Ultrastructural observations of the migration and early development of trigeminal motoneurons in chick embryos

The migration of immature chick trigeminal (V) motoneurons from a miclline medial column to a lateral nucleus was studied between days 3 and 7 of incubation. The somatic translocation of these cells is preceded by the lateral outgrowth of their axons. At days 3–4 the axons grow within extracellular spaces dorsal to pial end feet at the external limiting membrane. At later ages the axons grow deeper within the intermediate zone. Upon reaching the medial border of the V sensory tract the axons turn ventrally, exit from the neural tube, and grow toward the mandibular arch. The medial column somas rotate from their original radial orientation to lie parallel to the pial surface. They migrate laterally in close association with fiber bundles composed of axonal and trailing processes of medial column and other migrating V motoneurons. Several punctate contacts are made between the migrating cells and the axon bundles; these contacts may stabilize the somas while the growing tips of their axons elongate. During migration the cytoplasm of the V motoneuron matures, acquiring rough endoplasmic retic‐ulum, lysosomes, and multivesicular bodies. Immature synapses contact premigratory, migratory, and postmigratory cells on day 4. Their maturation on day 5 coincides with the onset of active jaw movements. Some V motoneurons secondarily migrate dorsally from the lateral nucleus while their trailing axons are in the head mesenchyme. Few punctate contacts are observed between these cells and their substrate, the longitudinal fibers of the lateral longitudinal fasciculus. Postmigratory dorsal V motoneurons are organelle poor and have few synaptic contacts by day 7. This undifferentiated state may reflect a later onset of the spontaneous movement of their target, the levator palpebrae muscle.