The UV action spectra for the clone-forming ability of cultured human melanocytes and keratinocytes.

Melanocytes (skin type 2) and keratinocytes were irradiated with UV light of 254, 297, 302, 312 and 365 nm and the survival was measured. Clone-forming ability was chosen as the parameter for cell survival. Melanocytes were found to be less sensitive to UV light than keratinocytes (a difference of a factor 1.22-1.92 for the UV-C and UV-B wavelengths (254, 297, 301 and 312 nm) and a factor 6.71 for the UV-A wavelength (365 nm). Because melanin does not appear to protect against the induction of pyrimidine dimers the difference between melanocytes and keratinocytes in the UV-C and UV-B region could not be explained by the presence of melanin in the melanocytes. The relatively small difference can be explained by the longer cell cycle of melanocytes, which provides more time for the melanocytes to repair UV damage. In the UV-A region the difference between melanocytes and keratinocytes was much larger, suggesting that besides the longer cell cycle some additional factors must be involved in protection against UV-A light.