Effects of genic substitution at the brown locus on the differentiation of epidermal melanocytes in newborn mouse skin.

In the dorsal epidermis of both C57BL/10J (black, BB) and C57BR/cdJ (brown, bb) mice, the number of melanocytes positive to the dopa reaction (melanocyte population) increases from birth to day 3 or 4, and then gradually decreases. However, the number of melanoblasts plus melanocytes positive to the combined dopa-premelanin reaction (melanoblast-melanocyte population) remains constant until day 3 or 4 and then decreases in the two strains. Despite the similarity of the developmental dynamics in both black and brown mice, there is a significant difference in the number of differentiated melanocytes. Melanocytes are more numerous and more dopa-reactive in brown mice than in black. The maximal density of the melanoblast-melanocyte population on day 3 or 4 does not differ in brown and black mice. Moreover, the maximal density of the melanocyte population in brown epidermis does not differ from that of the melanoblast-melanocyte population of both brown and black. These results indicate that b allele, when homozygous, enhances the differentiation of epidermal melanoblasts by inducing high tyrosinase activity.