Development of epiphyseal structure and function in Didelphis virginiana (Marsupiala, Didelphidae)

This study addressed the question of how the epiphyses of growing mammals c hange their external shape and internal architecture during postnatal devel opment. Ontogenetic transformations in the external form and internal struc ture of the fore- and hindlimb epiphyses were examined in a mixed cross-sec tional sample of Didelphis virginiana using two methods: morphometric analy sis of linear epiphyseal dimensions and histological staining of serially s ectioned epiphyses. Metric data indicate that Virginia opossums are born wi th relatively short hindlimbs and long forelimbs, but by the time they are weaned their hindlimbs are longer than their forelimbs. Functional integrat ion of the locomotor system in D. virginiana involves a decoupling of fore- and hindlimb growth rates so that between birth and weaning, femoral lengt h, diaphyseal cross-sectional area, and articular surface area increase at a significantly faster rate than the corresponding humeral dimensions. Hist ological results demonstrate that these differences in growth rate are refl ected in morphology of the humeral and femoral growth plate and epiphyseal cartilages. The humeral cartilages exhibit a level of cellular organization characteristic of more mature limb elements at earlier developmental stage s compared to the femoral cartilages, which assume this anisotropic structu re relatively later in postnatal development. Results presented here also r eveal that the formation of articular cartilage and the initiation of epiph yseal ossification in D. virginiana are both correlated with the developmen t of independent positional behaviors prior to weaning. These histological data, therefore, suggest that mechanical loading associated with the postna tal onset of locomotor and postural development may provide an important st imulus for the progression of ossification and the formation of articular c artilage in the epiphyses of growing mammals. (C) 1999 Wiley-Liss, Inc.