A natural defect in rat cerebellum postnatal development has been found in the fissura prima, consisting in various complex configurations of the cerebellar layers. We investigated the genesis of fissure malformations through immunoreactions for PCNA, GFAP, GABAA alpha6, and calbindin to label proliferating cells of the external granular layer (egl), radial glial fibers, mature granule cells, and Purkinje cells, respectively. Results on critical stages of rat postnatal development provided interesting evidences on how the malformation develops in fissures prima and secunda. Early (postnatal day 10) at the site of malformation, the Bergmann radial glia was often retracted and showed distortions and irregular running. The interruption of GFAP-positive radial glial fibers could fit in with the presence of clusters of PCNA-unlabeled cells in the sites of fusion of the egl; the clusters of cells are granule cells since their soma is labeled by GABAA alpha6. Moreover, an altered migration of granule cell precursors to the internal granular layer was evident which, in turn, affected Purkinje cell differentiation and the growth of their dendrites. In summary, the changed relationship among glial fiber morphology, granule cell migration, and Purkinje cell differentiation suggests how the Bergmann glial fibers have a basic role in the foliation process, being the driving physical force in directing migration of the granule cells at the base of fissure.
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