SGN nerve filaments develop synapses with IHCs earlier than with OHCs in C57BL/6 mouse inner ear.

OBJECTIVE To explore the connections between hair cells and spiral ganglion neurons (SGNs) during the development of the C57BL/6 mouse inner ear. MATERIALS AND METHODS The specimens of C57BL/6 mouse inner ear, from E15 (embryo day 15) to adult mouse, were collected; immunohistochemistry was employed to explore the frozen sections of specimens. RESULTS The development of cochlea starts sequentially from the basal turn to the apex turn. Morphological development of SGNs occurs mainly from E16 to P12 (postnatal day 12). Hair cells appear from E18 to P12, and inner hair cells (IHCs) develop earlier than outer hair cells (OHCs). The connections between hair cells and SGNs begin to develop during E18-P1, morphologically resemble mature synapses during P8-P12, and completely mature in adult mice. CONCLUSIONS The genesis of auditory ribbon synapse occurs from E18 to P1. Synchronized with the development of SGNs and hair cells, the functional filaments remain connected to hair cells, while the spare ones get disconnected from the surface of hair cells. Connections between SGN nerve filaments and IHCs occur earlier than those between SGN nerve filaments and OHCs.