Embryology of the epidermis: ultrastructural aspects. II. Period of differentiation in the mouse with mammalian comparisons.

A detailed light and electron microscopic study of the cellular morphology of the epidermis in the 13 through 16 day mouse fetus reveals that an occasional intermediate cell is interposed between the basal and peridermal layers on day 13. All layers are mitotically active. Tonofilaments, unassociated with desmosomes, are present within the basal cell cytoplasm and the mitotic axis of the basal cells has changed from a parallel to a perpendicular plane with respect to the epidermal surface. At 14 days, a complete stratum intermedium, composed of one or two cell layers, is present. Rarely, developing hemidesmosomes are observed. Pools of glycogen are present in all cells below the periderm. The periderm is dense and no longer mitotically active. The skein of filaments, present in the inferior cytoplasmic region of the basal cells on days 12 and 13, is now absent. In the 15 day fetus, numerous developing hemidesmosomes are present. The stratum intermedium contains three to four layers of cells, and filaments are located deep within the cytoplasm of these intermediate cells. Rarely, a few developing keratohyalin granules and keratinosomes are present. A stratum intermedium is no longer present in the 16 day fetus. This region is now composed of a stratum spinosum and a stratum granulosum. Numerous keratinosomes are located in the upper stratum spinosum and lower stratum granulosum. The cells in the stratum granulosum are nucleated and the uppermost cells contain large keratohyalin granules. Three heterogeneous and one homogeneous type of keratohyalin granule is described. Dense bodies are present within mitochondria, nuclei, glycogen pools and the peridermal cytoplasm. The periderm is no longer dense and glycogen and keratohyalin granules are not observed in this layer.