Migrating cells from the external germinal layer of the newborn rat cerebellum were studied by light and electron microscopy. Each migrating cell possessed a single, broad, leading process oriented perpendicular to the pial surface. These cell processes were sometimes associated with profiles of other granule cells, but were not necessarily associated with the electron-lucent Bergmann fibers present at these early developmental stages. Miograting granule cells could be observed circumventing both blood vessels and the perikarya/processes of other cells present in the developing molecular layer. Thus, during the early stages of cerebellar ontogeny, when the migration pathway through the molecular layer is sparsely populated with cells and processes, the vertical process of a granule cell may seek actively a path of least resistance, utilizing 'contacts' with surrounding objects for avoidance, rather than as guideposts imperative for directing migration. Cellular associations observed at this stage of cerebellar development may thus be more fortuitous than requisite.
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