DEVELOPMENT OF BRANCHIOMOTOR NEURONS IN ZEBRAFISH

The mechanisms underlying neuronal specification and axonogenesis in t he vertebrate hindbrain are poorly understood. To address these questi ons, we have employed anatomical methods and mutational analysis to ch aracterize the branchiomotor neurons in the zebrafish embryo. The zebr afish branchiomotor system is similar to those in the chick and mouse, except for the location of the nVII and nIX branchiomotor neurons. De velopmental analyses of genes expressed by branchiomotor neurons sugge st that the different location of the nVII neurons in the zebrafish ma y result from cell migration. To gain insight into the mechanisms unde rlying the organization and axonogenesis of these neurons, we examined the development of the branchiomotor pathways in neuronal mutants. Th e valentino (b337) mutation blocks the formation of rhombomeres 5 and 6, and severely affects the development of the nVII and nIX motor nucl ei, The cyclops (b16) mutation deletes ventral midline cells in the ne ural tube, and leads to a severe disruption of most branchiomotor nucl ei and axon pathways. These results demonstrate that rhombomere-specif ic cues and ventral midline cells play important roles in the developm ent of the branchiomotor pathways.