THE EFFECT OF TOPICAL TRETINOIN ON THE FACIAL PIGMENTED SPOTS OF THE STUMP-TAILED MACAQUE

The pigmented spots on the face of the post-adolescent stump-tailed ma caque (Macaca arc-toides) are species specific phenomenon. The facial pigmentation largely develops around the nose and spreads to the cheek , maxillar, and scalp regions along with ageing. The area and darkness of the pigmented skin shows marked individual variation. Histogically , the pigmented skin exhibits normal epidermal thickness and contains large dendritic melanocytes and numerous melanin granules spreading to the epidermal layer. The surrounding non-pigmented skin contains virt ually no dopa-positive melanocytes nor pigments in the epidermis. Neve rtheless, the darkness and extension of pigmented areas increases by e xposure to direct sunlight. Using this model we examined the effect of topical tretinoin (Retin A, 0.1%) on gross depigmentation, epidermal cell cycles melanocytes, and melanin granules. Pigmentation began to f ade as early as one month after application and significant fading of color occurred by 5 months after application. At 5 months, the epiderm is showed marked acanthosis and a 5-fold increase in DNA synthesis cel ls in the basal layer compared to those of vehicle-treated skin. The m elanin granules were scattered throughout the thickened epidermal laye r but large dendritic melanocytes exhibited no changes. The effect of reversal depigmentation by tretinoin appeared to be caused by a epider mal layer lamentation by tretinoin appeared to be caused by a dispersi on of melanin granules into proliferated epidermal keratinocytes and i ncreased cell cycles but not by a suppression of melanogenesis.