The zebrafish inner ear

This chapter discusses the development of the zebrafish inner ear. The inner ear arises from an ectodermal thickening, the otic placode, which, like other sensory placodes, originates in a common preplacodal region (PPR) surrounding the neural plate at the end of gastrulation. Specification of the PPR, which is marked by the expression of dlx, eya, six, and irx family genes, is known to be dependent on the correct levels of both bone morphogenetic protein (BMP) and fibroblast growth factor (FGF) signaling. Induction of the otic placode from the PPR occurs during early somite stages in zebrafish. The otic placode is morphologically evident from 16 hours post fertilization, when it begins to cavitate to form an elongated, hollow epithelial ball—that is, the “otic vesicle.” Two of the very earliest patterning events in the otic placode and vesicle are the establishment of the prosensory and proneural domains that give rise to sensory hair cells and neuroblasts, respectively. In the zebrafish otic vesicle, the atoh genes have an early proneural function in establishing the prosensory domain, and are also required later for hair cell differentiation.