Fgf-8 determines rostral-caudal polarity in the first branchial arch

In mammals, rostral ectomesenchyme cells of the mandibular arch give rise t o odontogenic cells, while more caudal cells form the distal skeletal eleme nts of the lower jaw Signals from the epithelium are required for the devel opment of odontogenic and skeletogenic mesenchyme cells. We show that rostr al-caudal polarity is first established in mandibular branchial arch ectome senchymal cells by a signal, Fgf-8, from the rostral epithelium. All neural crest-derived ectomesenchymal cells are equicompetent to respond to Fgf-8, The restriction into rostral (Lhx-7-expressing) and caudal (Gsc-expressing ) domains is achieved by cells responding differently according to their pr oximity to the source of the signal. Once established, spatial expression d omains and cell fates are fixed and maintained by Fgf-8 in conjunction with another epithelial signal, endothelin-1, and by positional changes in ecto mesenchymal cell competence to respond to the signal.