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

This study addressed the question of how the epiphyses of growing mammals change their external shape and internal architecture during postnatal development. Ontogenetic transformations in the external form and internal structure of the fore- and hindlimb epiphyses were examined in a mixed cross-sectional sample of Didelphis virginiana using two methods: morphometric analysis of linear epiphyseal dimensions and histological staining of serially sectioned epiphyses. Metric data indicate that Virginia opossums are born with relatively short hindlimbs and long forelimbs, but by the time they are weaned their hindlimbs are longer than their forelimbs. Functional integration of the locomotor system in D. virginiana involves a decoupling of fore- and hindlimb growth rates so that between birth and weaning, femoral length, diaphyseal cross-sectional area, and articular surface area increase at a significantly faster rate than the corresponding humeral dimensions. Histological results demonstrate that these differences in growth rate are reflected 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 stages compared to the femoral cartilages, which assume this anisotropic structure relatively later in postnatal development. Results presented here also reveal that the formation of articular cartilage and the initiation of epiphyseal ossification in D. virginiana are both correlated with the development of independent positional behaviors prior to weaning. These histological data, therefore, suggest that mechanical loading associated with the postnatal onset of locomotor and postural development may provide an important stimulus for the progression of ossification and the formation of articular cartilage in the epiphyses of growing mammals.