The effects of prolonged ultraviolet irradiation on the pigment system of PET/Wmr mouse epidermis

Repeated exposure to ultraviolet radiation brings about an increase in the DOPA-positive melanocyte population of the human. However, there is an age-dependent decline in the melanocyte population regardless of UV irradiation. In the rhesus monkey, a drop in the DOPA-positive epidermal melanocyte population has been observed after prolonged UV irradiation. The population of DOPApositive epidermal melanocytes in PET/Wmr mice reaches a peak during the first post-natal week after which the number of cells declines, fading almost completely within 3 weeks. Their occurrence in adult mice is rare. With UV irradiation (300 nm) the melanocyte population of newborn and adult PET mice is significantly increased over that of nonirradiated animals, reaching a peak during the first week of irradiation. Subsequently, there is a decline in melanocyte numbers which, in adults, extends through 26 days of irradiation. However, in the young mice the numbers begin to increase again on day 14 of irradiation and continue to do so through 18 days of irradiation. These results support the concept that a DOPA-negative amelanotic·melanocyte population exists in the interfollicular epidermis of these mice and can be stimulated by UV irradiation to become active. They further suggest the possibility of a cycle of melanogenic inhibition in the skin which is affected by UV irradiation