Use of retinoic acid for the analysis of dermal-epidermal interactions in the tarsometatarsal skin of the chick embryo.

Feet of chicks are normally covered with scales. Injection of retinoic acid into the amniotic cavity of 10-day chick embryos causes the formation of feathers on the foot scales. To elucidate whether retinoic acid affects primarily the epidermis or the dermis, heterotypic dermal-epidermal recombinants of tarsometatarsal skin were tested as to their morphogenetic capacity, when grafted to the chick chorioallantoic membrane. Recombinants involving treated epidermis and untreated dermis formed feathered scales, while the reverse recombinants of untreated epidermis and treated dermis led to the formation of scales only. Likewise the association of treated tarsometatarsal dermis with untreated epidermis from a non-appendage-forming region (the midventral apterium) resulted in the formation of scales only. These results show that retinoic acid affects primarily the epidermis. Further insight into the mechanism of dermal-epidermal interaction was gained by heterotopic recombinations of early (8.5- and 10-day) untreated tarsometatarsal dermis with epidermis from the midventral apterium. These recombinants formed scales, proving that tarsometatarsal dermis is endowed with scale-forming properties as early as 8.5 days of incubation. Finally, it is concluded that retinoic acid acts on the chick foot epidermal cells by temporarily inhibiting their scale placode-forming properties, allowing their latent feather placode-forming properties to be expressed.