Inductive and Morphogenetic Responses of the Inner Ear to FGF Signaling

Inner ear development entails a series of sequential inductive tissue interactions where one tissue signals to direct the development of adjacent tissue[s]. Inductive signals from mesodermal, endodermal, and neuroectodermal origin control multiple aspects of early inner ear formation. Signals arising from the endoderm and mesoderm that underlie the presumptive otic field in the cephalic surface ectoderm control induction of the otic placode from this region [1,2], whereas signals emanating from the neuroepithelium of the hindbrain direct invagination of the otic placode to form the otic vesicle. Subsequent morphogenesis of the otic vesicle into the elaborate three-dimensional inner ear occurs in response to yet another set of inductive interactions, this time between the epithelium of the otic vesicle, from which the cochlear and vestibular anlagen form, and the surrounding periotic mesenchyme. Thus all tissue layers in and around the otic region are involved in early inner ear development, and the likely molecular signals leading to these interactions are discussed below.