Expression of myosin VIIA in the developing chick inner ear neurons.

The auditory-vestibular ganglion (AVG) is formed by the division of otic placode-derived neuroblasts, which then differentiate into auditory and vestibular afferent neurons. The developmental mechanisms that regulate neuronal cell fate determination, axonal pathfinding and innervation of otic neurons are poorly understood. The present study characterized the expression of myosin VIIA, along with the neuronal markers, Islet1, NeuroD1 and TuJ1, in the developing avian ear, during Hamburger-Hamilton (HH) stages 16-40. At early stages, when neuroblasts are delaminating from the otic epithelium, myosin VIIA expression was not observed. Myosin VIIA was initially detected in a subset of neurons during the early phase of neuronal differentiation (HH stage 20). As the AVG segregates into the auditory and vestibular portions, myosin VIIA was restricted to a subset of vestibular neurons, but was not present in auditory neurons. Myosin VIIA expression in the vestibular ganglion was maintained through HH stage 33 and was downregulated by stage 36. Myosin VIIA was also observed in the migrating processes of vestibular afferents as they begin to innervate the otic epithelium HH stage 22/23. Notably, afferents targeting hair cells of the cristae were positive for myosin VIIA while afferents targeting the utricular and saccular maculae were negative (HH stage 26-28). Although previous studies have reported that myosin VIIA is restricted to sensory hair cells, our data shows that myosin VIIA is also expressed in neurons of the developing chick ear. Our study suggests a possible role for myosin VIIA in axonal migration/pathfinding and/or innervation of vestibular afferents. In addition, myosin VIIA could be used as an early marker for vestibular neurons during the development of the avian AVG.