Ultrastructural features of defective in vitro keratinization of chick embryonic skin.

Thirty-four explants of epithelium and subjacent mesenchyme from the hind limb buds of 5-day chick embryos were cultured for 1 to 14 days. Controls consisted of a series of in ovo limb bud specimens from 5 day's incubation to hatching. Both experimental and control specimens were examined by similar electron-microscopic methods. Differentiation of epidermis was precocious and that of dermis extremely retarded in vitro, contrasting with the events in ovo where epidermal differentiation occurred later and differentiation of dermis was early and vigorous. The two types of epidermal cells developed to a different degree in vitro. Pencytes and their derivatives at times developed precociously and reached full maturity, although their pattern of differentiation was asynchronous compared with that in ovo. Basal cells and their derivatives produced filaments precociously but mature keratrnized cells were not found. Absence of keratohyaline granules in the epithelial cells appeared to be the initial defect in the keratinization process. Hyperplasia of mitochondria in basal and parabasal cells was also noted. The underlying mesenchyme, which in ovo begins to produce large amounts of collagen at an exponentially accelerating rate at 5 days, synthesized only scant collagen under the in vitro conditions employed. The hypothesis is presented that the mesenchyme alters epithelial differentiation by affecting keratohyaline synthesis which in turn controls keratinization.