The location and orientation of mitotic spindle fibers in the developing cerebellar cortex was examined in rats. In both the transverse and sagittal planes, the mitotic spindle apparatus of cells in the superficial region (EGL-S) was generally oriented perpendicular to the pial surface, whereas that of cells in the deeper aspects (EGL-D) was oriented parallel. The mitotic frequency of cells in the EGL-S was 2- to 6-fold greater than was that in the EGL-D. The mitotic spindle arrangement of dividing cells located adjacent to Purkinje neurons, and presumed to be Bergmann glia, was always perpendicular to the pial surface. Autoradiographic studies revealed DNA synthesis in both the EGL-S and EGL-D. Mitotic cells do not migrate from the EGL-S to the EGL-D, as evidenced by disruption of the cytoskeletal matrix with vinblastine sulfate. Our results suggest that the orientation of the spindle apparatus may play an important role in the emergence of the precise geometry characteristic of the adult cerebellum, and could signify that the phenotypic fate of neural cell precursors is determined prior to the onset of overt differentiation.
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