Making connections in the inner ear: Recent insights into the development of spiral ganglion neurons and their connectivity with sensory hair cells

• New insights into the origins of the SGNs: the reemergence of the neural crest.
• New insights into the specification of CVG neuroblasts.
• Mechanisms of hair cell innervation by SGN peripheral axons.
• Tonotopic specializations of the SGNs.

In mammals, auditory information is processed by the hair cells (HCs) located in the cochlea and then rapidly transmitted to the CNS via a specialized cluster of bipolar afferent connections known as the spiral ganglion neurons (SGNs). Although many anatomical aspects of SGNs are well described, the molecular and cellular mechanisms underlying their genesis, how they are precisely arranged along the cochlear duct, and the guidance mechanisms that promote the innervation of their hair cell targets are only now being understood. Building upon foundational studies of neurogenesis and neurotrophins, we review here new concepts and technologies that are helping to enrich our understanding of the development of the nervous system within the inner ear.