These studies concluded that the W gene is expressed in these three stem cell lineages, while the Sl gene affects the microenvironment in which these cells divide and migrate. Because the product of the wild type W allele looked like a growth factor receptor, the search began for its ligand, a presumed growth factor. Moreover, it is encoded by the wild type Steel allele. When the melanocyte precursors forst leave the neural tube, they appear to be dependent on soluble stem cell factor. However, once they are in the dermatome, they need cell-bound stem cell factor.
Similar situations exist for mice and humans.
The fate map of the trunk neural crest cells. The central diagram represents the chick trunk, the somites on either side of the neural axis. The right-hand diagram shows the sympathetic ganglia formed from the neural crest cells from somite 5 and caudally. The adrenal medullary cells come from the crest between somites 18 and 24. Three genes have recently been found whose deficiency causes an absence of these enteric neurons.