Kit(+) melanocytes seem to contribute to melanocyte proliferation after UV exposure as precursor cells

Melanogenesis caused by UV exposure is characterized by proliferation and differentiation of functioning melanocytes in epidermis. Recently, the existence of precursor melanocytes in normal skin was proposed immunohistochemically. Thus, this precursor melanocyte seems to proliferate and differentiate into mature pigmented melanocytes after UV exposure. To elucidate how these processes occur, we examined C57BL mouse epidermal sheets after UV exposure immunohistochemically. In normal epidermis, TRP-1(+) cells and Kit(+) cells were easily identified and the cellularities were 41.0 and 31.7 cells per mm(2), respectively. Only a few Mitf(+) cells and no TRP-2(+) cells were observed. On the first day after UV exposure, Mitf(+) cells and TRP-2(+) cells appeared, whereas the numbers of TRP-1(+) cells and Kit(+) cells decreased. Some Kit+ cells also expressed Mitf, but no TRP-1 and Mitf double positive cells were seen. In this period, some Mitf(+) cells were labeled with BrdU. The next day, Mitf(+) cells and TRP-2(+) cells increased to the maximum cellularity, and they decreased thereafter and disappeared on the seventh day. An increase of TRP-1(+) cells was first identified on the fifth day after UV exposure, and reached a cellularity four times as great as that of the normal control on the fourteenth day. The subepidermal injection of Kit antibody attenuated the increase of Mitf+ cells and TRP-1(+) cells. These findings suggested that precursor cells that reside in the skin, first differentiated into Mitf(+) and TRP-2(+) melanocytes by the activation of the Kit receptor, then become mature TRP-1(+) melanocytes after UV exposure.