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Astrocyte
:''For the cell in the gastrointestinal tract, see Interstitial cell of Cajal.''
Astrocytes (Astro from Greek ''astron'' = star and ''cyte'' from Greek "kyttaron" = cell), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. The proportion of astrocytes in the brain is not well defined. Depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to 40% of all glia. They perform many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, and a role in the repair and scarring process of the brain and spinal cord following traumatic injuries.
Research since the mid-1990s has shown that astrocytes propagate intercellular Ca2+ waves over long distances in response to stimulation, and, similar to neurons, release transmitters (called gliotransmitters) in a Ca2+-dependent manner. Data suggest that astrocytes also signal to neurons through Ca2+-dependent release of glutamate. Such discoveries have made astrocytes an important area of research within the field of neuroscience.
==Structure==
Astrocytes are a sub-type of glial cells in the central nervous system. They are also known as ''astrocytic glial cells.'' Star-shaped, their many processes envelop synapses made by neurons. Astrocytes are classically identified using histological analysis; many of these cells express the intermediate filament glial fibrillary acidic protein (GFAP). Several forms of astrocytes exist in the Central Nervous System including ''fibrous'' (in white matter), ''protoplasmic'' (in grey matter), and ''radial''. The fibrous glia are usually located within white matter, have relatively few organelles, and exhibit long unbranched cellular processes. This type often has "vascular feet" that physically connect the cells to the outside of capillary walls when they are in close proximity to them. The protoplasmic glia are the most prevalent and are found in grey matter tissue, possess a larger quantity of organelles, and exhibit short and highly branched tertiary processes. The radial glia are disposed in planes perpendicular to axis of ventricles. One of their processes abut the pia mater, while the other is deeply buried in gray matter. Radial glia are mostly present during development, playing a role in neuron migration. Müller cells of retina and Bergmann glia cells of cerebellar cortex represent an exception, being present still during adulthood. When in proximity to the pia mater, all three forms of astrocytes send out processes to form the pia-glial membrane.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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