Expression and Functional Analysis of Glutamate Receptors in Glial Cells
The brain consists of a complex network in which neurones and glial cells are structurally and functionally interwoven. Astrocytes, the most numerous member of the glial family, were originally considered, along with the whole glial population, to be only of structural importance (Virchow, 1846). For example, during development the radial glia, the precursors of astrocytes, serve as a scaffold at which neurones migrate to form the layered structure of different brain regions such as the cortex, the hippocampus or the cerebellum. During the last two decades, considerable knowledge about astrocytes has accumulated regarding their physiological function. One exciting function is their contribution to the regulation of the extracellular space and, thereby, also of brain excitability (Walz, 1989). Qualities such as their capacity for uptake and metabolism of transmitters, buffering capacity of ions and ability to convey external signals via surface receptors to biological responses within the cells indicate an intimate crosstalk between glial cells and neurones. The other major glial population in the brain are the oligodendrocytes. As small cells with few processes they form the myelin sheath, a highly lipid enriched stack of cell membranes enwrapping 50 to 300αm long axonal segments to enhance the conduction of electrical signals and to inhibit electrical crosstalk between individual axons. Oligodendrocytes are capable of myelinating up to 50 axonal segments simultaneously. Mature oligodendrocytes develop from progenitors originating from the subventricular zone as the germinative layer (Miller, 1996). In vertebrates, progenitors start to migrate to their final destination regions, the presumptive white matter, during the first postnatal week.
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