Cochlear afferent innervation development.

Sound signal is detected by sensory hair cells located in the cochlear region of the inner ear, and transmitted to the central nervous system by the spiral ganglion neurons (SGNs). These bipolar neurons develop long peripheral processes to connect hair cells, forming ribbon synapses, specialised for the precision and speed required to process auditory information. The establishment of a complex innervation pattern relies on specific signals, intrinsic to SGNs or provided by neighbouring cells, which are tightly controlled in time and space. In this paper, we review recent advances about stepwise development of afferent auditory neuronal circuitries, from neuron specification within the early otic vesicle to definitive synaptic connections with target cells. We especially focus on the cellular and molecular developmental changes involved in fibre outgrowth and extension to the sensory epithelium, specific afferent targeting to hair cells, and synaptic pruning.