2 RHOMBOMERES ARE ALTERED IN HOXA-1 MUTANT MICE

This study provides a detailed description of the anatomical defects i n the Hoxa-1-/- mutant mice previously generated in our laboratory (T. Lufkin, A. Dierich, M. LeMeur, M. Mark and P. Chambon, 1991; Cell 66, 1105-1119). Three-dimensional reconstructions of the Hoxa-1-/- rhombe ncephalon reveals that it bears only five rhombomeric structures (ie. morphological segments) instead of the normal seven. The first three o f these rhombomeres appear normal as judged from the distribution patt ern of CRABPI transcripts in the neurectoderm and from the histologica l analysis of the cranial nerve components derived from these structur es. In contrast, the neural-crest-cell-free region normally located op posite rhombomere 5 is lacking in Hoxa-1-/-embryos, and motor neurons of the facial and abducens nerves, which normally differentiate within rhombomeres 4, 5 and 6, are missing in Hoxa-1-/- fetuses. These morph ological data, combined with the determination of the molecular positi onal identities of the rhombomeres 4 and 5 (P. Dolle, T. Lufkin, R. Kr umlauf, M. Mark, D. Duboule and P. Chambon, 1993; Proc. Natl. Acad. Sc i. USA, in press), suggest that rhombomere 4 is markedly reduced, wher eas rhombomere 5 is almost absent. Thus, the remnants of rhombomeres 4 and 5 appear to be fused caudally with rhombomere 6 to form a single fourth rhombomeric structure. Moreover, the migration of neural crest cells contributing to the glossopharyngeal and vagus nerves occurs in a more rostral position, resulting in abnormalities of these cranial n erves, which were visualized by whole-mount anti-neurofilament immunos taining. The mutual relationship along the rostrocaudal axis between t he otic pit and the neuroepithelial site of int-2 protein secretion (a putative otogenic cue) is not significantly changed in Hoxa-1-/-embry os. However, the abnormal relationship between the rhombencephalon and the epithelial inner ear may account for the aplasia and faulty diffe rentiation of the membranous labyrinth, the disruption of the cartilag inous otic capsule and the disorganisation of some middle ear structur es. This phenotype is compared with that of the Hoxa-1-/- mutants gene rated by O. Chisaka, T. S. Musci and M. R. Capecchi, 1992 (Nature 335, 516-520) and with that of the mice homozygous for the kreisler mutati on.