A qualitative and quantitative transmission electronmicroscopic study of the immediate pigment darkening reaction.

Five healthy Caucasians, 3 having skin type II and 2 having skin type III, were UVA irradiated up to 14.5 J/cm2. Punch biopsies were taken before and immediately after irradiation and processed for transmission electron microscopy. Qualitative observations of the keratinocytes were obtained from the 5 subjects, while morphometry was performed on 1 subject with immediate pigment darkening reaction (IPD) (skin type III) and 1 without IPD (skin type II). The number of preformed melanosomes was 10-fold higher in the melanocytes and keratinocytes of the subject showing IPD as compared with the one who did not. No statistically significant difference, however, was observed in either skin type regarding the number of mature (stage IV) melanosomes of basal keratinocytes or melanocytes, size of melanosomes or redistribution of melanosomes in keratinocytes after UVA irradiation. No changes of the ultrastructure in the keratinocytes could be observed. It is concluded that the IPD consists of a photochemical reaction in preformed melanosomes and that the absence of IPD in skin type II could be due to the presence of too few preformed melanosomes.