In vivo and in vitro morphological analysis of melanocytes homozygous for the misp allele at the murine microphthalmia locus.

The misp 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 Miwh (microphthalmia-white) the misp 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-misp/misp mice, C57BL/6J-Miwh/Miwh mice, C57BL/6J-Miwh/misp mice, and C57BL/6J-(+)/+ control mice. Melanocytes of the misp/misp mice appeared normal in situ. However, melanocyte cultures derived from neonatal skins of misp/misp 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 misp/misp melanocytes. These results demonstrate that while the misp allele has no obvious effect on the phenotype of the mouse, it does dramatically suppress the survival of melanocytes in normal culture conditions.