Synergy between Hoxa1 and Hoxb1: the relationship between arch patterning and the generation of cranial neural crest

Hoxa1 and Hoxb1 have overlapping synergistic roles in patterning the hindbr ain and cranial neural crest cells. The combination of an ectoderm-specific regulatory mutation in the Hoxb1 locus and the Hoxa1 mutant genetic backgr ound results in an ectoderm-specific double mutation, leaving the other ger m layers impaired only in Hoxa1 function. This has allowed us to examine ne ural crest and arch patterning defects that originate exclusively from the neuroepithelium as a result of the simultaneous loss of Hoxa1 and Hoxb1 in this tissue. Using molecular and lineage analysis in this double mutant bac kground we demonstrate that presumptive rhombomere 4, the major site of ori gin of the second pharyngeal arch neural crest, is reduced in size and has lost the ability to generate neural crest cells. Grafting experiments using wild-type cells in cultured normal or double mutant mouse embryos demonstr ate that this is a cell-autonomous defect, suggesting that the formation or generation of cranial neural crest has been uncoupled from segmental ident ity in these mutants. Furthermore, we show that loss of the second arch neu ral crest population does not have any adverse consequences on early patter ning of the second arch. Signalling molecules are expressed correctly and p haryngeal pouch and epibranchial placode formation are unaffected. There ar e no signs of excessive cell death or loss of proliferation in the epitheli um of the second arch, suggesting that the neural crest cells are not the s ource of any indispensable mitogenic or survival signals. These results ill ustrate that Hox genes are not only necessary for proper axial specificatio n of the neural crest but that they also play a vital role in the generatio n of this population itself. Furthermore, they demonstrate that early patte rning of the separate components of the pharyngeal arches can proceed indep endently of neural crest cell migration.