Cercbellar and cerebral cortex from human fetuses of 12ndash;20 weeks' gestation removed for the purpose of terminating pregnancy were studied in vitro and in tissue blocks by light and electron microscopy. In cultures, the morphologic features characterising the first three weeks of growth were examined. By use of multiple criteria, immature neurons and astrocytes and their processes could be identified with reasonable certainty. In cerebellar cultures, migrating immature neurons were similar to cells of the external granular layer observed in tissue blocks; whereas migrating young neurons in cerebral cultures were similar to cells of the cortical plate. Immature astrocytes developed abundant 100 Aring; filaments in vitro. Similar cells with abundant filaments were not observed in tissue blocks of either cercbellar or cerebral cortex.This study demonstrated not only the capacity of immature neurons of both human cerebellar and cerebral cortex to migrate in vitro, but also a consistent pattern of outgrowth of cells from explants during the first three weeks of growth in culture. Immature astrocytes first emerged from an explant followed by neuritic processes and subsequently by cell bodies of immature neurons from which the neurites arose. In addition to making contacts with other young neurons, intimate relationships were observed by light microscopy i between endings of neurites and the cytoplasm of astrocytes, and ii between cell bodies of migrating immature neurons and migrating immature astrocytes. The close relationship between migrating neurons and astrocytes was a particularly striking characteristic of the early outgrowth.
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