MELANOCYTE DEVELOPMENT IN-VIVO AND IN NEURAL CREST CELL-CULTURES - CRUCIAL DEPENDENCE ON THE MITF BASIC-HELIX-LOOP-HELIX-ZIPPER TRANSCRIPTION

The more than 20 different Mitf mutations in the mouse are all associa ted with deficiencies in neural crest-derived melanocytes that range f rom minor functional disturbances with some alleles to complete absenc e of mature melanocytes with others. In the trunk region of wild-type embryos, Mitf-expressing cells that coexpressed the melanoblast marker Dct and the tyrosine kinase receptor Kit were found in the dorsolater al neural crest migration pathway In contrast, in embryos homozygous f or an Mitf allele encoding a non-functional Mitf protein, Mitf-express ing cells were extremely rare, no Dct expression was ever found, and t he number of Kit-expressing cells was much reduced. Wild-type neural c rest cell cultures rapidly gave rise to cells that expressed Mitf and coexpressed Kit and Dct. With time in culture, Kit expression was incr eased, and pigmented, dendritic cells developed. Addition of the Kit l igand Mgf or endothelin 3 or a combination of these factors all rapidl y increased the number of Dct-positive cells. Cultures from Mitf mutan t embryos initially displayed Mitf-positive cells similar in numbers a nd Kit-expression as did wild-type cultures. However, Kit expression d id not increase with time in culture and the mutant cells never respon ded to Mgf or endothelin 3, did not express Dct, and never showed pigm ent. In fact, even Mitf expression was rapidly lost. The results sugge st that Mitf first plays a role in promoting the transition of precurs or cells to melanoblasts and subsequently, by influencing Kit expressi on, melanoblast survival.