The effects of ultraviolet exposure on skin melanin pigmentation.

The main clinical, histological, ultrastructural and biochemical changes to the pigmentary system following photo-exposure are reviewed. Acute exposure to ultraviolet (UV) radiation induces an immediate pigment-darkening reaction, due to photo-oxidation of preformed melanin, followed by delayed tanning, the mechanism of which is unknown. Chronic exposure to UV induces photo-ageing with uneven pigment distribution. The most common pigmented lesions on chronically sun-exposed skin include ephelides, solar lentigines and pigmented solar keratoses. Idiopathic guttate hypomelanosis is also common in sun-exposed skin and may be considered as a manifestation of photo-ageing. Chronic UV also appears to induce cutaneous melanomas. Psolaren UVA lentigines and sunbed lentigines provide good arguments for the fact that UV exposure can induce melanocyte dysplasia. In addition, various tumours involving the keratinocyte population are associated with increased pigmentation, suggesting a concomitant alteration in melanocyte function, as in the case of pigmented epitheliomas and pigmented actinic keratoses. The exact nature of the interactions between photo-exposure and melanocytes has yet to be fully established.