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NEUROGLIA

Glial cells which support neurones in the CNS and PNS are derived from three lineages, namely, neuroectoderm, neural crest and angioblastic mesenchyme. In the CNS, cells of the proliferating ventricular zone give rise to astrocytes and oligodendrocytes. After the proliferative phase the cells remaining at the ventricular surface differentiate into ependymal cells, which are specialized in many regions of the ventricular system as circumventricular organs. In the PNS, neural crest cells produce Schwann cells and astrocyte-like support cells in the enteric nervous system. Angioblastic mesenchyme gives rise to a variety of blood cell types including circulating monocytes which infiltrate the brain as microglial cells later in development.

The ventricular zone lining the early central canal of the spinal cord and the cavities of the brain gives rise to neurones and glial cells (Figs 24.4 and 24.5). One specialized form of glial cell is the radial glial cell, whose radial processes extend both outwards to form the outer limiting membrane deep to the pia mater, and inwards, to form the inner limiting membrane around the central cavity. The geometry of these cells may provide contact guidance paths for cell migrations, both neuronic and glioblastic. A secondary radial glial scaffold is formed in the late developing cerebellum and dentate gyrus and serves to translocate neurones, formed in secondary germinal centres, to their definitive adult locations. Radial glia eventually lose their connections with both inner and outer limiting membranes, except those persisting in the retina as Müller cells, in the cerebellum as Bergmann glia and in the hypothalamus as tanycytes. They can differentiate into neurones as well as astrocytes. They may partially clothe the somata of neighbouring developing neurones (between presumptive synaptic contacts), or similarly enwrap the intersynaptic surfaces of their neurites. Glial processes may expand around intraneural capillaries as perivascular end-feet. Other glioblasts retain an attachment (or form new expansions) to the pia mater, the innermost stratum of the meninges, as pial end-feet. Glioblasts also line the central canal and cavities of the brain as generalized or specialized ependymal cells, but lose their peripheral attachments. In some situations, as in the anterior median fissure of the spinal cord, ependymal cells retain their attachments to both the inner and outer limiting membranes. Thus, glia function as perineuronal satellites, and provide cellular channels interconnecting extracerebral and intraventricular cerebrospinal fluid, the cerebral vascular bed, the intercellular crevices of the neuropil and the cytoplasm of all neural cell varieties.

Microglia appear in the CNS after it has been penetrated by blood vessels and invade it in large numbers from certain restricted regions, whence they spread in what have picturesquely been called ‘fountains of microglia’, to extend deeply among the nervous elements.

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