The corticospinal tract attains a normal configuration in the absence of myelin: observations in jimpy mutant mice

The corticospinal projection was examined in dysmyelinated, jimpy mice and in unaffected littermates following cortical injections of either wheat germ agglutinin conjugated to horseradish peroxidase or biocytin. Corticospinal axons in both phenotypes traverse the medulla within a well-defined pyramidal tract, decussate within several fascicles at the spinomedullary junction, and extend down the spinal cord in a compact bundle in the ventral-most part of the dorsal funiculus. Very few labeled fibers are seen separated from the main bundle. This normal configuration of the corticospinal tract is attained despite the virtual absence of CNS myelin in jimpy mice. It seems unlikely then that the myelin normally present in fiber bundles adjacent to this relatively late emerging projection can significantly influence pathway selection during its development.