Plurisegmental vestibulocerebellar projections and other hindbrain cerebellar afferents in midterm chick embryos:...

The vestibular neuronal groups that project to the cerebellum were mapped in midterm chick embryos (10-11 days in "ovo") through "in vitro " retrograde tracing experiments. Massive unilateral deposits of biotin-dextranamine were placed at the basis of the cerebellum to label the cerebellar peduncles. Separate rostral and caudal vestibulo-cerebellar groups were identified, with predominance of contralateral neurons. We tentatively identified the rhombomeric location of both groups, as well as their topography within the conventional cytoarchitectonically-defined vestibular nuclei, by comparison with previously established segmental fate maps. The rostral group extended from rhombomeres 1-4 (r1-r4) and was restricted mainly to the superior vestibular nucleus. The caudal group stretched from r6 to pseudorhombomere "r8" and was related to the descending and medial vestibular nuclei. The less abundant ipsilateral vestibulocerebellar neurons had a similar topography. The crossing axons of the rostral vestibulocerebellar neurons formed a distinct rostral vestibulocerebellar decussation, restricted to the floorplate of rhombomere 2. The axons of the caudal vestibulocerebellar population mostly decussated associated to the deep cochlear commissure. The present results extend the "segmental hodological mosaic" of defined projection-neuron groups identified within the avian vestibular nuclear complex: The vestibulocerebellar projecting neurons as a type appear iterated from r1 to r4 and from r6 to pseudorhombomere "r8," albeit showing in their arrangement peculiarities related to single segmental domains, particularly rostrally. In contrast, the vestibulospinal groups are located more restrictedly in r4-r6, while the vestibulo-ocular projecting neurons extend from r1 to "r7." Only r4 and r6 contain elements of all three hodological types. The organization of the three vestibular projection populations studied to date seems comparable in chicken and frogs and may be a conserved feature in vertebrates.