Confocal laser microscopy of DiI-labeled slices of postnatal rat cerebellum (postnatal Day 4-10; P4-10) was compared to infrared microscopy and the rapid Golgi method (P0-14) to investigate postnatal migration of granule neurons. Vertical migration of the granule neurons occurred already at birth (P0). Surprisingly, mossy fibers often reached the external granule cell layer and were in close contact with the external granule cells. These mossy fibers may play a role in initiating granule cell migration. At this age, cell bodies of the immature neurons were attached to the external basal lamina by a process and extended down toward the presumptive internal granule cell layer. At P14, some granule cells remained attached to the surface, although their cell bodies exhibited the typical morphology of mature granule neurons and were located deep in the internal granule cell layer. These cells extended their endfeet-like processes all the way to the surface of the brain. These results indicate that the vertical pathways of granule cell migration form early and persist throughout the period of granule cell migration. Confocal infrared microscopy of DiI-labeled sections and the rapid Golgi method also allowed demonstration of tangentially migrating neurons that made one or more turns on the way to the internal granule cell layer. The rapid Golgi method confirmed that many Bergmann glial processes end at the level of the tangentially migrating granule cells whereas others project to the surface. These observations show that migratory granule cells take several different routes to their final destination, which cannot be explained by so-called radial glial guidance. The only mode of migration in evidence is consistent with process elongation and translocation of the nucleus within the preformed processes.
Copyright 2003 Wiley-Liss, Inc.