Presence of displaced neural elements within rat cerebellar fissures.

Clusters of neural tissue have been observed within the cerebellar fissures of normal and experimentally treated albino rats and hooded rats, between the ages of 5 and 90 days. The observations of these intra-fissural clusters were made in conjunction with several series of experiments dealing with the effects of various treatments on cerebellar morphogenesis. More important is the fact that similar displaced clusters are present in normal, untreated animals. Clusters can show a considerable variation in size, ranging from a few closely packed cells, to clusters with many cells completely occupying the area between the two folial surfaces, The clusters consist of immature or mature granule cells, depending on the age. With increasing age a well-developed neuropil is seen with bundles of parallel fibers coursing among the granule cells, along with occasional Bergmann glial fibers, capillaries and pericytes. Synaptic contacts, possibly granule cells, are present within the cluster. The continued differentiation of the cells in the clusters is exemplified by the observation of a mossy fiber terminal at the base of a cluster. Connecting each cluster with the nearby parenchyma is a stalk consisting of granule cells, Bergmann glial fibers, and neurites. The basal lamina of the folial surface is interrrupted at this point. The presence of these clusters implies a failure of normal morphogenetic and migrational control mechanisms. Importantly, the cells of the clusters continue to differentiate and to make synaptic connections, despite their abnormal location.