Biocytin was injected into the oculomotor, trochlear, or abducens nucleus on one side using isolated chicken brainstem preparations or brain slices to identify the medial vestibular nucleus (MVN) neurons projecting to these targets. Oculomotor nucleus injections produced retrogradely labeled neurons in the contralateral ventro- lateral MVN (MVNVL), with few labeled neurons in the ipsilateral MVNVL and rarely in the dorsomedial MVN on either side. Labeled MVNVL neurons were identified as stellate (95%) and elongate (5%) cells. Trochlear nu- cleus injections produced a similar pattern of MVN neu- ron labeling. Abducens nucleus injections resulted in retrogradely labeled stellate (87%) and elongate (13%) neurons in the MVNVL, which had smaller cell bodies than those projecting to the oculomotor nucleus. Ante- roposteriorly, labeled MVNVL neurons were coextensive with the tangential nucleus, with neurons projecting to the oculomotor nucleus distributed lateral to and inter- mixed with the more medially situated neurons projec- ting to the abducens nucleus. The fundamental pattern of vestibuloocular projecting neurons was similar at both embryonic ages studied, E16 and E13. In contrast to the case in mammals, where most vestibuloocular projection neurons reside within the MVN, most retro- gradely labeled neurons in these chicken preparations were found within the ventrolateral vestibular, descend- ing vestibular, and tangential nuclei. The morphological identification and mapping of vestibuloocular projection neurons in the chicken MVN described here represents the first step in a systematic evaluation of the relation- ship between avian vestibuloocular neuron structure and function. V C 2009 Wiley-Liss, Inc.
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