Embryology of the epidermis: ultrastructural aspects. III. Maturation and primary appearance of dendritic cells in the mouse with mammalian comparisons.

The epidermis of mice ranging in age from prenatal day 17 through postnatal day 4 and, in addition, postnatal day 18, was studied with the electron microscope. In the 17 day fetus, the periderm may or may not be present and a stratum corneum is developing in the latter case. The cells of the strata spinosum and granulosum contain much glycogen and many keratinosomes and homogeneous keratohyalin granules which appear to line up in a row near the distal cell membranes, fuse and form keratinized cells which contain material similar in density to that of the individual keratohyalin granules. In the 18 to 20 day fetuses, no periderm is present. The stratum corneum becomes well-developed. The quantity of glycogen decreases but the number of keratinosomes and keratohyalin granules increases. Some cells in the basal region contain necrotic phagocytosed cells. Few changes occur in the epidermis of the neonate, although the stratum corneum increases in thickness. In the early postnatal period, the mouse epidermis is mature in appearance and resembles that of man. By postnatal day 18, the whole epidermis is much thinner, although all classical strata can usually be identified. The primary appearance and subsequent maturation of epidermal dendritic cells was also studied in the mouse during the embryonic, fetal, early postnatal and 18 day postnatal periods. Melanocytes which lacked cell processes were first identified in the 15 day fetus. Melanocyte cell processes and an increasing number of melanosomes, as well as melanocytes, are present between prenatal day 18 and postnatal day 3. On postnatal day 4, fewer melanocytes are present and they contain vacuoles. Phagocytosed melanocytes are also present within basal keratinocytes. Undifferentiated dendritic cells or indeterminate cells are observed beginning on prenatal day 16. These cells do not appear to increase significantly in number subsequently. On postnatal day 18, Langerhans cells and indeterminate cells are present but no melanocytes can be identified. No Merkel cells were observed. Mitotic dendritic cells or dendritic cells traversing the basal lamina were not observed.