Retinoic acid and pigment cells: a review of in-vitro and in-vivo studies.

Topical tretinoin improves mottling and hyperpigmented lesions of photodamaged skin. The basic mechanisms underlying these effects are not known. It is demonstrated that retinoids inhibit the growth and enhance the differentiation of melanoma cells in vitro, and stimulate the constitutive melanogenesis in melanoma cells in vitro. On the other hand, they inhibit hormonally or pharmacologically induced melanogenesis in these cells. Very few data are available concerning the effect of retinoic acid on normal human melanocytes, but there is some inhibition of growth as in melanoma cells. Retinoic acid appears to have little effect on the melanogenesis of normal human melanocytes grown in vitro using serum-free culture medium. Changes in the shape of these melanocytes suggest that retinoic acid acts on cytoskeleton proteins. Further studies, both in vitro and in vivo, are needed to clarify the effects of retinoic acid on melanocytes.