| Description |
Bones that exhibit marked changes in structural and material characteristics during ontogeny are potentially useful for studying the mechanisms that produce functional adaptations. The diaphyseal region of the sheep radius was examined because it is relatively simply loaded in cranial-caudal bending (net tension in cranial cortex and net compression in caudal cortex). Newborn, 4-5 month, 8-10 month, and 2- year-old animals (n=36 total) were examined for: (1) longitudinal curvature, (2) crosssectional shape, and major and minor axes of second moments of area, and (3) regional variations in cortical thickness, mineralization (percent ash), and predominant collagen fiber orientation (CFO). Longitudinal curvature was highly developmentally constrained, changing little from newborn to adult. By contrast, from newborn to 4-5 months the cross-sectional shape changed from quasi-annular to elliptical (relatively narrowed in cranial-caudal (Cr-Cd) direction). Cr-Cd cortical thickness also changed: the Cd "compression" cortex was thicker in newborns but the Cr "tension" cortex became disproportionately thicker by 4-5 months and this persisted in adults. By 4-5 months age, a marked Cr-Cd CFO difference and a less marked mineralization difference also emerged. Because CFO and mineralization only weakly correlated with cortical thickness, and Cr-Cd load predictability (which has cardinal importance in ensuring an adequate whole-bone safety factor) can readily be ensured by longitudinal curvature and Cr-Cd narrowing, the emergence of CFO and mineralization variations likely represent functional ("extra-genetic") adaptations at the local level, especially for the relatively more deleterious tension strains. Without these histocompositional adaptations the potentially adverse consequences (e.g., microdamage formation) of Cr-Cd load predictability would likely lead to failure (fracture) of the bone during normal use. |
