About 8 layers of differing cells constructed the epidermis of the opossum specimen 2 days after birth. An outmost layer of keratinocytes, which contained keratohyaline granules, was the same thickness as in the previous stage, but the inner layer of keratinocytes had thickened. These observations suggest that a keratinized layer was complete from this stage. The squamous cells became larger and thinner, and were piled upon each other with their projections. Increasing numbers of squamous cells caused the thickness a squamous layer to grow compared to the previous stage. Basal cells arranged in a layer at the base of the epidermis divided an epidermis and a dermis (Fig. 2A
Fig. 2. A. Electron micrograph of an epidermis of a baby of 2 days after birth. Squamous cells show platy shape, and basal cells appear polygonal. The basal cells have many projections at an apical and both sides of the cell. SC; squamous cell, BC; basal cell. scale bar 1 µm B. Electron micrograph of a dermis of a baby 2 days after birth. Most fibroblasts are oval or polygonal. OF; oval or polygonal type fibroblast, SF; slim fibroblast. Scale bar 5 µm. C. Electron micrograph of fibroblasts from a baby 2 days after birth. Fibroblast has well-developed endoplasmic reticula in its cytoplasm. Fibers are thick, and are gathered into bundles in the intracellular space. Scale bar 1 µm.). The basal cells of this stage had shapes typical of this cell type. The basal cells were tall, and contacted a basement membrane at their base. The cell lacked projections at the base, but featured many projections at their apical portions, binding them to each other, or to neighboring squamous cells (Fig. 2A). In the dermis, the slim type of fibroblast decreased in number, and the oval or polygonal type increased. The intercellular space in the dermis began to expand in proportion with the increase of cell number. However, with the expansion of the intercellular space and the increase in cell number, overall cell count per unit area did not differ from the previous stage (Fig. 2B), with cell count per unit area of about 16 cells/40 µm × 40 µm. Endoplasmic reticula were well-developed in fibroblasts, and thin fibers in intercellular space were thickened and gathered into bundles (Fig. 2C).