In vivo and in vitro morphological analysis of melanocytes homozygous for the mi(sp) allele at the murine microphthalmia locus

The mi(sp) allele (microphthalmia-spotted), a mutant allele at the murine microphthalmia (mi) locus, when homozygous, results in a normal phenotype in which there is no apparent alteration in pelage pigmentation or ocular development. However, when heterozygous with other mi locus alleles, specifically Mi(uh) (microphthalmia-white) the mi(.8p) allele exerts an affect on the phenotype. We examined the ultrastructure of melanocytes in the anagen hair bulb and the choroid plus the retinal pigmented epithelium of C57BL/6J-mi(.sp)/mi(.8p) mice, C57BL/6J-Mi(.uh)/Mi(.uk) mice, C57BL/6J-Mi(.uh)/mi2(.8p) mice, and C57BL/6J-(+)/(+) control mice. Melanocytes of the mi(.8p)/mi(.8p) mice appeared normal in situ. However, melanocyte cultures derived from neonatal skins of mi(.sp)/mi(.8p) mice exhibited small primary colonies that did not dramatically expand in size. Occasionally, abnormalities in the structure of the Golgi apparatus were observed in primary cultures of mi(.8p)/mi(.8p) melanocytes. These results demonstrate that while the mi(.8p) allele has no obvious effect on the phenotype of the mouse, it does dramatically suppress the survival of melanocytes in normal culture conditions.