A new origin for the maxillary jaw

One conserved feature of craniofacial development is that the first pharyngeal arch has two components, the maxillary and mandibular, which then form the upper and lower jaws, respectively. However, until now, there have been no tests of whether the maxillary cells originate entirely within the first pharyngeal arch or whether they originate in a separate condensation, cranial to the first arch. We therefore constructed a fate map of the pharyngeal arches and environs with a series of dye injections into stage 13-17 chicken embryos. We found that from the earliest stage examined, the major contribution to the maxillary bud is from post-optic mesenchyme with a relatively minor contribution from the maxillo-mandibular cleft. Cells labeled within the first pharyngeal arch contributed exclusively to the mandibular prominence. Gene expression data showed that there were different molecular codes for the cranial and caudal maxillary prominence. Two of the genes examined, Rarβ (retinoic acid receptor β) and Bmp4 (bone morphogenetic protein) were expressed in the post-optic mesenchyme and epithelium prior to formation of the maxillary prominence and then were restricted to the cranial half of the maxillary prominence. In order to determine the derivatives of the maxillary prominence, we performed focal injections of CM-DiI into the stage 24 maxillary prominence. Labeled cells contributed to the maxillary, palatine, and jugal bones, but not the other elements of the upper beak, the premaxilla and prenasal cartilage. We also determined that the cranial cells give rise to more distal parts of the upper beak, whereas caudal cells form proximal structures. Grafts of stage 24 maxillary prominences were also analyzed to determine skeletal derivatives and these results concurred with the DiI maps. These early and later fate maps indicate that the maxillary prominence and its skeletal derivatives are not derived from the first pharyngeal arch but rather from a separate maxillary condensation that occurs between the eye and the maxillo-mandibular cleft. These data also suggest that during evolution, recession of the first pharyngeal arch-derived palatoquadrate cartilage to a more proximal position gave way to the bony upper jaw of amniotes.