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-[[Image:FluorescentCells.jpg|thumb|right|300px|The [[eukaryotic]] cytoskeleton. [[Actin filaments]] are shown in red, [[microtubules]] in green, and the [[cell nucleus|nuclei]] are in blue.]]The '''cytoskeleton''' (also CSK) is a cellular [[scaffolding]] or [[skeleton]] contained within a [[Cell (biology)|cell]]'s [[cytoplasm]]. The cytoskeleton is present in all cells; it was once thought to be unique to [[eukaryote]]s, but recent research has identified the [[prokaryotic cytoskeleton]]. It forms structures such as [[flagellum|flagella]], [[cilium|cilia]] and [[lamellipodia]] and plays important roles in both intracellular transport (the movement of [[vesicle (biology)|vesicle]]s and organelles, for example) and [[Cell division|cellular division]]. In 1903 [[Nikolai Koltsov|Nikolai K Koltsov]] proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton. The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929 <ref>{{cite journal |author=Peters RA |title= The Harben Lectures, 1929. Reprinted in: Peters, R. A. (1963) Biochemical lesions and lethal synthesis,p. 216. Pergamon Press, Oxford. }}</ref> while the term (''cytosquelette'', in French) was first introduced by French embryologist [[Paul Wintrebert]] in 1931.<ref>{{cite journal |author=Frixione E |title=Recurring views on the structure and function of the cytoskeleton: a 300-year epic |journal=Cell motility and the cytoskeleton |volume=46 |issue=2 |pages=73–94 |year=2000 |month=June |pmid=10891854 |doi=10.1002/1097-0169(200006)46:2<73::AID-CM1>3.0.CO;2-0}}</ref>
-==The Eukaryotic Cytoskeleton==
-[[Image:MEF microfilaments.jpg|thumb|right|200px|Actin cytoskeleton of [[mus musculus|mouse]] [[embryo]] [[fibroblast]]s, stained with [[phalloidin]].]]
-[[Eukaryotic]] cells contain three main kinds of cytoskeletal filaments, which are [[microfilaments]], [[intermediate filaments]], and [[microtubules]]. The cytoskeleton provides the cell with structure and shape, and by [[excluded volume|excluding]] [[macromolecules]] from some of the [[cytosol]] it adds to the level of [[macromolecular crowding]] in this compartment.<ref>{{cite journal |author=Minton AP |title=Confinement as a determinant of macromolecular structure and reactivity |journal=Biophys. J. |volume=63 |issue=4 |pages=1090–100 |year=1992 |month=October |pmid=1420928 |pmc=1262248 |url=http://www.biophysj.org/cgi/reprint/63/4/1090 |doi=10.1016/S0006-3495(92)81663-6 |bibcode=1992BpJ....63.1090M}}</ref> Cytoskeletal elements interact extensively and intimately with cellular membranes.<ref>{{cite journal |author=Doherty GJ and McMahon HT |title=Mediation, Modulation and Consequences of Membrane-Cytoskeleton Interactions |journal=Annual Review of Biophysics |volume=37 |pages=65–95 |year=2008 |pmid=18573073 |url=http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.biophys.37.032807.125912 |doi=10.1146/annurev.biophys.37.032807.125912}}</ref> A number of small molecule [[cytoskeletal drugs]] have been discovered that interact with actin and microtubules. These compounds have proven useful in studying the cytoskeleton and several have clinical applications.
-===Microfilaments (actin filaments)===
-{{main|microfilament}}
-These are the thinnest filaments of the cytoskeleton. They are composed of linear polymers of [[actin]] subunits, and generate force by elongation at one end of the filament coupled with shrinkage at the other, causing net movement of the intervening strand. They also act as tracks for the movement of [[myosin]] molecules that attach to the microfilament and "walk" along them. Actin structures are controlled by the [[Rho family]] of small GTP-binding proteins such as Rho itself for contractile acto-myosin filaments ("stress fibers"), Rac for lamellipodia and Cdc42 for filopodia.
-===Intermediate filaments===
-[[Image:KeratinF9.png|thumb|right|200px|Microscopy of keratin filaments inside cells.]]
-{{main|intermediate filament}}
-These filaments, averaging 10 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton. Like actin filaments, they function in the maintenance of cell-shape by bearing tension ([[microtubules]], by contrast, resist compression. It may be useful to think of micro- and intermediate filaments as cables, and of microtubules as cellular support beams<!-- microfilaments, not microtubules= "cellular support beams?" -->). Intermediate filaments organize the internal tridimensional structure of the cell, anchoring [[organelle]]s and serving as structural components of the [[nuclear lamina]] and [[sarcomere]]s. They also participate in some cell-cell and cell-matrix junctions.
-Different intermediate filaments are:
-* made of [[vimentin]]s, being the common structural support of many cells.
-* made of [[keratin]], found in [[skin]] cells, [[hair]] and [[nail (anatomy)|nails]].
-* [[neurofilament]]s of neural cells.
-* made of [[lamin]], giving structural support to the nuclear envelope.
-===Microtubules===
-[[Image:Btub.jpg|thumb|right|200px|Microtubules in a gel fixated cell.]]
-{{main|microtubule}}
-Microtubules are hollow cylinders about 23&nbsp;nm in diameter (lumen = approximately 15&nbsp;nm in diameter), most commonly comprising 13 protofilaments that, in turn, are polymers of alpha and beta [[tubulin]]. They have a very dynamic behaviour, binding [[Guanosine triphosphate|GTP]] for polymerization. They are commonly organized by the [[centrosome]].
-In nine triplet sets (star-shaped), they form the [[centrioles]], and in nine doublets oriented about two additional microtubules (wheel-shaped) they form cilia and flagella. The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by the protein [[dynein]]. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton.
-They play key roles in:
-* intracellular transport (associated with [[dynein]]s and [[kinesin]]s, they transport [[organelles]] like [[mitochondria]] or [[vesicle (biology)|vesicle]]s).
-* the [[axoneme]] of [[cilium|cilia]] and [[flagellum|flagella]].
-* the [[mitotic spindle]].
-* synthesis of the cell wall in plants.
-===Comparison===
-{|class=wikitable
-|-
-! Cytoskeleton type<ref name=boron25Unless>Unless else specified in boxes, then ref is:{{cite book |author=Walter F., PhD. Boron |title=Medical Physiology: A Cellular And Molecular Approaoch |publisher=Elsevier/Saunders |location= |year=2003 |pages=1300 |isbn=1-4160-2328-3 |oclc= |doi=}} Page 25</ref> !! Diameter ([[nanometre|nm]])<ref>{{cite journal |author=Fuchs E, Cleveland DW |title=A structural scaffolding of intermediate filaments in health and disease |journal=Science |volume=279 |issue=5350 |pages=514–9 |year=1998 |month=January |pmid=9438837 |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=9438837 |doi=10.1126/science.279.5350.514|bibcode = 1998Sci...279..514F }}</ref>  !! Structure !! Subunit examples<ref name=boron25Unless/>
-|-
-! [[Microfilaments]]
-|&nbsp; &nbsp;&nbsp;6 ||&nbsp;[[double helix]] || &nbsp;[[actin]]
-|-
-! [[Intermediate filament]]s
-|&nbsp; &nbsp;10 ||&nbsp;two anti-parallel [[helix|helices]]/dimers, forming tetramers ||
-*[[vimentin]] ([[mesenchyme]])
-*[[glial fibrillary acidic protein]] ([[glial cell]]s)
-*[[neurofilament]] proteins (neuronal processes)
-*[[keratin]]s ([[epithelial cell]]s)
-*[[nuclear lamins]]
-|-
-![[Microtubule]]s
-|&nbsp; &nbsp;23 ||&nbsp;[[protofilament]]s, in turn consisting of tubulin subunits in complex with [[Stathmin_protein_domain|stathmin]]<ref name="pmid17029844">{{cite journal| author=Steinmetz MO| title=Structure and thermodynamics of the tubulin-stathmin interaction. | journal=J Struct Biol | year= 2007 | volume= 158 | issue= 2 | pages= 137-47 | pmid=17029844 | doi=10.1016/j.jsb.2006.07.018 | pmc= | url=http://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=17029844}}</ref> || &nbsp;[[α-bubulin|α-]] and [[β-tubulin]]
-|}
-==The prokaryotic cytoskeleton==
-{{main|Prokaryotic cytoskeleton}}
-The cytoskeleton was previously thought to be a feature only of [[eukaryote|eukaryotic]] cells, but [[homology (biology)|homologues]] to all the major proteins of the eukaryotic cytoskeleton have recently been found in [[prokaryotes]].<ref name=Shih>{{cite journal |author=Shih YL, Rothfield L |title=The bacterial cytoskeleton |journal=Microbiol. Mol. Biol. Rev. |volume=70 |issue=3 |pages=729–54 |year=2006 |pmid=16959967 |doi=10.1128/MMBR.00017-06 |pmc=1594594}}</ref> Although the evolutionary relationships are so distant that they are not obvious from protein sequence comparisons alone, the similarity of their three-dimensional [[protein structure|structures]] and similar functions in maintaining cell shape and polarity provides strong evidence that the eukaryotic and prokaryotic cytoskeletons are truly homologous.<ref>{{cite journal |author=Michie KA, Löwe J |title=Dynamic filaments of the bacterial cytoskeleton |journal=Annu. Rev. Biochem. |volume=75 |issue= |pages=467–92 |year=2006 |pmid=16756499 |url=http://www2.mrc-lmb.cam.ac.uk/SS/Lowe_J/group/PDF/annrev2006.pdf |doi=10.1146/annurev.biochem.75.103004.142452 |format=}} {{dead link|date=January 2010}}</ref> However, some structures in the bacterial cytoskeleton may have yet to be identified.<ref>{{cite journal |author=Briegel A, Dias DP, Li Z, Jensen RB, Frangakis AS, Jensen GJ |title=Multiple large filament bundles observed in Caulobacter crescentus by electron cryotomography |journal=Mol. Microbiol. |volume=62 |issue=1 |pages=5–14 |year=2006 |month=October |pmid=16987173 |doi=10.1111/j.1365-2958.2006.05355.x}}</ref>
-===FtsZ===
-[[FtsZ]] was the first protein of the prokaryotic cytoskeleton to be identified. Like tubulin, FtsZ forms filaments in the presence of [[Guanosine triphosphate|GTP]], but these filaments do not group into tubules. During [[cell division]], FtsZ is the first protein to move to the division site, and is essential for recruiting other proteins that synthesize the new [[cell wall]] between the dividing cells.
-===MreB and ParM===
-Prokaryotic actin-like proteins, such as [[MreB]], are involved in the maintenance of cell shape. All non-spherical bacteria have [[gene]]s encoding actin-like proteins, and these proteins form a helical network beneath the cell membrane that guides the proteins involved in cell wall [[biosynthesis]].
-Some [[plasmid]]s encode a partitioning system that involves an actin-like protein [[ParM]]. Filaments of ParM exhibit [[dynamic instability]], and may partition plasmid DNA into the dividing daughter cells by a mechanism [[Analogy (biology)|analogous]] to that used by microtubules during eukaryotic [[mitosis]].
-===Crescentin===
-The bacterium ''[[Caulobacter crescentus]]'' contains a 3rd protein, [[crescentin]], that is related to the intermediate filaments of eukaryotic cells. Crescentin is also involved in maintaining cell shape, such as helical and [[vibrio]]id forms of bacteria, but the mechanism by which it does this is currently unclear.<ref>{{cite journal |author=Ausmees N, Kuhn JR, Jacobs-Wagner C |title=The bacterial cytoskeleton: an intermediate filament-like function in cell shape |journal=Cell |volume=115 |issue=6 |pages=705–13 |year=2003 |month=December |pmid=14675535 |doi=10.1016/S0092-8674(03)00935-8}}</ref>
-==History==
-===Microtrabeculae===
-A fourth eukaryotic cytoskeletal element, ''microtrabeculae'', was proposed by Keith Porter based on images obtained from high-voltage [[electron microscopy]] of whole cells in the 1970s.<ref>{{cite journal |author=Wolosewick JJ, Porter KR |title=Microtrabecular lattice of the cytoplasmic ground substance. Artifact or reality |journal=J. Cell Biol. |volume=82 |issue=1 |pages=114–39 |year=1979 |month=July |pmid=479294 |pmc=2110423 |url=http://www.jcb.org/cgi/pmidlookup?view=long&pmid=479294 |doi=10.1083/jcb.82.1.114}}</ref> The images showed short, filamentous structures of unknown molecular composition associated with known cytoplasmic structures. Porter proposed that this microtrabecular structure represented a novel filamentous network distinct from microtubules, filamentous actin, or intermediate filaments. It is now generally accepted that microtrabeculae are nothing more than an artifact of certain types of fixation treatment, although we have yet to fully understand the complexity of the cell's cytoskeleton.<ref>{{cite journal | author=Heuser J | title = Whatever happened to the 'microtrabecular concept'? | journal = Biol Cell | year = 2002 | volume = 94| issue = 9| pages = 561–96  | doi=10.1016/S0248-4900(02)00013-8 | pmid=12732437}}</ref>
-==References==
-{{reflist|2}}
-==External links==
-{{Commons category|Cytoskeleton}}
-* [http://www.biochemweb.org/cytoskeleton.shtml Cytoskeleton, Cell Motility and Motors - The Virtual Library of Biochemistry and Cell Biology]
-* [http://www.cytoskeletons.com Cytoskeleton database, clinical trials, recent literature, lab registry ...]
-* [http://aimediaserver.com/studiodaily/videoplayer/?src=harvard/harvard.swf&width=640&height=520 Animation of leukocyte adhesion] (Animation with some images of actin and microtubule assembly and dynamics.)
-* http://cellix.imba.oeaw.ac.at/ Cytoskeleton and cell motility including videos
-* [http://www.tandfonline.com/doi/full/10.1080/00018732.2013.771509 Open access review article] on the emergent complexity of the cytoskeleton (appeared in Advances in Physics, 2013)
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-{{Cytoskeletal Proteins}}
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-[[Category:Cytoskeleton|*]]