Morphology of migrating trigeminal motor neuroblasts as revealed by horseradish peroxidase retrograde labeling techniques

The trigeminal motor sensory roots were severed in chick embryos on days 2.5 4.5 of incubation and horseradish peroxidase applied to the wound. This procedure retrogradely labels developing trigeminal motor neuroblasts whose axons are at the level of the incision. In day 2.5 embryos, migrating and lateral trigeminal motor neuroblasts were labeled only when the incision was 8 μm from the metencephalon. Migrating cells did not have somatic processes whereas cells of the lateral nucleus had one dendritc-like process extending dorsally. No cells of the medial column, a cluster of premigratory trigeminal motor neuroblasts, were labeled at this age.In the 3-, 3.5-, 4- and 4.5-day embryos, medial column cells, migratory cells and lateral nucleus cells were retrogradely labeled by this procedure. At all these ages, medial column cells tend to have few somatic filopodia migratory cells tend to have increased filopodia as they proceed laterally, and lateral nucleus cells arc characterized either by a single, long dendrite-like process dorsally directed, or a radiation of short, stubby processes. Axons of medial column and migratory cells take a sinuous course across the metencephalon and frequently exhibit small branches and localized swellings. Axons of lateral nucleus cells rarely have branches within the brain stem and usually enter the motor root at an acute angle from their origin at the soma.The central processes of the trigeminal ganglion cells are also labeled with this procedure. In all embryos these fibers were confined to the trigeminal spinal tract at the level of the trigeminal motor nucleus. Caudally, small fibers were observed to exit this tract in the presumed region of the developing trigeminal spinal nucleus.This study demonstrates that axonal outgrowth into the periphery precedes somatic migration and translocation in the trigeminal motor nucleus During their migration, trigeminal motor neuroblasts appear to be in proximity to axons of adjacent migrating cells. Considerable differentiation occurs during this process.