Cell lineage and mutational studies of cranial neural crest development in the zebrafish

As vertebrates develop, cells of the neural crest migrate extensively throughout the body to form a wide variety of cell types occupying many different body segments. How does a crest cell acquire its tissue or segment-specificity? Previous studies in vitro and in vivo have suggested that subsets of cranial neural crest cells may be specified, having different developmental properties from their neighbors, before migration from the neuroepithelium. To address this problem I describe the fates of individual premigratory crest cells in the zebrafish embryo, Brachydanio rerio. Single cells, injected with a lineage tracer dye, generate clonal progeny that are confined to particular pharyngeal segments and particular cell types, consistent with the specification hypothesis. To address possible genetic bases for crest cell specification I have characterized a lethal mutation, chinless wonder $(chwsp{b146}),$ that disrupts a subset of cranial crest derivatives, pharyngeal cartilage. Mutant embryos have neither cartilage nor striated muscles in the head. Mosaic analyses suggest that chw functions directly within a subset of premigratory neural crest, the cartilage lineage, but that the requirement for chw may be bypassed when mutant neural crest cells are in close contact with wild-type cartilage precursors. The chw mutation appears to act indirectly to block muscle cell development, since mutant mesoderm will form muscle in the presence of wild-type cartilage. This suggests an additional role of neural crest cells or their cartilage derivatives in muscle induction.