Migration of neuroblasts along preexisting axonal tracts during prenatal cerebellar development.

During prenatal development of the cerebellum in rats, a secondary stem cell layer, the external granular, or germinal, layer (EGL) develops at the pial surface of the cerebellar anlage. The EGL cells arise in a region at the caudolateral margin of the fourth ventricle, and the cells migrate over the surface of the developing cerebellum. A key question is how this migration is guided. We have investigated the possible role of the extracellular matrix protein, fibronectin, and find none present in the EGL during the migration. Instead, we find that the EGL neuroblasts migrate in close contact with axons that are present prior to the onset of migration. It appears that these axons serve as the substrate for EGL neuroblast migration and that axonal guidance of cell migration may be a third general mechanism to be added to the previously studied guidance by glial processes and extracellular matrix.