Previous work has emphasized the crucial role of retinoic acid (RA) in the ontogenesis of the vast majority of mesenchymal structures derived from the neural crest cells (NCC), which migrate through, or populate, the frontonasal process and branchial arches. Using somatic mutagenesis in the mouse, we have selectively ablated two or three retinoic acid receptors (i.e., RARa/RARb, RARa/RARc and RARa/RARb/ RARc) in NCC. By rigorously analyzing these mutant mice, we found that survival and migration of NCC is normal until gestational day 10.5, suggesting that RAR-dependent signaling is not intrinsically required for the early steps of NCC development. However, ablation of Rara and Rarg genes in NCC yields an agenesis of the median portion of the face, demonstrating that RARa and RARc act cell-autonomously in postmigratory NCC to control the development of structures derived from the frontonasal process. In contrast, ablation of the three Rar genes in NCC leads to less severe defects of the branchial arches derived structures compared with Rar compound null mutants. Therefore, RARs exert a function in the NCC as well as in a separated cell population. This work demonstrates that RARs use distinct mechanisms to pattern cranial NCC.
INTRODUCTIONThe morphogenesis of the craniofacial region results from complex developmental processes involving the migration of neural crest cells (NCC) and their subsequent differentiation through interactions with endoderm and ectoderm (Veitch et al., 1999;Trainor and Krumlauf, 2001). Cranial NCC arise from fore-, mid-, and hindbrain in distinct migratory streams. Fore-and midbrain NCC give rise to the frontonasal skeleton, whereas pre-otic hindbrain populations colonize the first two branchial arches (BAs), where they differentiate to form skeletal elements of the jaw, middle ear and tongue. Cardiac NCC participate in the development of aortic arch arteries, as well as thymus, thyroid, and parathyroid glands (Le Douarin et al., 1993;Santagati and Rijli, 2003). Because patterning and morphogenesis of these craniofacial structures require a finely orchestrated series of tissue interactions (Graham and Smith, 2001;Trainor and Krumlauf, 2001), perturbation of different processes may yield similar developmental abnormalities. For instance, inactivation of genes expressed in either NCC, endoderm, or ectoderm, results in similar cardiovascular defects, as remodeling of BA arteries into the definitive arterial vessels and heart outflow tract requires coordinated communication between NCC, endoderm and ectoderm (Depew ABBREVIATIONS BA branchial arch E embryonic day NCC neural crest cells RA retinoic acid ALDH1A retinaldehyde dehydrogenase RAR retinoic acid receptor RXR rexinoid receptor VAD vitamin A-deficient