Regulation of SC1/DM-GRASP during the migration of motor neurons in the chick embryo brain stem

The hindbrain of the chick embryo contains three classes of motor neurons: somatic, visceral, and branchial motor. During development, somata of neurons in the last two classes undergo a laterally directed migration within the neuroepithelium; somata translocate towards the nerve exit points, through which motor axons are beginning to extend into the periphery. All classes of motor neuron are immunopositive for the SC1/DM-GRASP cell surface glycoprotein. We have examined the relationship between patterns of motor neuron migration, axon outgrowth, and expression of the SC1/DM-GRASP mRNA and protein, using anterograde or retrograde axonal tracing, immunohistochemistry, and in situ hybridization. We find that as motor neurons migrate laterally, SC1/DM-GRASP is down-regulated, both on neuronal somata and axonal surfaces. Within individual motor nuclei, these lateral, more mature neurons are found to possess longer axons than the young, medial cells of the population. Labelling of sensory or motor axons growing into the second branchial arch also shows that motor axons reach the muscle plate first, and that SC1/DM-GRASP is expressed on the muscle at the time growth cones arrive.