Effects of vitamin A acid in skin: in vivo and in vitro studies.

Profound metabolic changes take place in keratinizing epithelia in the presence of retinoic acid. In vivo as well as in vitro the proliferative activity of epidermal cells is greatly enhanced. Together with the increased rate of new cell production cellular differentiation (keratinization and cornification) is also altered. The effects are species-unspecific, probably tissue-specific and dose-dependent. The precise action of retinoic acid, however, still remains unknown.