Dlx5 and Dlx6 expression in the anterior neural fold is essential for patterning the dorsal nasal capsule

Abstract

SUMMARYMorphogenesis of the vertebrate facial skeleton depends upon inductive interactions between cephalic neural crest cells (CNCCs) and cephalic epithelia. The nasal capsule is a CNCC-derived cartilaginous structure comprising a ventral midline bar (mesethmoid) overlaid by a dorsal capsule (ectethmoid). Although Shh signalling from the anterior-most region of the endoderm (EZ-I) patterns the mesethmoid, the cues involved in ectethmoid induction are still undefined. Here, we show that ectethmoid formation depends upon Dlx5 and Dlx6 expression in a restricted ectodermal territory of the anterior neural folds, which we name NF-ZA. In both chick and mouse neurulas, Dlx5 and Dlx6 expression is mostly restricted to NF-ZA. Simultaneous Dlx5 and Dlx6 inactivation in the mouse precludes ectethmoid formation, while the mesethmoid is still present. Consistently, siRNA-mediated downregulation of Dlx5 and Dlx6 in the cephalic region of the early avian neurula specifically prevents ectethmoid formation, whereas other CNCC-derived structures, including the mesethmoid, are not affected. Similarly, NF-ZA surgical removal in chick neurulas averts ectethmoid development, whereas grafting a supernumerary NF-ZA results in an ectopic ectethmoid. Simultaneous ablation or grafting of both NF-ZA and EZ-I result, respectively, in the absence or duplication of both dorsal and ventral nasal capsule components. The present work shows that early ectodermal and endodermal signals instruct different contingents of CNCCs to form the ectethmoid and the mesethmoid, which then assemble to form a complete nasal capsule. phenotype has not been yet described in detail (Beverdam et al., 2002; Depew et al., 2002;Levi et al., 2003;Robledo et al., 2002). Noticeably, nasal defects in Dlx5/6 -/-heads are not shared with either single or compound mutations of other Dlx family members (Depew et al., 2005). A recent study has demonstrated that the olfactory pit plays a late role in controlling the morphogenesis of skeletal elements of the avian nasal capsule (Szabo-Rogers et al., 2009). These data were obtained from embryos in which CNCCs had already migrated into the developing olfactory territory and do not provide information about the initial interactions directing CNCCs towards ectethmoid formation. As the olfactory pit derives from the Dlx5/Dlx6-positive anterior part of the neural fold, we decided to explore the possibility that this territory could be endowed with early inductive capacities. Using mouse mutants and surgical and molecular analyses of chick embryos, we demonstrate that an anterior region of the neural fold ectoderm (NF-ZA) is the territory responsible for ectethmoid induction through a Dlx5/Dlx6-dependent mechanism.We conclude that the complete nasal capsule results from the juxtaposition of two independent structures formed by CNCCs competent to respond to different endodermal or ectodermal cues. MATERIALS AND METHODS Avian and mouse embryosAll animal procedures were approved by national and institutional ethical committees. Fertil...

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