Development of second-order vestibular projections in the chicken embryo

The results reviewed here demonstrate that vestibulospinal and vestibulo-ocular interneurons in the chicken embryo are organized into coherent clusters, each of which occupies a unique spatial domain and has a characteristic axon trajectory. Moreover, the vestibulo-ocular clusters have specific termination patterns in the extraocular motoneuron pools and may to some extent have specific functions. The axon trajectories and termination patterns are established accurately during early development. This suggests the presence of selective mechanisms that ensure the generation of an appropriately specific connectivity pattern. The correlation of cluster domains with unique fields of regulatory gene expression at early stages suggests a mechanistic link between the position of a vestibular interneuron and the differentiation of its axon trajectory, termination pattern, and neurotransmitter phenotype.