The photoadaptive response to ultraviolet exposure in human skin using ultraviolet spectrophotometry

Background: Both pigmentation and non-pigmentary processes contribute to the development of photoadaptation yet the exact contribution of either in the resting state and in response to ultraviolet (UV) radiation is unclear. The purpose of this study was to estimate independently these changes occurring in the epidermis following repeated exposure to UV in two groups with differing degrees of constitutive pigmentation. Methods: We describe a mathematical model for explaining the spectral absorbance of excised human epidermis based on the absorbance of constituent chromophores. The model was applied to spectral absorbance data measured on samples of epidermis excised from pre-irradiated skin and from skin obtained following UV irradiation on 3 successive days. Results: We found that in Asian skin there was only a mild photoadaptive response, principally by a small increase in pigmentation. On the other hand, the significant adaptive response in Caucasian skin was through hyperplasia of the epidermis, with tanning contributing only to a much smaller degree. Conclusion: This study has enabled us to study independently the pigmentary and non-pigmentary pathways and has shown that in those people with a lower degree of constitutive pigment, the primary mechanism of photoadaptation is via the non-pigmentary route.