Developmental Guidance of Neuronal Migration

The neuroglia are crucial to CNS histogenesis. Primitive glial cells, the radial glia, provide a structural scaffolding for migratory young neurons. Their migration involves translocation of cell bodies along a pre-existing neuronal process: outward, as for neurons leaving the ventricular zone for the cerebral cortex, or inward, as for cells of the transient external granular layer of the cerebellar cortex. These descend along inward processes past the Purkinje cell layer into the internal granular layer. In the developing cerebral hemisphere, the distances involved may be great (3000 mm in the case of the neocortex in primates). Hence, glial guidance and assistance are necessary, as follows.

Migrating young neurons shifting their somata outward in the intermediate zone of the hemisphere are apposed to radial glial fibers. At its leading and trailing processes and at its bipolar cell body, a neuron may partly encircle the vertical glial shaft (like someone climbing a rope, wrapping arms and legs around it and hugging it tightly). At such points, interstitial junctions are seen: widening of the space between, with filamentous material in it. Arrangements for descending cerebellar granule cells are similar: neuroglial contact guidance for neurons. This assistance is much enhanced by astrotactin, a glycoprotein neural receptor offering control of cell position along the radial fiber system.

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