Migration and proliferation of cultured neural crest cells in W mutant neural crest chimeras.

Chimeric mice, generated by aggregating preimplantation embryos, have been instrumental in the study of the development of coat color patterns in mammals. This approach, however, does not allow for direct experimental manipulation of the neural crest cells, which are the precursors of melanoblasts. We have devised a system that allows assessment of the developmental potential and migration of neural crest cells in vivo following their experimental manipulation in vitro. Cultured C57Bl/6 neural crest cells were microinjected in utero into neurulating Balb/c or W embryos and shown to contribute efficiently to pigmentation in the host animal. The resulting neural crest chimeras showed, however, different coat pigmentation patterns depending on the genotype of the host embryo. Whereas Balb/c neural crest chimeras showed very limited donor cell pigment contribution, restricted largely to the head, W mutant chimeras displayed extensive pigmentation throughout, often exceeding 50% of the coat. In contrast to Balb/c chimeras, where the donor melanoblasts appeared to have migrated primarily in the characteristic dorsoventral direction, in W mutants the injected cells appeared to migrate in the longitudinal as well as the dorsoventral direction, as if the cells were spreading through an empty space. This is consistent with the absence of a functional endogenous melanoblast population in W mutants, in contrast to Balb/c mice, which contain a full complement of melanocytes. Our results suggest that the W mutation disturbs migration and/or proliferation of endogenous melanoblasts. In order to obtain information on clonal size and extent of intermingling of donor cells, two genetically marked neural crest cell populations were mixed and coinjected into W embryos. In half of the tricolored chimeras, no co-localization of donor crest cells was observed, while, in the other half, a fine intermingling of donor-derived colors had occurred. These results are consistent with the hypothesis that pigmented areas in the chimeras can be derived from extensive proliferation of a few donor clones, which were able to colonize large territories in the host embryo. We have also analyzed the development of pigmentation in neural crest cultures in vitro, and found that neural tubes explanted from embryos carrying wt or weak W alleles produced pigmented melanocytes while more severe W genotypes were associated with deficient pigment formation in vitro.