Glial cell 518989 224152999 2008-07-07T15:43:31Z 218.19.163.124 /* References */ {{refimprove|date=February 2008}} [[Image:Neuroglia.png|thumb|250px|Neuroglia of the brain shown by [[Golgi's method]].]] [[Image:Gfapastr5.jpg|thumb|250px|Astrocytes can be visualized in culture because, unlike other mature glia, they express [[glial fibrillary acidic protein]].]] '''Glial cells''', commonly called '''neuroglia''' or simply '''glia''' (Greek for "glue"), are non-[[neuron]]al [[cell (biology)|cells]] that provide support and nutrition, maintain [[homeostasis]], form [[myelin]], and participate in signal transmission in the [[nervous system]]. In the [[human brain]], glia are estimated to outnumber neurons by about 10 to 1.<ref name="sfn">[http://www.sfn.org/index.cfm?pagename=brainBriefings_astrocytes sfn.org Society for Neuroscience, 2000]</ref> Glial cells provide support and protection for [[neuron]]s, the other main type of cell in the nervous system. They are thus known as the "glue" of the nervous system. The four main functions of glial cells are to surround neurons and hold them in place, to supply [[nutrients]] and [[oxygen]] to neurons, to insulate one neuron from another, and to destroy [[pathogens]] and remove dead neurons. == Function of the glial cell == Some glia function primarily as the physical support for neurons. Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and provide nutrition to nerve cells. Glia have important developmental roles, guiding migration of neurons in early development, and producing molecules that modify the growth of [[axon]]s and [[dendrite]]s. Recent findings in the [[hippocampus]] and [[cerebellum]] have indicated that glia are also active participants in [[synapse|synaptic transmission]], regulating clearance of neurotransmitter from the synaptic cleft, releasing factors such as [[Adenosine triphosphate|ATP]] which modulate presynaptic function, and even releasing neurotransmitters themselves. Unlike the neuron, which is generally considered permanently [[mitosis|post-mitotic]]<ref>Nature Reviews Neuroscience 8, 368-378 (May 2007) | {{doi|10.1038/nrn2124}}</ref>, glia are capable of [[mitosis]]. Traditionally glia had been thought to lack certain features of neurons. For example, glia were not believed to have [[synapse|chemical synapses]] or to release [[neurotransmitter]]s. They were considered to be the passive bystanders of neural transmission. However, recent studies have disproved this. For example, [[astrocyte]]s are crucial in clearance of [[neurotransmitter]] from within the [[synaptic cleft]], which provides distinction between arrival of action potentials and prevents toxic build up of certain neurotransmitters such as [[glutamate]] ([[excitotoxicity]]). Furthermore, at least [[in vitro]], [[astrocytes]] can release neurotransmitter glutamate in response to certain stimulation. Another unique type of glia, the [[oligodendrocyte precursor cell]]s or OPCs, have very well defined and functional synapses from at least two major groups of neurons. The only notable differences between neurons and glia, by modern scrutiny, are the ability to generate [[action potential]]s and the polarity of neurons, namely the [[axon]]s and [[dendrite]]s which glia lack. It is inaccurate to consider glia as 'glue' in the nervous system as the name implies; rather, it is more of a partner to neurons. They are also crucial in the development of the nervous system and in processes such as [[synaptic plasticity]] and [[synaptogenesis]]. Glia have a role in the regulation of repair of neurons after injury. In the [[Central nervous system|CNS]] glia suppress repair. [[Astrocytes]] enlarge and proliferate to form a scar and produce inhibitory molecules that inhibit regrowth of a damaged or severed axon. In the [[Peripheral nervous system|PNS]] [[Schwann cell]]s promote repair. After axon injury Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This difference between [[Peripheral nervous system|PNS]] and [[Central nervous system|CNS]] raises hopes for the regeneration of nervous tissue in the [[Central nervous system|CNS]], for example a spinal cord injury or severance. ==Types of glial cells== ===Microglia=== {{details|Microglia}} [[Microglia]] are specialized [[macrophage]]s capable of [[phagocytosis]] that protect neurons of the [[central nervous system]]. They are derived from hemopoietic precursors rather than [[germ layer|ectodermal]] tissue; they are commonly categorized as such because of their supportive role to neurons. These cells comprise approximately 15% of the total cells of the central nervous system. They are found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, macroglia and blood vessels). ===Macroglia=== {| class="wikitable" | '''Location''' || '''Name''' || '''Description''' |- | [[central nervous system|CNS]] || [[Astrocyte]]s || The most abundant type of macroglial cell, ''astrocytes'' (also called ''astroglia'') have numerous projections that anchor neurons to their blood supply. They regulate the external [[chemical]] environment of neurons by removing excess [[ion]]s, notably [[potassium]], and recycling [[neurotransmitter]]s released during [[synaptic transmission]]. The current theory suggests that astrocytes may be the predominant "building blocks" of the [[blood-brain barrier]]. Astrocytes may regulate vasoconstriction and vasodilation by producing substances such as [[arachidonic acid]], whose metabolites are vasoactive. Astrocytes signal each other using [[calcium]]. The [[gap junction]]s (also known as electrical synapses) between astrocytes allow the messenger molecule [[Inositol triphosphate|IP3]] to diffuse from one astrocyte to another. IP3 activates calcium channels on cellular organelles, releasing calcium into the cytoplasm. This calcium may stimulate the production of more IP3. The net effect is a calcium wave that propagates from cell to cell. Extracellular release of [[Adenosine triphosphate|ATP]], and consequent activation of [[purinergic receptors]] on other astrocytes, may also mediate calcium waves in some cases. There are generally two types of astrocytes, protoplasmic and fibrous, similar in function but distinct in morphology and distribution. Protoplasmic astrocytes have short, thick, highly branched processes and are typically found in gray matter. Fibrous astrocytes have long, thin, less branched processes and are more commonly found in white matter. |- | CNS || [[Oligodendrocyte]]s || ''Oligodendrocytes'' are cells that coat axons in the [[central nervous system]] (CNS) with their cell membrane forming a specialized membrane differenciation called [[myelin]], producing the so-called [[myelin sheath]]. The myelin sheath provides [[Electrical insulation|insulation]] to the axon that allows [[electricity|electrical]] signals to propagate more efficiently. |- | CNS || [[Ependymal cells]] || ''Ependymal cells'', also named ''ependymocytes'', line the cavities of the CNS and make up the walls of the ventricles. These cells create and secrete [[cerebrospinal fluid]](CSF) and beat their [[cilia]] to help circulate that CSF. |- | CNS || [[Radial glia]] || ''Radial glia cells'' arise from neuroepithelial cells after the onset of neurogenesis. Their differentiation abilities are more restricted than those of neuroepithelial cells. In the developing nervous system, radial glia function both as neuronal progenitors and as a scaffold upon which newborn neurons migrate. In the mature brain, the cerebellum and retina retain characteristic radial glial cells. In the cerebellum, these are [[Bergmann glia]], which regulate [[synaptic plasticity]]. In the retina, the radial [[Müller cell]] is the principal glial cell, and participates in a bidirectional communication with neurons. |- | [[peripheral nervous system|PNS]] || [[Schwann cell]]s || Similar in function to oligodendrocytes, ''Schwann cells'' provide myelination to axons in the [[peripheral nervous system]] (PNS). They also have [[phagocytosis|phagocytotic]] activity and clear cellular debris that allows for regrowth of PNS neurons. |- | PNS || [[Satellite cells (glial)|Satellite cells]] || ''Satellite cells'' are small cells that line the exterior surface of PNS neurons and help regulate the external chemical environment. |} ==Capacity to divide== Glia retain the ability to undergo cell division in adulthood, while most neurons cannot. The view is based on the general deficiency of the mature nervous system in replacing neurons after an injury, such as a [[stroke]] or trauma, while very often there is a profound proliferation of glia, or [[gliosis]] near or at the site of damage. However, detailed studies found no evidence that 'mature' glia, such as astrocytes or [[oligodendrocyte]]s, retain the ability of mitosis. Only the resident [[oligodendrocyte precursor cell]]s seem to keep this ability after the nervous system matures. On the other hand, there are a few regions in the mature nervous system, such as the [[dentate gyrus]] of the [[hippocampus]] and the [[subventricular zone]], where generation of new neurons can be observed. ==Embryonic development== Most glia are derived from [[germ layer|ectodermal]] tissue of the developing [[embryo]], particularly the [[neural tube]] and [[neural crest|crest]]. The exception is [[microglia]], which are derived from hemopoietic stem cells. In the adult, microglia are largely a self-renewing population and are distinct from macrophages and monocytes which infiltrate the injured and diseased CNS. In the central nervous system, glia develop from the ventricular zone of the neural tube. These glia include the oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia derive from the neural crest. These PNS glia include Schwann cells in nerves and satellite cells in ganglia. ==History== Glia were discovered in [[1856]] by the pathologist [[Rudolf Virchow]] in his search for a 'connective tissue' in the brain. The human brain contains about ten times more glial cells than neurons.<ref name="sfn"> </ref> Following its discovery in the late 19th century, this fact underwent significant media distortion, emerging as the famous myth claiming that "we are using only 10% of our brain". The role of glial cells as managers of communications in the synapse gap, thus modifying learning pace, has been discovered only very recently ([[2004]]). ==Additional images== <gallery> Image:Oligodendrocyte.png|Oligodendrocyte Image:Gray667.png|Section of central canal of medulla spinalis, showing ependymal and neuroglial cells. Image:Gray706.png|Transverse section of a cerebellar folium. </gallery> ==References== {{reflist}} Oligodendrocyte Section of central canal of medulla spinalis, showing ependymal and neuroglial cells. Transverse section of a cerebellar folium. ==External links== * [http://pfrieger.gmxhome.de/work/publications/pfrieger_2002.pdf Role of glia in synapse development] * {{cite journal | author = Overstreet L | title = Quantal transmission: not just for neurons | journal = Trends Neurosci | volume = 28 | issue = 2 | pages = 59–62 | year = 2005 | pmid = 15667925 | doi = 10.1016/j.tins.2004.11.010}} [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T0V-4F05G3W-1&_coverDate=02%2F01%2F2005&_alid=242540802&_rdoc=1&_fmt=&_orig=search&_qd=1&_cdi=4872&_sort=d&view=c&_acct=C000006078&_version=1&_urlVersion=0&_userid=75682&md5=a6462b73c196ee912e6ea3407462f0b3 article] * {{cite journal | author = Peters A | title = A fourth type of neuroglial cell in the adult central nervous system | journal = J Neurocytol | volume = 33 | issue = 3 | pages = 345–57 | year = 2004 | pmid = 15475689 | doi = 10.1023/B:NEUR.0000044195.64009.27}} * {{cite journal | author = Volterra A, Steinhäuser C | title = Glial modulation of synaptic transmission in the hippocampus | journal = Glia | volume = 47 | issue = 3 | pages = 249–57 | year = 2004 | pmid = 15252814 | doi = 10.1002/glia.20080}} * {{cite journal | author = Huang Y, Bergles D | title = Glutamate transporters bring competition to the synapse | journal = Curr Opin Neurobiol | volume = 14 | issue = 3 | pages = 346–52 | year = 2004 | pmid = 15194115 | doi = 10.1016/j.conb.2004.05.007}} * [http://www.livescience.com/humanbiology/060816_neural_progenitors.html New Source of Replacement Brain Cells Found] - glial cells can transform into other cell types and reproduce indefinitely — tricks once thought exclusive to stem cells. * [http://www.anioman.com/Profile.php?viewedArtistID=1793201250&gallery=&page=1&back=1 Artist ADSkyler](uses concepts of neuroscience and found inspiration from Glia) {{Nervous tissue}} [[Category:Glial cells| ]] [[Category:Neurobiology]] [[de:Gliazelle]] [[es:Célula glial]] [[eo:Glia ĉelo]] [[eu:Glia]] [[fr:Cellule gliale]] [[io:Glia celulo]] [[it:Cellula della glia]] [[he:תאי גליה]] [[hr:Glija]] [[nl:Gliacel]] [[no:Gliaceller]] [[ja:グリア細胞]] [[pl:Komórki glejowe]] [[pt:Célula da glia]] [[ru:Нейроглия]] [[sk:Ependýmová bunka]] [[fi:Gliasolu]] [[sv:Gliacell]] [[th:เซลล์เกลีย]] [[uk:Нейроглія]] [[zh:神經膠質細胞]] [[sr:глијалне ћелије]]