Fig. 9. Model for Ednra functions in the arches. (A) Edn1-Ednra1 interactions in the pharyngeal ectoderm initiate or maintain ventral hand2 and more dorsal expression of bapx1 through signaling events transduced by both Ednra1 and Ednra2. This establishes NC domains, each of which is patterned into distinct dorsal and ventral skeletal elements. (B) In Ednra1 morphants, edn1 expression is reduced in the ectoderm, and initiation or maintenance of these domains rely entirely on signals transduced by Ednra2. This is sufficient to maintain ventral identity; however, intermediate fates such as the bapx1-expressing joints are almost completely abolished. A dorsal mesodermal domain of eng2 expands slightly into the intermediate and/or ventral regions. (C) In Ednra1;Ednra2 morphants as well as in suc;edn1^-/- mutants, Edn1 signaling completely fails, resulting in loss of joint and ventral identities and expansion of eng2.