In contrast to Rara and Rarb, the murine Rarg gene shows no detectable expression within the developing brain and spinal cord neuroepithelium, apart from its early expression in the caudalmost neural plate (see above). From E9 onwards, it is expressed throughout the cranial mesenchyme, both at the level of the frontonasal mass and the branchial arches (Figure 3A,B) [Ruberte et al., 1990]. The cranial mesenchyme is composed of two cell components, the primary mesenchyme and the neural crest cells originating from forebrain, midbrain and hindbrain levels. The homogeneous pattern of Rarg expression (Figure 3A) suggests that both components may express this receptor. Eventually, Rarg expression becomes localized to precartilaginous cell condensations (Figure 3B), which in the head, derive from neural crest cells. Rarg expression is found in precartilaginous condensations throughout the body (including non-skeletal presumptive cartilages, e.g., laryngeal and tracheal 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
