Postnatal development of the fore- and hindlimbs in the grey short-tailed opossum, Monodelphis domestica.

Marsupials are good experimental animals for developmental studies as their offspring are born at a stage comparable to embryonic stages of eutherian species. The South American opossum, Monodelphis domestica, is particularly useful because of its small size and easy maintenance. This study was carried out to compare development of opossum fore- and hindlimbs during postnatal life, using light microscopy and whole mount alizarin staining. At birth, well-developed mobile forelimbs show cartilage models of bones and myotubular striated muscle fibres. However, hindlimbs are relatively underdeveloped paddle-like outgrowths. Two days later mesodermal condensations form models of the future hindlimb bones and mononucleate myoblast aggregates are present; by 6 days post partum (dpp) the hindlimb has reached a stage of development similar to that of the forelimb at birth. At this stage, periosteal buds have invaded forelimb long bones and nuclei in forelimb muscle fibres have become displaced to the periphery. The 16 dpp hindlimb shows long bones invaded by periosteal buds and closely packed, striated muscle fibres. Epiphyseal plates are now seen in the forelimb long bones and forelimb muscle fibres show mature characteristics. Musculoskeletal development is well correlated with the functional demands of the limbs during postnatal development in the opossum, which provides an excellent model for investigations into the genes and molecules controlling limb development.