Plasticity in mouse neural crest cells reveals a new patterning role for cranial mesoderm

The anteroposterior identity of cranial neural crest cells is thought to be preprogrammed before these cells emigrate from the neural tube. Here we test this assumption by developing techniques for transposing cells in the hindbrain of mouse embryos, using small numbers of cells in combination with genetic and lineage markers. This technique has uncovered a surprising degree of plasticity with respect to the expression of Hox genes, which can be used as markers of different hindbrain segments and cells, in both hindbrain tissue and cranial neural crest cells. Our analysis shows that the patterning of cranial neural crest cells relies on a balance between permissive and instructive signals, and underscores the importance of cell-community effects. These results reveal a new role for the cranial mesoderm in patterning facial tissues. Furthermore, our findings argue against a permanently fixed prepatterning of the cranial neural crest that is maintained by passive transfer of positional information from the hindbrain to the periphery.