Rapid modulation of keratinocyte differentiation by the external environment.

The ultrastructural differentiation of epidermal keratinocytes cultured in the presence of 3T3 feeder cells is significantly different from that of the epidermis in vivo. Several markers of keratinization, including keratohyaline granules (KGs), membrane-coating granules (MCGs), and an enucleated stratum corneum are essentially absent from cultured cells. When cultured rabbit epidermal cells were trypsinized and injected subcutaneously into athymic mice, the cells reaggregated and formed cysts lined with a stratified squamous epithelium morphologically resembling the in vivo epidermis. In this paper, we examined the differentiation of the injected cells by electron microscopy. Within 24 h after injection, MCGs and KGs appeared in the reaggregated epidermal cells. Horny cells were noted within 48 h. Since basement membrane formation was not completed until much later (between 4-9 days), a direct contact between epidermal cells and a continuous basal lamina structure was not required for the formation of various keratinization markers. Glycogen and lipid droplets, which were abundant in the early (10-48 h) cystic epithelia, gradually disappeared from the basal through the granular layers during days 2-9. By 16 days, the ultrastructure of the cystic epithelium appeared similar to the in vivo rabbit epidermis including the formation of "rabbit-type" KGs, MCGs, and normal enucleated horny cells. These results provided further evidence that the differentiation of epidermal cells can be modulated significantly and reversibly by the external environment. Moreover, the expression of certain morphologic markers of keratinization (KGs and MCGs) can be modulated rapidly by the growth environment.