Studies on the development of the definitive cell type of embryonic epidermis using the cross-linked envelope as a differentiation marker

Abstract

During their late stages of differentiation, keratinocytes of epidermis form beneath the plasma membrane a protein envelope cross-linked by ϵ-(γ-glutamyl)lysine bonds. This property is a reliable and specific marker for the keratinocyte and has been used to study the development of this cell type in the skin during the embryonic life of the rabbit. Up to halfway through gestation, virtually no cells of the epithelium have the ability to make such an envelope. Between the 16 and 20th days of gestation, some cells acquire this ability, and by birth most can make envelopes. It may be concluded that prior to 16 days' gestation, the external epithelium consists of a cell type that is a precursor of the keratinocyte. In culture, colonies of typical keratinocytes develop from single epithelial cells of embryos 23 days or older, but not from the cells of epithelia of 14-day or younger embryos. Colonies of cells resembling keratinocytes can be grown from epithelia of 16- to 20-day embryos; the cells of these colonies develop envelopes with a somewhat higher frequency than the cells in the epithelia from which they were derived but not nearly as well as the cells grown from older epithelia. The epithelium during this period contains cells at an intermediate stage of development. The epithelial cells of 14-day embryos will not differentiate into keratinocytes in surface culture. In contrast, cultured explants of the 14-day epithelium do support the differentiation of the precursor cells into keratinocytes that are able to form cross-linked envelopes with a high frequency, like those of advanced epithelium. When primitive epithelium and underlying connective tissue of early embryos are disaggregated with trypsin and the cells reassembled in the form of aggregates, a significant though small number of keratinocytes develops from epithelial cells of 14-day embryos, and a considerably larger number from those of 16-day embryos. The failure of the differentiation in surface culture could be due to the absence of proper mesenchymal influence.