Brain Research, 213 (1981) 30%318 307 © Elsevier/North-Holland Biomedical Press ULTRASTRUCTURE OF MIGRATING SPINAL MOTONEURONS IN ANURAN LARVAE I-WU CHU-WANG, RONALD W. OPPENHEIM and PAUL B. FAREL* Neuroembryology Laboratory, North Carolina Department of Mental Health, Research Division, Raleigh, N.C. 2761 l and (P.B.F.) Department of Physiology, University of North Carolina, School of Medicine, Chapel Hill, N.C. 27514 (U.S.A.) (Accepted October 16th, 1980) Key words': spinal motoneuron - - migration - - ultrastructure - - anuran larva - - lateral motor column SUMMARY Motoneurons in the course of migration to the lateral motor column (LMC) were identified by horseradish peroxidase (HRP) applied to lumbar ventral roots in anuran larvae (Rana catesbeiana). The ultrastructural characteristics of these migrating moto- neurons were compared with motoneurons in the LMC that had completed their migration. Both migrating and LMC motoneurons tended to be bipolar with one process extending toward the ependyma and the other out towards the ventral root. Typically, centrally and peripherally directed processes in both migrating and LMC motoneurons contained many microtubules, mitochondria and rosette-like clusters of ribosomes, as did the soma. Four of the five types of synapses found on adult frog motoneurons were also found on migrating motoneurons in tadpoles. Radial glia, whose cell bodies are located in the ependyma, have processes extending to the spinal cord periphery. In less than 10 ~ of the preparations, radial glia were labeled with the HRP reaction product. These labeled glia were further distinguished from migrating motoneurons at the ultra- structural level by the presence of abundant filaments. INTRODUCTION Neuronal development is typically considered to be a sequential process in which cell birth and migration of the immature nerve cell are followed by differentiation and axon elongation. In contrast to this generalization, Levi-Montalcini 11 clearly demon- strated axonal outgrowth from sympathetic preganglionic neurons before they assumed * To whom correspondence should be addressed.