In earlier experiments, Fell & Mellanby (1953) showed that the simple, two-layered epidermis of the 7-day embryonic chick underwent mucous metaplasia when grown in medium containing excess vitamin A. The present investigation was undertaken to see whether epidermis at more advanced stages of development would undergo a similar transformation
in vitro
under the influence of vitamin A. Skin from the shank and foot of 13-, 14- and 18-day chick embryos was grown on rayon acetate cloth by Shaffer’s modification of the watch-glass method, in medium (cock plasma and embryo extract) to which natural or synthetic vitamin A alcohol had been added. For purposes of comparison, one experiment was made with skin from the trunk and limbs of 7-day embryos. A dose of 1500 i. u. vitamin A /100 ml. of culture medium completely inhibited keratinization in all the + A explants, whatever the age of the embryo from which they were obtained. This concentration induced mucous metaplasia in all the explants from 7- and 13-day chicks, and in a minority of those from 18-day embryos. In the 13- and 18-day explants, the outer strata of epidermal cells degenerated and were sloughed, and the secretory epithelium was formed from the deepest and least differentiated layers. The dermis also was affected by the vitamin. When the explants were transferred from + A to normal medium, mucin secretion at first increased, often becoming astonishingly copious; later the mucous tissue was shed and the deeper cells regenerated a squamous, keratinizing epidermis. In all the controls grown on normal medium, the epidermis retained its squamous structure and formed increasing amounts of keratin, except at the margin of the 7- and 13-day cultures; here the newly formed epithelium, which had spread beyond or below the dermis, often failed to cornify and in one 7-day control, which elsewhere was heavily keratinized, it even developed some secretory cells. This peripheral effect is thought to be due to the close and prolonged contact of the outwandering epithelium with the fairly high level of vitamin A normally present in fowl blood plasma. The concentrations of vitamin A used in the present experiments were much less than those that can be produced in the blood of fowls fed on a high vitamin A diet. The vitamin A in the culture medium, however, may be much more readily available to the epidermis than the same concentrations of vitamin in the blood stream of a hypervitaminotic bird. It is also probable that the vitamin is in a more active state in the culture medium than it is
in vivo
(cf. Fell & Mellanby 1952).
Correction of abnormal matrix formed by cmd/cmd chondrocytes in culture by exogenously added cartilage proteoglycan.