Development of branchiomotor neurons in zebrafish

The mechanisms underlying neuronal specification and axonogenesis in the vertebrate hindbrain are poorly under-stood. To address these questions, we have employed anatomical methods and mutational analysis to character-ize the branchiomotor neurons in the zebrafish embryo. The zebrafish branchiomotor system is similar to those in the chick and mouse, except for the location of the nVII and nIX branchiomotor neurons. Developmental analyses of genes expressed by branchiomotor neurons suggest that the different location of the nVII neurons in the zebrafish may result from cell migration. To gain insight into the mechanisms underlying the organization and axonogenesis of these neurons, we examined the development of the branchiomotor pathways in neuronal mutants. The valentinob337 mutation blocks the formation of rhombomeres 5 and 6, and severely affects the development of the nVII and nIX motor nuclei. The cyclopsb16 mutation deletes ventral midline cells in the neural tube, and leads to a severe disruption of most bran-chiomotor nuclei and axon pathways. These results demon-strate that rhombomere-specific cues and ventral midline cells play important roles in the development of the bran-chiomotor pathways.