Ch

3,4-Didehydroretinol (and its esters) CH CH3 CH

CH3 CH3 3

(Vitamin A2 and its esters)

All - frans-retinaldehyde

AN-frans-retinoic acid FIGURE 1. The structures of selected retinoids found in skin.

grow in popularity. Data available from FDA's Voluntary Cosmetics Registration Program on cosmetic products in the US market indicate that 102 cosmetic formulations in 1981, 355 cosmetic formulations in 1992, 667 formulations in 2000, and more than 700 products in 2004 contained retinyl palmitate (FDA, 2004). It may, therefore, be anticipated that the skin will remain a focus for studies on the biological activity of vitamin A and other retinoids.

The biological effects of vitamin A on the skin have been reviewed by a number of authors. Reviews emphasizing the use of retinol and retinyl palmi-tate in cosmetic products have appeared (Cosmetic Ingredient Review, 1987;

Ries and Hess, 1999). Clinical management of specific dermatological disorders by retinoids is the subject of a vast number of reviews. The specific molecular mechanisms underlying the biological effects of vitamin A and other retinoids on the skin appear to be less frequently reviewed. Reviews of the general role played by retinoids in the skin and of retinoid-mediated molecular events in the skin include those by Fisher and Voorhees (1996), Roos et al. (1998), Randolph and Siegenthaler (1999), and Kang et al. (2000). The present chapter is intended to update information regarding the physiological role of vitamin A and its biologically active metabolites in the skin. Throughout the discussion, retinyl palmitate will play a central role since retinyl palmitate is the primary retinyl ester found in the diet, stored in the body, and used in topically applied consumer products.

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