Early Development of the Spiral Ganglion

Abstract

The neurons and sensory epithelia of the inner ear are produced from a proneurosensory domain in the early otocyst. Embryological observations together with analysis at the molecular level have uncovered the origin of spiral ganglion neurons and detailed the series of events that underlie their differentiation. During early development, extrinsic signals position the proneurosensory domain in the anterior of the otic cup, restricting formation of non-sensory regions to the posterior. Transcriptional networks reinforce the neurosensory fate and steer progenitors towards neuronal or sensory fates. Neuronal progenitors delaminate from the otic epithelium, proliferate, and then differentiate to form a contiguous cochlear-vestibular ganglion. Spiral and vestibular ganglion neurons arise from spatially and temporally distinct neuronal progenitors, relying in part on the action of cell-type specific transcription factors. Through unclear mechanisms, spiral ganglion neurons (SGNs) are further subdivided into Type I and Type II SGNs, followed by further diversification of Type I SGNs to produce the heterogeneous population necessary for the sense of hearing.