Meeting Abstract
The skeleton can respond to forces encountered over an individual’s lifetime by altering bone structure or shape to resist commonly encountered loads, thereby decreasing fracture risk. To explore the effects of ground-reaction (GRF) and muscle forces on the skeleton, we examined the cross-sectional growth and gross morphologies of the femur under varying loading regimes. Four-week-old outbred (ICR) mice were divided into four activity treatment groups, each exposed to a different source of loading for 21 days: downward jumping to increase GRF, swimming to increase muscle loading, wheel running to mimic the combination of muscle and GRF in natural locomotion, and non-exercised controls. Fluorochrome labeling during the last week of treatment allowed bone growth assays. Mid-diaphyseal thin sections of the left femur were examined for cortical growth patterns and mineral apposition rate, and gross shape changes were assessed using 3D landmark analysis of right hind limb micro-CT scans. Histological analysis indicated that, in all groups, mineral apposition occurred most consistently on the posterior-facing endosteal and periosteal surfaces, indicating posterior cortical drift. Additional apposition on other cortical surfaces characterized each experimental loading regime, with the apposition pattern in the running treatment most resembling that of swimming. Mineral apposition rates were also found to differ from controls in all three treatment groups, especially in the lateral sectors of the cross-section, and again the running and swimming groups were most similar. These results reveal the intricate patterns of apposition on both cortical envelopes that lead to changes in geometric properties and gross morphology of bone following exercise.
