[Genetic expression and morphogenesis of the skin in vertebrates]

Retinoic acid is an exception amongst teratogenic agents: indeed, it not only interferes with morphogenesis, but it also alters positional values, which leads to the formation of additional structures. The latter effect, similar to that of homeotic mutations, which have been characterized in drosophila, is the most instructive way to study morphogenetic processes. Intramniotic injection of retinoic acid into 10-day-old chick embryos causes feather formation on the scales of the anterior side of tarsometatarsus. Likewise, when the upper-lip skin of 13.5-day mouse embryos is treated in vitro with retinoic acid, there is a development of mouse vibrissa hair buds into glands. The expression of retinoic acid nuclear receptors (RARs) has been studied by in situ hybridization during normal morphogenesis of hair vibrissa follicles as well as during retinoic acid-induced glandular metaplasia. During the normal development of hair follicle, only RAR alpha and RAR gamma genes are transcribed, whereas RAR beta gene expression remains undetected by the techniques used. Retinoic acid skin treatment brings about RAR beta expression in the dermis, leading to the formation of glomerular glands. Now, hair, gland, scale or feather differentiation implies dermal-epidermal interactions which have been extensively studied. However, little is known about the language of communication used by the two tissues. Even if the inductive role of the dermis seems dominant, the competence of the epidermis must not be neglected: the latter determines in some cases the type of cutaneous appendage which is formed.(ABSTRACT TRUNCATED AT 250 WORDS)