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==Astrocytes==

==Astrocytes==

Astrocytes, or astroglia, are star-shaped glial cells within the brain which have many processes that envelope synapses made between neurons. Astrocytes have several functions including the biochemical support of the endothelial cells in forming the blood-brain barrier, provision of nutrients to nervous tissues and to form scar tissue during repair of the brain. The fluorescent image of astrocytes on the right is possible because astrocytes express glial fibrillary acidic protein (GFAP) which facilitates their identification.

Astrocytes, or astroglia, are star-shaped glial cells within the brain which have many processes that envelope synapses made between neurons. Astrocytes have several functions including the biochemical support of the endothelial cells in forming the blood-brain barrier, provision of nutrients to nervous tissues and to form scar tissue during repair of the brain. The fluorescent image of astrocytes on the right is possible because astrocytes express glial fibrillary acidic protein (GFAP) which facilitates their identification. The following paragraphs provide a more detailed overview of the many functions of the astrocyte within the brain.

[[Image:astrocyte.jpg|thumb|right|150px|Astrocyte (immunofluorescence staining). Image courtesy of Alliance Européenne Dana pour le Cerveau (EDAB)]]

[[Image:astrocyte.jpg|thumb|right|150px|Astrocyte (immunofluorescence staining). Image courtesy of Alliance Européenne Dana pour le Cerveau (EDAB)]]

Another key role of astrocytes is within the brain where they play an important structural role as well as performing the roles described above. Astrocytes are involved in the physical structuring of the brain and the interplay between astrocytes and mesenchymal cells is important in sculpting [[Forebrain_-_Anatomy_%26_Physiology#Cerebral_Cortex|sulci]] in development. Astrocytes also delineate the  CNS from the PNS by creating the CNS-PNS interface. The cells cover the surface of the brain and where this occurs the astrocytes' surface is coated by a basal lamina.

Another key role of astrocytes within the brain where they play an important structural role as well as performing the roles described above. Astrocytes are involved in the physical structuring of the brain and the interplay between astrocytes and mesenchymal cells is important in sculpting [[Forebrain_-_Anatomy_%26_Physiology#Cerebral_Cortex|sulci]] in development. Astrocytes also delineate the  CNS from the PNS by creating the CNS-PNS interface. The cells cover the surface of the brain and where this occurs the astrocytes' surface is coated by a basal lamina.

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Where astocytes are involved in repair within the brain they take over from fibroblasts to play an important role in abscess formation, as it is difficult to induce fibrosis within the CNS. It is due to this role that astrocytes are the only differentiated CNS cell that retains the ability to divide.

Where astocytes are involved in repair within the brain they take over from fibroblasts to play an important role in abscess formation, as it is difficult to induce fibrosis within the CNS. It is due to this role that astrocytes are the only differentiated CNS cell that retains the ability to divide.

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Some classifications of neurotransmitters, namely [[Neurotransmitters_-_Anatomy_%26_Physiology#Small-molecule_Neurotransmitters|small-molecule neurotransmitters]] are reabsorbed and subsequently recycled. Astrocytes are represent an element of the system that enables this neurotransmitter re-uptake. Astrocytes are able to effectively isolate synapses an control their potassium levels as well as expressing several types of transporter for the numerous types of neurotransmitters, including glutamate, ATP and GABA. Glutamate is particularly well taken up, and converted to glutamine using glutamine synthase. The role played in controlling potassium ion concentrations within the extracellular spaces involves astrocytes releasing K⁺  when neurons are active, increasing the local extracellular K⁺ concentration. Astrocytes have a high density of K⁺ channels, allowing them to rapidly clear the excess accumulation in the extracellular space. If this function fails extracellular K⁺ concentration rises leading to neuronal depolarisation, and may be a cause of epilepsy.

** Astrocytes:

**# Isolate synapses

**# Control potassium levels

**#* Astrocytes express transporters for several neurotransmitters, including glutamate, ATP and GABA.

**#** Glutamate is particularly well taken up, and converted to glutamine using glutamine synthase.

* '''Regulation of ion concentration in the extracellular space'''

** Astrocytes release K⁺  when neurons are active, increasing the local extracellular K⁺ concentration.  

** Astrocytes have a high density of K⁺ channels, allowing them to rapidly clear the excess accumulation in the extracellular space.

*** If this function fails extracellular K⁺ concentration rises. This leads neuronal depolarisation, and may be a cause of epilepsy.

* '''Vasomodulation'''

* '''Vasomodulation'''

** Astrocytes may serve as intermediaries in neuronal regulation of blood flow.

** Astrocytes may serve as intermediaries in neuronal regulation of blood flow.