Differential expression of bone morphogenetic proteins in the developing vestibular and auditory sensory organs

The genes responsible for the formation of various sensory organs in the inner ear are not known. There are eight sensory organs in the chick inner ear, and our previous study showed that all presumptive sensory organs initially express bone morphogenetic protein 4 (BMP4), a member of the transforming growth factor (TGF)-beta gene family. To address the potential role of BMPs in the patterning of different sensory organ structures, we investigated the expression of BMP4, BMP5, and BMP7 during sensory organ differentiation in the chick inner ear. The gene expression pattern of BMP5, although similar to that of BMP4, was transient and disappeared by embryonic day 3.5 (E3.5). In contrast, BMP7 gene expression was quite extensive, starting in the otic placode. By E5, gene expression patterns of BMP4 and BMP7 differed among vestibular and auditory sensory organs. In the vestibular sensory organs, BMP7 gene expression segregated from the main sensory tissue areas at the onset of differentiation, whereas BMP4 expression concentrated in supporting cells. In the cochlea, however, BMP7 gene expression became restricted to sensory tissue over time and eventually concentrated in supporting cells, whereas BMP4 gene expression was localized to hair cells. The different BMP expression patterns in developing auditory and vestibular sensory organs may help to shape each respective sensory structure. Furthermore, the expression of BMP4 in the cochlea also revealed an interesting pattern of sensory cell differentiation: the distal portion of the cochlea differentiates first, and the tall hair cells develop before the short hair cells.