The skin of the newborn opossum at 0 day after birth was clearly divided into an epidermis and a dermis. Three types of cells were apparent. One or two layers of flat keratinocytes containing keratohyaline granules were arranged at the outermost part of the epidermis, forming a keratinized layer (Figs. 1A and 1B
Fig. 1. A. Electron micrograph of a keratinized layer of an opossum baby 0 day after birth: Many keratohyaline granules are included in the cells (arrow). Scale bar 1 µm. B. Electron micrograph of an epidermis and a dermis of a baby 0 day after birth: border between the epidermis and the dermis is clear, and two types of fibroblasts are observed within the dermis. OF; oval or polygonal fibroblast, SF; slim fibroblast. Scale bar 10 µm. C. Electron micrograph of a fibroblast and the neighboring intercellular space from a baby 0 day after birth. The fibroblast contains compact packed endoplasmic reticula in its cytoplasm. Bundles of Collagen fibers are observed at the space. Scale bar 1 µm. D. Enlarged view of the bundle which marked in Fig. 1C. The bundle is composed of some collagen fibers. Scale bar 0.25 µm.). Two or three layers of squamous cells with projections were arranged just beneath part of the keratinized layer. A layer of basal cells was distributed in lower part of the epidermis. Therefore, it was evident that in the skin at this stage, the epidermis consisted of 5–8 layers of several types of cells. The epidermis of this stage was composed of thick gathering of some types of cells, and was divided into a definite lower dermis, but typical basal cells were not observed. However, cells that seemed to be basal cells were distributed in an irregular line between the epidermis and dermis (Fig. 1B). Two types of fibroblasts were observed in the dermis, judging from their fine structures. Both fibroblast types bore well-developed endoplasmic reticular systems, suggesting they were in an active stage to create much protein, such as protocollagen. Bundles of collagen fiber were distributed between the fibroblasts in a disordered but connected arrangement, forming a three-dimensional network (Figs. 1C and 1D).