precartilages) (Figure 3C). Both Rara and Rarb are also expressed in cranial mesenchyme. Rara (mainly the Rara1 isoform) is expressed in a relatively diffuse manner, whereas Rarb (mainly Rarb2) is strongly expressed in the frontonasal and periocular mesenchyme (see Figure 2B); it is not expressed in most of the maxillo-mandibular region derived from the first branchial arch, and weaker levels are found in more posterior branchial arches [Mollard et al., 2000; Ruberte et al., 1991]. Gene knockout experiments revealed that RARα and RARγ are the prevalent receptors in neural crest-derived cranial mesectoderm [Lohnes et al., 1994]. Rarb (especially Rarb2) is also strongly expressed in the embryonic foregut endoderm (Figure 1E,F). This expression appears before formation of the branchial arches [Ruberte et al., 1991; Smith, 1994] and is eventually found in the branchial pouch endoderm (Figure 2E), another important target tissue of retinoid signaling ([Niederreither et al., 2003; Wendling et al., 2000]; also see accompanying review by [Mark et al., 2009]).
