Visible light and human skin pigmentation: The importance of skin phototype

Melanin is synthesised within melanocytes and transferred to keratinocytes in human skin, thereby regulating skin colour and protecting skin cells against UVR-induced damage. We commonly divide human skin into six phototypes (SPT)-I to -VI (Fitzpatrick scale) according to the skin's tanning response to UVR. In this pilot study, we investigated the impact of UVR (maximum 311nm), blue (peak 450nm) and green visible light (peak 530nm) on melanin production and type in healthy human skin histocultures (SPT-I, -II and -III). UVR, blue and green light stimulated a surface tanning response in SPT-II and -III, but not SPT-I. Using the Warthin-Starry stain for sensitive melanin detection, all three light treatments induced melanogenesis in SPT-II and -III skin. Surprisingly, blue and green light (but not UVR) stimulated melanin synthesis in SPT-I skin. Moreover, melanin synthesis induced by blue and green visible light in SPT-I, SPT-II, and SPT-III skin was not associated with a detectable increase in DNA damage or cell apoptosis. By contrast, both responses were detected after UVR. These data suggest that blue and green visible light can stimulate melanin production in fair-skinned individuals without, at least some of, the harmful consequences of UVR-induced pigmentation. We are currently examining the molecular basis of UVR-independent melanogenesis in fair skin.