Function of the retinoic acid receptors (RARs) during development: (I) Craniofacial and skeletal abnormalities in RAR double mutants

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Abstract
Numerous congenital malformations have been observed in fetuses of vitamin A-deficient (VAD) dams [Wilson, J. G., Roth, C. B., Warkany, J., (1953), Am. J. Anat. 92, 189-217]. Previous studies of retinoic acid receptor (RAR) mutant mice have not revealed any of these malformations [Li, E., Sucov, H. M., Lee, K.-F., Evans, R. M., Jaenisch, R. (1993) Proc. Natl. Acad. Sci. USA 90, 1590-1594; Lohnes, D., Kastner, P., Dierich, A., Mark, M., LeMeur, M., Chambon, P. (1993) Cell 73, 643-658; Lufkin, T., Lohnes, D., Mark, M., Dierich, A., Gorry, P., Gaub, M. P., LeMeur, M., Chambon, P. (1993) Proc. Natl. Acad. Sci. USA 90, 7225-7229; Mendelsohn, C., Mark, M., Dollé, P., Dierich, A., Gaub, M.P., Krust, A., Lampron, C., Chambon, P. (1994a) Dev. Biol. in press], suggesting either that there is a considerable functional redundancy among members of the RAR family during ontogenesis or that the RARs are not essential transducers of the retinoid signal in vivo. In order to discriminate between these possibilities, we have generated a series of RAR compound null mutants. These RAR double mutants invariably died either in utero or shortly after birth and presented a number of congenital abnormalities, which are reported in this and in the accompanying study. We describe here multiple eye abnormalities which are found in various RAR double mutant fetuses and are similar to those previously seen in VAD fetuses. Interestingly, we found further abnormalities not previously reported in VAD fetuses. These abnormalities affect ocular glands, salivary glands and their associated ducts, the axial and limb skeleton, and all skeletal elements derived from the mesectoderm of the frontonasal mass and of the second and third pharyngeal arches. RAR double mutants also exhibit supernumerary cranial skeletal elements that are present in the ancestral reptilian skull. The role of retinoic acid (RA) and of the RARs in the ontogenesis of the affected structures, particularly of those that are derived from mesenchymal neural crest cells, is discussed.