Patterning the vertebrate head: murine Hox 2 genes mark distinct subpopulations of premigratory and migrating cranial neural crest.

The structures of the face in vertebrates are largely derived from neural crest. There is some evidence to suggest that the form of the facial pattern is determined by the crest, and that it is specified before migration as to the structures that is is able to form. The neural crest is able to control the form of surrounding, non-neural crest tissues by an instructive interaction. Some of this cranial crest is derived from a region of the hindbrain that expresses Hox 2 homeobox genes in an overlapping and segment-restricted pattern. We have found that neurogenic and mesenchymal neural crest expresses Hox 2 genes from its point of origin beside the neural plate, during migration and after migration has ceased and that rhombomeres 3 and 5 do not have any expressing neural crest beside them. Each branchial arch expresses a different combination or code of Hox genes in a segment-restricted way. The surface ectoderm over the arches initially does not express Hox genes, and later adopts an expression pattern that reflects that of neural crest that has come to underlie it. We suggest that initially the neural plate and neural crest are spatially specified, while the surface ectoderm is unpatterned. Subsequently some positional information could be transferred to the surface ectoderm as a result of an interaction with the neural crest. Given that the role of the homologous genes in insects is position specification, and that neural crest is imprinted before migration, we suggest that Hox 2 genes are providing part of this positional information to the neural crest and hence are involved in patterning the structures of the branchial arches.