Specification and migration of melanoblasts at the vagal level and in hyperpigmented Silkie chickens

The final pattern of neural crest derivatives used to be believed to be the result of unspecified neural crest cells haphazardly entering migratory pa ths and then receiving cues unique to that path that direct their different iation. An alternative model, which we have coined the phenotype-directed m odel, is that neural crest cells are fate-specified first and then select a migratory pathway based on their developmental specification. Support for this model comes from recent studies demonstrating that, at the thoracic le vel, neural crest cells are specified as melanocyte precursors (melanoblast s) prior to entering the dorsolateral path, and that only melanoblasts have the ability to migrate dorsolaterally. Here we examine two examples of mel anocyte patterning in birds that apparently contradict this model. The firs t is neural crest at the vagal level, where early crest cells migrate dorso laterally and enter the branchial arches. Despite the fact that these cells migrate dorsolaterally (suggesting that they are melanoblasts), branchial arch-derived neural crest cells fail to differentiate as melanocytes in vit ro. These observations suggest that the branchial arch environment may not support the survival or differentiation of melanogenic neural crest cells. The second example is the hyperpigmented Silkie chickens, which exhibit ext ensive internal pigmentation. The Silkie defect has been linked to a differ ence in the neural crest migratory environment that potentially causes (or allows) unspecified neural crest cells to undergo melanogenesis in the vent ral path. In both of these situations, it appears that the final distributi on of pigment cells is controlled by environmental factors, which would con tradict the phenotype-directed model. Here we show that the final pattern o f melanocytes at the vagal level and in Silkie chickens reflects the migrat ory behavior of lineage-specified melanoblasts, as predicted by the phenoty pe-directed model. At the vagal level, the early, dorsolaterally migrating crest cells that colonize the branchial arches are not melanoblasts and are biased against melanogenesis in vitro. Melanoblasts are not specified unti l later, just prior to a second wave of dorsolateral migration, and althoug h these cells migrate dorsolaterally they do not invade the branchial arche s. In Silkie embryos, melanoblasts are specified late and only invade the d orsolateral path after they have been specified. Unlike quail and White leg horn melanoblasts, however, Silkie melanoblasts also migrate ventrally, but again only after they are specified. Dev. Dyn. 1998;213:476-485. (C) 1998 Wiley-Liss, Inc.