This series of experiments examined the arrival and organization of cochlear nerve axons in the primary auditory brainstem nucleus, nucleus magnocellularis (NM), of the chick. DiI and DiD were injected into the cochlear nerve, cochlear ganglion, and basilar papilla (i.e., avian cochlea) in fixed tissue and labeled axons were studied in NM and its vicinity. Cochlear nerve axons first penetrate NM between stages 29 (E6) and 36 (E10). Axons penetrate NM in a middle-to-posterior-to-anterior developmental sequence; the anterior, high-frequency region of NM receives axons last. When cochlear nerve axons arrive in the NM, they are already organized in a topographic map related to the position of their cell bodies along the basilar papilla, foreshadowing the tonotopic mapping observed between NM and the basilar papilla later in development. Evidence of a topographic map was also observed in the other primary auditory brainstem nucleus, nucleus angularis. These results indicate that topographic mapping of position (and ultimately characteristic frequency) between the basilar papilla and NM is established as cochlear nerve axons arrive in the NM prior to the onset of synaptic activity. .

A previous study showed that class III ␤-tubulin, a widely used neuron-specific marker, is expressed in mature and regenerating hair cells but not the support cells of the avian inner ear. We investigated the expression of this marker in the developing avian inner ear. We found that class III ␤-tubulin is not neuron-specific in the avian embryo, but appears to accumulate in neuronal cell types, including hair cells, about the time of their differentiation. In the developing inner ear, some degree of class III ␤-tubulin immunoreactivity is found in all regions of the otic epithelium from its formation as the otic placode (stage 10 [embryonic day, E1.5]) until about stage 21 (E3.5), when the prospective tegmentum vasculosum begins to lose its staining. By stage 35 (E8-9), most of the nonsensory epithelia have lost their class III ␤-tubulin staining, leaving distinct regions of staining between the morphological compartments of the inner ear. Concurrent with the loss of staining from nonsensory regions, the hair cells of the sensory epithelia accumulate class III ␤-tubulin, whereas the supporting cells decrease their staining. We also observed a similar pattern of development in another hair cell organ, the paratympanic organ. Double labeling with the 275 kD hair cell antigen (HCA) indicated that the majority of hair cells identifiable with class III ␤-tubulin are HCA-positive. Additionally, presumptive hair cells were identified which were not within defined sensory epithelia. Our findings show that class III ␤-tubulin can be used as an early marker for hair cell differentiation in all hair cell sensory epithelia in the chicken.

A previous study showed that class III beta-tubulin, a widely used neuron-specific marker, is expressed in mature and regenerating hair cells but not the support cells of the avian inner ear. We investigated the expression of this marker in the developing avian inner ear. We found that class III beta-tubulin is not neuron-specific in the avian embryo, but appears to accumulate in neuronal cell types, including hair cells, about the time of their differentiation. In the developing inner ear, some degree of class III beta-tubulin immunoreactivity is found in all regions of the otic epithelium from its formation as the otic placode (stage 10 [embryonic day, E1.5]) until about stage 21 (E3.5), when the prospective tegmentum vasculosum begins to lose its staining. By stage 35 (E8-9), most of the nonsensory epithelia have lost their class III beta-tubulin staining, leaving distinct regions of staining between the morphological compartments of the inner ear. Concurrent with the loss of staining from nonsensory regions, the hair cells of the sensory epithelia accumulate class III beta-tubulin, whereas the supporting cells decrease their staining. We also observed a similar pattern of development in another hair cell organ, the paratympanic organ. Double labeling with the 275 kD hair cell antigen (HCA) indicated that the majority of hair cells identifiable with class III beta-tubulin are HCA-positive. Additionally, presumptive hair cells were identified which were not within defined sensory epithelia. Our findings show that class III beta-tubulin can be used as an early marker for hair cell differentiation in all hair cell sensory epithelia in the chicken.

No abstract