Morphology and development of spinal motoneurons of the zebrafish (segments, axogenesis, growth cones)

The application of horseradish peroxidase to lesions of the muscles and the central nervous system of the larval zebrafish Brachydanio rerio was used to identify several types of neurons present in the spinal cord. The spinal cord was found to contain three distinct motoneuronal types, primary and secondary motoneurons that innervate the axial muscles, and pectoral fin motoneurons that innervate the muscles of the pectoral girdle. Only three primary motoneurons are present in each hemisegment; they are individually identifiable by their position within the segment and peripheral arbor. I have directly observed the development of spinal motoneurons in embryos by means of Nomarski and epifluorescence microscopy, and have examined the timing of the final mitosis, axon initiation, and axon outgrowth for primary and secondary motoneurons. Primary motoneurons have their birthdays 5 to 6 hours earlier than secondary motoneurons, and initiate axon outgrowth approximately 5 hours earlier. The identified primary motoneuron types did not have significantly different birthdays, but there is a strong suggestion that the most caudal of the identified primary motoneurons within a segment begins axon outgrowth earlier than the other primary motoneurons in the same segment. No evidence of a rostrocaudal sequence of development could be found. Within the range of segments 5 to 15, the first primary motoneuron to have its birthday or initiate axon outgrowth could appear in any segment. I have also examined the kinetics of growth cone and soma migration in these animals. Growth cones of primary motor axons move through the periphery at a slow overall rate (approximately 7 (mu)m/hour) that is punctuated by sporadic pauses that can last for up to 2 hours. Simultaneously with axon outgrowth, the cell bodies of primary motoneurons migrate from their initial position on the ventral floor of the spinal cord to a more dorsal position at the rate of 0.7 (mu)m/hour.