The effect of weight loading and subsequent release from loading on the postnatal skeleton

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 bonesʹ 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.