Neuronal migration was investigated in rodent cerebellum in vitro and in vivo. Time-lapse video recording showed that cultured neurons migrated on laminin by first extending neurites that formed contacts with other neurons. This was followed by movement of the cell nucleus inside the preformed process. No guidance cues other than laminin were required. When the rodent premigratory (E18-P0) cerebellum was examined by immunocytochemistry, the radial glial cells were found to have extracellular punctate deposits of laminin along their fibers. Such punctate deposits of laminin were more numerous in the premigratory cerebellum than during the peak of neuronal migration (e.g., at 7-10 days postnatally). At the same time (E18-P0) L1 antigen- and neurofilament-positive, presumably granule cell processes extend radially from the external granule cell layer (EGL). These results imply that neuronal migration on laminin in vitro involves neuronal contact formation followed by nuclear movement inside a preformed process. That this mode of neuronal migration may occur in vivo is indicated by the fact that L1 antigen- and neurofilament-positive "pioneer neurites" colocalize with the punctate deposits of laminin deposited along the radial glial processes in the premigratory EGL. Taken together these results imply that the established glial dependency of the granule cell migration may in fact be dependency of the granule cells and their pioneer neurites on the punctate deposits of laminin produced and laid down by the glial cells.
Animals Animals, Newborn