Key roles of retinoic acid receptors alpha and beta in the patterning of the caudal hindbrain, pharyngeal arches and otocyst in the mouse

Mouse fetuses carrying targeted inactivations of both the RAR alpha and the RAR beta genes display a variety of malformations in structures known to b e partially derived from the mesenchymal neural crest originating from post otic rhombomeres (e,g, thymus and great cephalic arteries) (Ghyselinck, N,, Dupe, V,, Dierich, A., Messaddeq, N,, Garnier, J,M,, Rochette-Egly, C,, Ch ambon, P, and Mark M, (1997), Int, J, Dev. Biol. 41, 425-447), In a search for neural crest defects, we have analysed the rhombomeres, cranial nerves and pharyngeal arches of these double null mutants at early embryonic stage s, The mutant post-otic cranial nerves are disorganized, indicating that RA Rs are involved in the patterning of structures derived from neurogenic neu ral crest, even though the lack of RARa and RAR beta has no detectable effe ct on the number and migration path of neural crest cells, Interestingly, t he double null mutation impairs early developmental processes known to be i ndependent of the neural crest e,g,, the initial formation of the 3rd and 4 th branchial pouches and of the 3rd, 4th and 6th arch arteries, The double mutation also results in an enlargement of rhombomere 5, which is likely to be responsible for the induction of supernumerary otic vesicles, in a disa ppearance of the rhombomere 5/6 boundary, and in profound alterations of rh ombomere identities, In the mutant hindbrain, the expression domain of krei sler is twice its normal size and the caudal stripe of Krox-20 extends into the presumptive rhombomeres 6 and 7 region, In this region, Hoxb-1 is ecto pically expressed, Hoxb-3 is ectopically up-regulated and Hoxd-4 expression is abolished, These data, which indicate that retinoic acid signaling thro ugh RAR alpha and/or RAR beta is essential for the specification of rhombom ere identities and for the control of caudal hindbrain segmentation by rest ricting the expression domains of kreisler and of Krox-20, also strongly su ggest that this signaling plays a crucial role in the posteriorization of t he hindbrain neurectoderm.