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Astrocytes (also known collectively as astroglia) are characteristic star-shaped glial cells in the brain. They perform many functions, including biochemical support of endothelial cells which form the blood-brain barrier, the provision of nutrients to the nervous tissue, and a principal role in the repair and scarring process in the brain.
Additional recommended knowledge
Astrocytes are a sub-type of the glial cells in the brain. They are also known as astrocytic glial cells. Star-shaped, their many processes envelope synapses made by neurons. Astrocytes are classically identified histologically by their expression of glial fibrillary acidic protein (GFAP). Previously in medical science, the neuronal network was considered the only important one, and astrocytes were looked upon as gap fillers. But recently they have been reconsidered and are now thought to play a number of active roles in the brain. Although they aid in the maintenance of the blood-brain barrier, they do not actually form it.
In the 1990s, following persistent study, a small group of scientists began to uncover evidence that astrocytes signal to neurons and influence their activity. First, cell experiments in petri dishes found that following an increase of the element calcium in astrocytes, there is an increase of calcium in surrounding neurons. This implied some form of communication between the two cell types. Next, scientists found that indeed the calcium increase in astrocytes directly links to changes in neuron activity. In one study of rat cells, microelectrodes measured the electrical impulses that neurons use to signal to each other. In response to the calcium increase in astrocytes, the majority of neurons tested slowed down their signaling activity. A few increased their signaling activity.
Other research is uncovering key molecules that aid in communication. Several studies indicate that following the rise of calcium, astrocytes release the amino acid glutamate, which helps them talk to the neurons. The communication flows both ways, with neurons also being able to "talk" to the astrocytes through their own glutamate release. Signaling molecules, such as ATP and prostaglandins, also appear to promote the cell-to-cell communication, according to other new investigations.
Determining why the astrocyte chatting occurs and whether it actually affects the neurons' ability to process information, is another area of research. Early studies hint that some of the chatting may aid memory. Adding glutamate to cell samples of astrocytes prompts them to produce special molecules that nourish neurons, known as trophic factors. Other research has found that these molecules are key to memory function. In one recent study, injections of trophic factors into the brains of rats boosted the biological mechanisms known to relate to memory and improved the rats' performance in a memory task. This all may mean that glutamate release from neurons triggers astrocytes to produce trophic factors, which then help neurons process information for memory. Scientists currently are testing this theory.
Together the research is not only making researchers rethink how the brain operates, but also how to treat it when it malfunctions. For one, if the research on astrocytes' connection to memory pans out, then the cells may make good targets for treatment of memory disorders such as Alzheimer's disease. Astrocytes' relationship to glutamate also may make them good targets for clinical intervention since several brain disorders have been tied to glutamate problems. For example, some scientists believe that when the brain is infected by the human immunodeficiency virus or is deprived of oxygen from lack of blood flow due to a stroke, a release of excess glutamate causes neurons to die. Agents that target astrocytes might help limit the glutamate overflow and prevent cell death.
Furthermore, studies are underway to determine whether astroglia play an instrumental role in depression, based on the link between diabetes and depression. Altered CNS glucose metabolism is seen in both these conditions, and the astroglial cells are the only cells with insulin receptors in the brain.
Astrocytes are linked by gap junctions, creating an electrically coupled syncytium.
An increase in intracellular calcium concentration can propagate outwards through this syncytium. Mechanisms of calcium wave propagation include diffusion of IP3 through gap junctions and extracellular ATP signalling. Calcium elevations are the primary known axis of activation in astrocytes, and are necessary and sufficient for some types of astrocytic glutamate release.
There are several different ways to classify astrocytes:
by Lineage and antigenic phenotype
These have been established by classic work by Raff et al in early 1980s on Rat optic nerves.
by Anatomical Classification
by Transporter/receptor classification
Bergmann glia, a type of glia also known as radial epithelial cells (as named by Camillo Golgi), are astrocytes in the cerebellum that have their cell bodies in the Purkinje cell layer and processes that extend into the molecular layer, terminating with bulbous endfeet at the pial surface. Bergmann glia express high densities of glutamate transporters that limit diffusion of the neurotransmitter glutamate during its release from synaptic terminals. Besides their role in early development of the cerebellum, Bergmann glia are also required for the pruning or addition of synapses.
Astrocytomas are primary intracranial tumors derived from astrocytes cells of the brain.
Categories: Nervous system | Glial cells
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Astrocyte". A list of authors is available in Wikipedia.|