The differentiation of epidermis. II. Alternative pathways of differentiation of embryonic chicken epidermis in organ culture.

In the present study, two series of experiments have been made to investigate the role of the dermis in determining alternative pathways of differentiation in the epidermis. Previous work has shown that the scaly metatarsal epidermis of 12-day chicken embryos, when isolated in culture on a plasma clot or various other substrata, fails to develop normally and undergoes a characteristic sequence of differentiative and degenerative changes (McLoughlin, 1961a; Wessells, 1962;Dodson, 1963,1966). On the other hand, the separated epidermis, when cultivated in combination with either its own dermis or a gel of collagen, survives and forms a stratum corneum (Dodson, 1963, 1966). The questions arise of whether the changes produced in the epidermis by growth in isolation can be reversed by subsequent recombination of the epithelium with the dermis, and if so at what stage the degeneration becomes irreversible. Accordingly, in the first series of experiments, epidermis was cultivated in isolation for various periods of time, then recombined with dermis and cultivated further; the explants were examined histologically for signs of a keratinizing epithelium. In a similar experiment, Wessells (1963) found that proliferating columnar basal cells appeared in epidermis isolated for 10 or 24 h, then recombined with dermis for a further 1 or 2 days, but longer periods of segregation were not tested, nor was further differentiation described. The differentiation of the epidermis of embryonic chicken skin in culture can also be altered profoundly by treatment with excess of vitamin A (Fell & Mellanby, 1953; Fell, 1957). When the whole skin of 7-, 13-, or 18-day embryos was treated, keratinization was inhibited and a mucous metaplasia was induced; the latter change, however, was less extensive and less frequent in the oldest skin. Originally it was not known whether this action of the vitamin was directly on the epidermis or whether it was mediated through changes produced in the dermis. McLoughlin (1961a) noted, however, that in isolated limb