Development of the avian inner ear and acoustic-vestibular ganglion and their connection to the primary auditory brainstem nuclei

The experiments described in this dissertation investigated the development of the inner ear and acoustic-vestibular ganglion (AVG) and the formation of connections from the peripheral auditory system to the primary auditory brainstem nuclei in chicken embryos. The first study examined the development of the otic epithelium and the differentiation of the auditory and vestibular sensory receptors using a neuron-specific marker, class III beta-tubulin. I found that class III beta-tubulin is upregulated in the sensory receptors and down regulated in non-sensory cell types (e.g., support cells and non-sensory epithelia) as they differentiate. The reduction in class III beta-tubulin expression in the tegmentum vasculosum, around stage 21 (E3.5), indicates that non-sensory and sensory structures in the cochlear duct are being determined during the same time period. The second study used antibodies to class III beta-tubulin to identify and examine the migration of AVG precursors from the otic epithelium and the formation of the AVG. This study showed that AVG precursors migrate from the same or adjacent regions of the otic epithelium from which the vestibular sensory epithelia develop and that migration continues to occurs as the sensory epithelia are determined and differentiate. Migration does not appear to occur from the cochlear duct, however, suggesting that the neurons of the cochlear ganglion derive from vestibular structures. In addition, the cochlear ganglion appears to form as an outgrowth of the posterior ampullary nerve. The third study in this dissertation examined the arrival and topography of the central processes of cochlear ganglion cells as they penetrate the primary auditory brainstem nucleus, nucleus magnocellularis (NM). Cochlear nerve fibers penetrate NM over approximately a four-day period from stage 29 (E6) to 36 (E10), prior to the formation of functional synapses. When cochlear nerve fibers arrive in NM, they are already organized in a topographic map related to the position of their cell bodies along the basilar papilla, foreshadowing the tonotopic map observed later in development. These results support the hypothesis that neural activity is not required for the formation of topographic maps.