Postnatal development of the epidermis in a marsupial, Didelphis virginiana.

At birth the epidermis of the opossum is 43 micron thick and consists of a basal layer of columnar cells, an intermediate layer of fusiform cells, a layer of incompletely cornified cells and a single surface layer of cells (the periderm). The latter shows central nuclei and distinct cell boundaries. Adjacent surface cells. are contiguous and show extensive interdigitations of the lateral cell membranes. The periderm is lost during the first week of postnatal development. The epidermis attains its greatest thickness (58 micron) at the 4.0 cm stage (18 days postnatum), and this is due primarily to an increase in thickness of the spinous layer. After this the epidermis thins to 14 micron in the adult. The epidermis of the adult consists of a thin Malphighian layer and a desquamating cornified layer. Hair follicles begin to differentiate at the 2.5 cm stage (7 days postnatum). They continue to differentiate and develop while the epidermis is increasing, and then decreasing, in thickness. The young are fully furred prior to the time they first venture from the protection of the pouch.