Fonseca et al., 2013; Sharir, Barak, and Shahar, 2008; Walker et al., 2015; Turner, 2002 

Measurements of bone mechanical resistance indicates changes in bone integrity possibly due to bone loss, as weaker bones are unable to withstand to as much mechanical force as healthy bones. Often measures Young’s modulus (E) which indicates the property of an object to stretch and deform and is defined as the ratio of applied stress to measured strain on an object. 


Measurements of bone connectivity:  

  • Euler’s characteristic  

  • Betti numbers 

  • Connectivity density (Conn. D) 

Odgaard and Gundersen, 1993  

These mathematical models allow for the 3D reconstruction of connectivity in cancellous bone. Bone loss, as seen as a decrease in strength and bone stiffness, can result from a decrease in connective bone tissue.  


Domain of Applicability

A description of the scientific basis for the indicated domains of applicability and the WoE calls (if provided).  More help

Taxonomic applicability: Bone loss is applicable to all vertebrates such as humans, mice and rats.   

Life stage applicability: There is insufficient data on life stage applicability of this KE.  

Sex applicability: According to a study of astronauts who spent 170 days living in the international space station, women demonstrated greater preservation of their musculoskeletal tissues during the mission compared to males, (33 men, 9 women) (Lang et al., 2017). However, other studies have indicated that the rates of regional and whole-body bone loss were similar in male and female astronauts (Lang et al., 2017).

Evidence for perturbation by a stressor: Multiple studies showed that many types of stressors including ionizing radiation and altered gravity (Bikle and Halloran, 1999; Donaubauer et al., 2020) can interfere with bone remodeling.  

Regulatory Significance of the Adverse Outcome

An AO is a specialised KE that represents the end (an adverse outcome of regulatory significance) of an AOP. More help


List of the literature that was cited for this KE description. More help

Bikle, D. D., & B. P. Halloran (1999), “The response of bone to unloading”, Journal of Bone and Mineral Metabolism, Vol. 17, Nature, 

Carter, D. R., M. L. Bouxsein, and R. Marcus. (1992), “New Approaches for Interpreting Projected Bone Densitometry Data”, Journal of Bone and Mineral Research, Vol. 7/2, Wiley,  

Cummings, S. R., D. Bates, and D. M. Black (2002), “Clinical Use of Bone Densitometry: Scientific Review”, Journal of the American Medical Association, Vol. 288/15,  

Dempster, D. W., et al. (2013), “Standardized Nomenclature, Symbols, and Units for Bone Histomorphometry: A 2012 Update of the Report of the ASBMR Histomorphometry Nomenclature Committee”, American Society for Bone and Mineral Research, Vol. 28/1, Wiley,  

Fonseca, H., et al. (2013), “Bone Quality: The Determinants of Bone Strength and Fragility”, Sports Medicine, Vol. 44/1, Nature, 

Kulak, C. A. M and D. W. Dempster (2010), “Bone histomorphometry: a concise review for endocrinologists and clinicians”, Arquivos Brasileiros de Endocrinologia & Metabologia, Vol. 54/2, Sociedade Brasileira de Endocrinologia e Metabologia, Sao Paulo, 

 Lang, T. et al. (2017), "Towards human exploration of space: The THESEUS review series on muscle and bone research priorities", npj Microgravity, Vol. 3/1, Nature,

Odgaard, A. and H. J. G. Gundersen (1993), “Quantification of connectivity in cancellous bone, with special emphasis on 3-D reconstructions”, Bone, Vol. 14, Elsevier, Amsterdam, 

Rho, J. Y., R. B. Ashman, and C. H. Turner (1993), “Young’s modulus of trabecular and cortical bone material: Ultrasonic and microtensile measurements”, Journal of Biomechanics, Vol. 26/2, Elsevier, Amsterdam, 

Russo, C. R. (2009), “The effects of exercise on bone. Basic concepts and implications for the prevention of fractures”, Clinical Cases Mineral and Bone Metabolism, Vol. 6/3, pp. 223-228. 

Sharir, A., M. M. Barak, and R. Shahar (2008), “Whole bone mechanics and mechanical testing”, The Veterinary Journal, Vol. 177/1, Elsevier, Amsterdam, 

Turner, C. H. (2002), “Biomechanics of Bone: Determinants of Skeletal Fragility and Bone Quality”, Osteoporosis International, Vol. 13, Nature, 

Walker, A. H. et al. (2015), “Changes in Mechanical Properties of Rat Bones under Simulated Effects of Microgravity and Radiation” Physics Procedia, Vol. 66, Elsevier, Amsterdam, 

Wang, Y. H., et al. (2006), “Examination of Mineralized Nodule Formation in Living Osteoblastic Cultures Using Fluorescent Dyes”, Biotechnology Progress, Vol. 22/6, Wiley,