Patterning the cranial neural crest

The pattern of skeletal elements and musculature in the branchial region of the chick embryo is profoundly influenced by the neural crest which migrates in separate streams. Neural crest emigration from the hindbrain into the branchial arches is discontinuous, and regions of crest production are separated from each other by two regions of crest apoptosis, rhombomeres (r) 3 and 5. Both r3 and r5 produce large numbers of crest cells when they are cultured in isolation; but, when co-cultured with their neighbouring segment neural crest apoptosis is restored. The expression of the signalling molecule Bmp-4 and the gene msx-2 in apoptotic domains of r3 and r5 is also dependent upon neighbour interactions and the addition of recombinant Bmp-4 to explant cultures of r3/r5 upregulates both Bmp-4 and msx-2 expression and restores apoptosis. This interactive mechanism acts to ensure the sculpting of crest outflow into non-mixing streams and, consequently, the fidelity of pattern transfer into the branchial arches.