Development of Auditory Nerves and Postsynaptic Glutamate Receptors (GlutR2/R3) of Mouse Cochlea

  Objective  To study the development of auditory nerves and postsynaptic glutamate receptors (GlutR2/R3) between inner hair cells and auditory nerve fibers of mouse cochlea.

  Methods  The development and distribution of auditory nerves and postsynaptic glutamate receptors (GlutR2/R3) between inner hair cells and auditory nerve fibers of mouse cochlea were observed by immunohistochemical techniques in the embryos and newborns of mice with C57BL/6J and 129S6 mixed genetic background.

  Results  At 9.5-day embryo (E9.5) of mouse inner ear, high signals of NeuroD were detected on otic epithelia, vestibulocochlear ganglia, and facial ganglia. Innervation connections between vestibulocochlear ganglia and otic epithelia and between vestibulocochlear ganglia and neural tubes were observed at stage as early as E12.5. At E13.5, highly expressed signals of L1 and Tuj1 were detected in spiral ganglia and nerve fibers in cochlea, which suggested that efferent fibers further developed after the division of inner ear into cochlear and vestibular parts. At E16.5, nerve fibers connecting the spiral ganglia and hair cells were clearly visible. L1 and Tuj1 were highly expressed and rich clusters of nerve fibers were visible around the inner hair cells. However, GlutR2/R3 signals were not detected until E18.5 between inner hair cells and auditory nerve fibers and became stronger at P1. Postsynaptic GlutR2/R3 signals were focused around the bottom of inner hair cells.

  Conclusions  The auditory nerves appear to establish fiber connections with the sensory epithelium and central nervous system as early as the period of embryo development. The establishment of nerve fiber domination is earlier than the maturation of inner hair cells. The synapse connection between the inner hair cells and auditory nerves gradually develops at the later stages of gestation and after birth.