The physiology and biochemistry of retinoic acid.

: The ability of the fat soluble Vitamin A to modulate cellular differentiation has been known for over 60 years. Numerous studies have shown that Vitamin A and its 2 major metabolites play a key role in vision (retinal) and cell differentiation (retinoic acid, RA). The control of cellular differentiation may be exercised on a variety of tissues including simple and stratified epithelia, the latter will be the focal point of this article, with emphasis being given to the skin. In the development of the limb bud, RA has been proposed as a morphogen controlling development via a concentration gradient established by a mechanism thought to involve specific nuclear receptors and a cytosolic binding protein (CRABP). In the hemopoietic system, RA is known to modulate several functions of polymorphonuclear leucocytes (PMN) in vitro and the differentiation of granulocyte macrophage-precursors. Moreover it has been shown to exert immunomodulatory properties.