Immunohistochemical survey of the distribution of epidermal melanoblasts and melanocytes during the development of UVB-induced pigmented spots

BackgroundRepeated exposures to ultraviolet B radiation (UVB) induce pigmented spots on dorsal skin of (HR-1 × HR/De) F1 hairless mouse. We showed previously that this mouse is suitable for studies of melanocyte function.ObjectiveTo clarify the mechanism of development of pigmented spots induced by chronic UVB exposure.MethodsWe used light and fluorescence microscopy to quantify changes in the numbers of differentiated melanocytes containing melanin pigments (MM) and melanoblasts/melanocytes immunohistochemically positive for tyrosinase-related protein (TRP)-1, TRP-2 (dopachrome tautomerase), and c-kit in epidermis during the development of pigmented spots in hairless mice chronically exposed to UVB (99 mJ/cm2, 3 times/week, 8 weeks).ResultsThe change in the number of TRP-1-positive cells during chronic UVB exposure was similar to that of MM: both increased dramatically during the stage of acute pigmentation, then decreased sharply after cessation of UVB, concomitantly with depigmentation; subsequently they increased gradually with the development of pigmented spots. In contrast, after two UVB exposures, no c-kit-positive cells were detected, then the number gradually increased during UVB irradiation, and continued to increase after cessation of irradiation; TRP-2-positive cells showed a rather similar pattern, except that they did not disappear initially.ConclusionOur results indicate that chronic UVB irradiation induces differentiation and proliferation of melanoblasts, followed by an increase of differentiated melanocytes, leading to the development of pigmented spots. The sequence of expression of markers appeared to be c-kit, TRP-2, TRP-1, and finally melanin, as it is during normal melanocyte differentiation.