Fig. 11.

A, Shift of the geography of developing cerebellar cortex over time. At P10, the granule cell precursors proliferate within ∼19–21 hr of the cell cycle within levels I and II of the EGL. Approximately 21 hr after their final cell division, tangentially migrating granule cells change the orientation of their somata from horizontal to vertical and start to leave the EGL. As a result, their horizontal processes become parallel fibers, and the EGL–ML border moves up toward the pial surface (as seen at P11). Therefore, the granule cell, which initially migrates tangentially at level III of the EGL at P10, will be located at the level V after 24 hr (at P11), without undergoing downward movement. B, Schematic drawing illustrating the hypothesis that granule cells may be specified to settle in a particular A-P compartment and have to travel tangentially over relatively long distances in the middle and the bottom of the EGL to reach a target compartment. Granule cells that underwent final cell division in the same area at the top level of the EGL may settle in different compartments in the IGL. For example, green granule cells may be instructed to enter a green compartment, whereas red granule cells may be instructed to enter ared compartment. Furthermore, postmitotic granule cells located in the left or right side of the hemisphere tend to migrate toward the midline region, whereas the cells located in the midline region do not exhibit a preference in the movement direction between the left and right side.