EXPRESSION OF FGF-3 IN RELATION TO HINDBRAIN SEGMENTATION, OTIC PIT POSITION AND PHARYNGEAL ARCH MORPHOLOGY IN NORMAL AND RETINOIC ACID-EXPOSED MOUSE EMBRYOS

The gene Fgf-3 is expressed in rhombomeres 5 and 6 of the hindbrain an d has been functionally implicated in otic development. We describe ne w sites of expression of this gene in mouse embryos in the forebrain, the midbrain-hindbrain junction region, rhombomere boundaries, a crani al surface ectodermal domain that includes the otic placode, and in th e most recently formed somite. In the early hindbrain, high levels of Fgf-3 transcripts are present in rhombomere 4. The surface ectodermal domain at first (day 8 1/2) extends laterally from rhombomeres 4 and 5 (prorhombomere B), in which neuroepithelial levels of expression are highest, to the second pharyngeal arch ventrally; at day 9, when the r egion of highest level of neuroepithelial Fgf-3 expression is in rhomb omeres 5 and 6, the dorsal origin of the surface ectodermal domain is also at this level, extending obliquely to the otic placode and the se cond arch. The initially high level of Fgf-3 transcripts in the otic p lacode is downregulated as the placode invaginates to form the otic pi t. Fgf-3 is a good marker for the epithelium of pharyngeal arches 2 an d 3, and our in situ hybridization results confirm the dual identity o f the apparently fused first and second arches in some retinoic acid-e xposed embryos, and the fusion of the first arch with the maxillary re gion in others. Correlation between Fgf-3 expression and morphological pattern in craniofacial tissues of normal and retinoic acid-exposed e mbryos indicates that prorhombomere B, the second arch and the otic ec toderm represent a cranial segment whose structural integrity is maint ained when hindbrain morphology and phamal Fgf-3 expression domains wi th those of Fgf-4 and with the phenotype of Fgf-3-deficient mutant emb ryos suggests that there is some functional redundancy between Fgf-3 a nd Fgf-4 in otic induction and second arch development.