Astrocyte

Astrocytes (also known collectively as astroglia) are characteristic star-shaped glial cells in the brain.

Description
Astrocytes are sub-type of the glial cells in the brain. They are also known as astrocytic glial cells. Star-shaped, their many arms span all around neurons. They outnumber the neurons ten to one. 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.

Functions

 * Structural. A commonly accepted function is to physically structure the brain.
 * Metabolic support. A second function is to provide neurons with nutrients such as glucose.
 * Blood-brain barrier. The astrocyte end-feet encircling endothelial cells form part of the blood-brain barrier.
 * Transmitter reuptake and release. Astrocytes express plasma membrane transporters such as glutamate transporters for several neurotransmitters, including glutamate, ATP and GABA. More recently, astrocytes were shown to release glutamate or ATP in a vesicular, Ca2+-dependent manner.
 * Regulation of ion concentration in the extracellular space. Astrocytes express potassium channels at a high density. When neurons are active, they release potassium, increasing its extracellular concentration. Because astrocytes are so permeable to potassium, they rapidly clear its excess accumulation in the extracellular space. If this function is interfered with, the extracellular concentration of potassium will rise, leading to neuronal depolarization by the Goldman equation. Abnormal accumulation of extracellular potassium is well known to result in epileptic neuronal activity.
 * Modulation of synaptic transmission. In the supraoptic nucleus of the hypothalamus, rapid changes in astrocyte morphology have been shown to affect heterosynaptic transmission between neurons.
 * Vasomodulation. Astrocytes may serve as intermediaries in neuronal regulation of blood flow.
 * Promotion of the myelinating activity of oligodendrocytes. Electrical activity in neurons causes them to release ATP, which serves as an important stimulus for myelin to form. Surprisingly, the ATP does not act directly on oligodendrocytes. Instead it causes astrocytes to secrete LIM, a regulatory protein that promotes the myelinating activity of oligodendrocytes.  This suggest that astrocytes have an executive-coordinating role in the brain.

Calcium waves
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.

Classification
There are several different ways to classify astrocytes:

by Location

 * Type I: Those astrocytes are in direct contact with blood capillaries through astrocytique pod. They are actively helping neuronal metabolism and glucose delivery.
 * Type II: Type II astrocytes surrounds neurones and synaptic gap. This coverage varies from 1 to 100%.

by Anatomical Classification

 * Protoplasmic: found in grey matter and have many branching processes whose end-feet envelop synapses
 * Fibrous: found in white matter and have long thin unbranched processes whose end-feet envelop nodes of Ranvier

by Transporter/receptor classification

 * GluT type: express glutamate transporters (EAAT1/ and EAAT2/) and respond to synaptic release of glutamate by transporter currents
 * GluR type: express glutamate receptors (mostly mGluR and AMPA type) and respond to synaptic release of glutamate by channel-mediated currents and IP3-dependent Ca2+ transients

Pathology
Astrocytomas are primary intracranial tumors derived from astrocytes cells of the brain.