Epidermal metaplasia of proamnionic epithelium induced by dorsal skin dermis in the chick embryo.

Proamnionic epithelium of the chick embryo cultivated directly on Wolff and Haffen's medium in the absence of mesenchymes fails to differentiate. Cultivation of the dorsal dermis of 6-5-day chick embryos in the absence of epithelium also results in lack of differentiation of dermal cells. When proamnionic epithelium taken from embryos before the 10-somite stage is cultivated combined with dorsal dermis of 6-5-day embryos for 6 days, the epithelium invariably undergoes metaplastic changes, forming stratified epidermis, sometimes with keratinized superficial layer. The underlying dermal cells are condensed and this often leads to the formation of feather germ-like structures. The competence of the epithelium for changing into the epidermis is gradually lost after the 10-somite stage, and the dorsal dermis from 8-5-day embryos is not very effective in inducing the epidermal metaplasia. Proamnionic epithelium cultivated on heat-killed dorsal dermis seems healthy but shows no sign of differentiation. Dorsal dermis combined with heat-killed proamnionic epithelium spreads and remains almost undifferentiated. These observations suggest that reciprocal induction mechanisms are involved in the epithelial and dermal differentiation. Cultivation of proamnionic epithelium with various heterologous mesenchymes or fragments of embryonic organs shows that this epithelium is only competent for epidermal differentiation when combined with dorsal dermis. When proamnion (proamnionic epithelium plus hypoblast) is directly combined with 6-5-day dorsal dermis it undergoes metaplastic changes. The same result is obtained when inverted (upside-down) proamnion is combined with the dermis. Hypoblast does not seem to affect the inductive interaction between the epithelium and the dorsal dermis.