Diversity of contralateral commissural projections in the embryonic rodent spinal cord.

In vertebrate embryos, the axons of spinal commissural neurons grow toward and across the floor plate, a specialized structure located at the ventral midline. Although the initial segment of this trajectory has been intensively studied, relatively little is known about commissural axon pathfinding on the contralateral side of the floor plate in higher vertebrates. We recently demonstrated that many embryonic mouse and chick spinal commissural axons follow a complex trajectory once they cross the ventral midline. Here we use focal applications of 1,1'-dioctadecyl-3,3,3',3' tetramethylindocarbocyanine perchlorate (DiI) to identify four different contralateral commissural trajectories, two of which have not previously been described in the embryonic rodent spinal cord. Intermediate longitudinal commissural (ILC) axons travel away from the floor plate along an arcuate trajectory into intermediate regions of the spinal cord. In contrast, medial longitudinal commissural (MLC) axons grow alongside the floor plate, projecting primarily in the rostral direction. Bifurcating longitudinal commissural (BLC) axons branch into rostrally and caudally directed projections. Forked transverse commissural (FTC) axons either execute two orthogonal turns before crossing the floor plate or extend directly across the floor plate. We also show a variation in the relative frequencies of individual contralateral commissural projections along the dorsoventral and anteroposterior axes of the spinal cord. In addition, using a novel culture system, we demonstrate that commissural axons elaborate ILC-, MLC-, BLC-, and FTC-like trajectories in vitro. These results provide a basis for examining the mechanisms that regulate commissural axon pathfinding on the contralateral side of the floor plate in the embryonic rodent spinal cord.