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FIG 13. Drawings illustrate normal cerebellar development. During the 5th gestational week, cellular proliferation within the alar plates in conjunction with formation of the pontine flexure forms the rhombic lips. The neuroepithelial zones, in the roof of the fourth ventricle and the rhombic lips, are the locations of the germinal matrices where the cells of the cerebellum and many brain stem nuclei will form. Between 9 and 13 postconceptional weeks, the Purkinje cells of the cerebellar cortex and the neurons of the deep cerebellar nuclei migrate radially outward from this germinal matrix. In contrast, the neurons of the granular layer of the cerebellar cortex migrate tangentially from the germinal zone of the rhombic lips, over the cerebellar surface to form a transient external (Ext.) granular layer, which acts as a secondary germinal matrix. The external granular layer forms between the 10th and 11th postconceptional weeks and persists until approximately 15 months postnatal. The cells in the external granular layer proliferate, and the granule cell neuroblasts begin to migrate inward between clusters of homophilic Purkinje cells with the presumed aid of radial glial (Bergman) fibers, forming the internal (Int.) granular layer (reprinted with permission from Lippincott Williams & Wilkins [27]).
