The neural crest provides a population of pluripotent, highly migratory cells which delaminate from the neuroepithelium and differentiate into a wide range of cell types. Neural crest cells originate from the mid- and hindbrain and migrate ventrally to populate the developing head. Once they have reached their targets they differentiate into cartilage, connective tissue, sensory neurons and pigment cells. Although neural crest development has been studied for some time by elegant embryological and in vitro studies very little is known about the genes which regulate this process. As neural crest cells are one of the few vertebate cell types which have no clear analogue in invertebrate species, it is more difficult to apply results from genetic screens in Drosophila and C.elegans to this problem. We are currently using mutants isolated in a large scale zebrafish screen to identify and, eventually, molecularly characterise genes involved in neural crest development in vivo.

During the large screen, 109 mutants, comprising at least 26 genes, were found that affect distinct processes during jaw and branchial arch development. For example, mutations in four genes specifically affect the development of the two anterior arches, whereas other mutations affect only posterior arches. Mutations in about 8 genes affect cartilage differentiation and will be useful in elucidating the distinct regulatory events which underly cartilage formation. We are currently analyzing mutants which show defects in only subsets of arches using a variety of techniques like in situ hybridization, antibody staining, DiI labeling, histological analysis, cell transplantation as well as molecular techniques. As the branchial arches, which are lined by endoderm, become populated by paraxial mesoderm (giving rise to endothelia and pharyngeal muscles) we are also studying how these different tissues interact during the patterning process.

The neural crest cells which arise in the trunk migrate to widely dispersed embryonic locations where they give rise to many differentiated cell fates, such as pigment cells, and neurons and glia of the peripheral nervous system. Although it is a question which has long been considered of some importance, very little is known about the molecules which guide the migrating crest cells and to what extent their final destinations affect their differentiation. We have isolated more than two hundered and eighty mutations, in at least 92 genes, which affect the positioning, differentiation or morphology of pigment cells, a cell type which is derived from the trunk neural crest. We are currently trying to determine to what extent these mutations affect the other crest derived lineages, in the hope that we will find genes which are essential for general neural crest migration and morphogenesis.