Transitory expression of Dlx5 and Dlx6 in maxillary arch epithelial precursors is essential for upper jaw morphogenesis.

Asymmetric, articulated jaws are characteristic of most vertebrate species; they derive from the first pharyngeal arch (PA1) which generates both maxillary and mandibular components. PA1 is colonized by cranial neural crest cells (CNCCs) which give rise to most bones and tendons of the jaws. The elements formed by different CNCCs contingents are specified by the combinatorial expression of Dlx genes. Dlx5 and Dlx6 are predominantly expressed by mandibular CNCCs. Analysis of the phenotype of Dlx5 and Dlx6 double mutant mice has suggested that they are necessary and sufficient to specify mandibular identity. Here, using 3D reconstruction, we show that inactivation of Dlx5 and Dlx6 does not only affect the mandibular arch, but results in the simultaneous transformation of mandibular and maxillary skeletal elements which assume a similar morphology with gain of symmetry. As Dlx5- and Dlx6-expressing cells are not found in the maxillary bud, we have examined the lineage of Dlx5-expressing progenitors using an in vivo genetic approach. We find that a contingent of cells deriving from epithelial precursors transiently expressing Dlx5 participate in the formation of the maxillary arch. These cells are mostly located in the distal part of the maxillary arch and might derive from its lambdoidal junction with the olfactory pit. Our observations provide the first genetic demonstration of the 'Hinge and Caps' model[1]. We support the notion that 'cap' signals could originate from epithelial derivatives of Dlx5-expressing progenitors which migrate and colonize the maxillary arch epithelium. Our results imply that Dlx5 and Dlx6 control upper and lower jaw morphogenesis through different coordinated mechanisms to generate functional, articulated jaws.