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The effect of weight loading and subsequent release from loading on the postnatal skeleton
Adi Reich1, Amnon Sharir, Elazar Zelzer
1Institute of Biochemistry and Nutrition, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Israel. reich@agri.huji.ac.il
Insights
External loading on young chicks’ bones negatively impacts growth plate development and bone properties. Despite a temporary catch-up, mechanical strength is compromised, highlighting risks during skeletal development.
Area of Science:
- Orthopedics
- Developmental Biology
- Biomechanics
Background:
- The relationship between mechanical load and bone properties is well-established for mature bones.
- This relationship is less understood in immature bones, particularly the cartilaginous growth plate during postnatal development.
Purpose of the Study:
- To investigate the effects of external loading on the growth plate and subsequent bone structural and mechanical properties in young, fast-growing chicks.
Main Methods:
- Tibial growth plates and bones from loaded and control chicks were analyzed using histological staining and quantitative PCR.
- Mechanical testing (three-point bending) and micro-computed tomography (micro-CT) were used to assess structural and mechanical properties.
Main Results:
- Externally loaded tibias were shorter with narrower growth plates initially.
- A 'catch-up' phenomenon was observed after load release, with increased growth plate thickness but unchanged chondrocyte differentiation gene expression.
- Upregulation of genes related to ossification and calcification occurred during catch-up.
- Loaded bones exhibited inferior mechanical and structural properties compared to controls after the catch-up period.
Conclusions:
- External loading during bone elongation adversely affects skeletal mechanical and structural integrity.
- The primary impact is on the growth plate, with consequences for bone phenotype appearing after a delay.
Introduction:
The relationship between load and the structure and mechanical properties of mature bones has been thoroughly described. In contrast, this relationship has been studied much less in immature bones, which consist of bony tissue and cartilaginous growth plate, during the postnatal period. This paper describes the effect of an externally applied load on the bones of young fast-growing chicks; in particular, we examine the effect on the growth plate, which regulates longitudinal bone growth, and the consequences in terms of bone structural and mechanical properties.
Materials And Methods:
The tibial growth plates from chicks subjected to external load and control chicks, immediately after loading and following 5 days of load release, were studied by histological staining and quantitative PCR. The contralateral tibiae were mechanically tested by three-point bending and their structural features determined by micro-CT.
Results:
At the end of the external loading period, the tibias of the experimental group were shorter and their growth plate narrower than in controls. However, at this time point, effects were not yet apparent in the bones' structural or mechanical parameters. After a further 5 days of no external load, bones and growth plates of the experimental group demonstrated the phenomenon of 'catch-up': the thickness of the growth plate exceeded that of the control; however the relative expression of genes controlling chondrocyte differentiation (collagen II and X) did not change, while the expression of factors related to growth-plate ossification (osteopontin, alkaline phosphatase) and cartilage and bone calcification (matrix and bone Gla proteins) was upregulated as a result of the catch-up process. At this time, however, the tibiae of the experimental group showed inferior mechanical and structural properties relative to the control group.
Conclusion:
External loading during bone elongation negatively affects the mechanical and structural properties of the skeleton. The effect is first noticeable in the growth plate, which regulates bone growth, and is exhibited in the bone phenotype after a lag period.
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