Lamellipodia
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<!-- Commented out because image was deleted: [[Image:Lamellipodium.jpg|right|frame|A [[Green fluorescent protein|GFP]] label of the actin cytoskeleton in a melanoma cell, showing a ruffled-edge lamellipodium, from [http://cellix.imolbio.oeaw.ac.at/Videotour/video_tour_5.html Cell Biology at IMB Salzburg]. {{deletable image-caption|1=Saturday, 26 January 2008}}]] -->
The '''lamellipodium''' (pl. '''Lamellipodia''') is a [[cytoskeleton|cytoskeletal]] [[actin]] projection on the mobile edge of the [[cell (biology)|cell]]. It contains a two-dimensional actin mesh; the whole structure pulls the cell across a substrate (Alberts, et al, 2002). Within the lamellipodia are ribs of actin called [[microspikes]], which, when they spread beyond the lamellipodium frontier, are called [[filopodia]] (Small, et all, 2002). The lamellipodium is born of actin [[nucleation]] in the [[plasma membrane]] of the cell (Alberts, et al, 2002) and is the primary area of actin incorporation or [[microfilament]] formation of the cell.
Lamellipodia are found primarily in very mobile cells, in particular the [[keratinocytes]] of [[fish]] and [[frog]]s, which are involved in the quick [[wound healing|repair of wounds]], crawling at a speeds of 10-20μm/minute over [[epithelium|epithelial]] surfaces. A lamellipodium separated from the main part of a cell by scratching across the cell with a [[pipette]] tip can continue to crawl freely about on its own.
Lamellipodia are a characteristic feature at the front, leading edge, of motile cells. They are believed to be the actual motor which pulls the cell forward during the process of [[cell migration]]. The tip of the lamellipodium is the site where [[exocytosis]] occurs in migrating mammalian cells as part of their [[clathrin]]-mediated [[endocytic cycle]]. This, together with actin-polymerisation there, helps extend the lamella forward and thus advance the cell's front. It thus acts as a steering device for cells in the process of [[chemotaxis]]. It is also the site from which particles or aggregates attached to the cell surface migrate in a process known as [[cap formation]].
Structurally, the plus ends of the microfilaments (localized actin [[monomer]]s in an [[Adenosine triphosphate|ATP]]-bound form) face the "seeking" edge of the cell, while the minus ends (localized actin monomers in an [[Adenosine diphosphate|ADP]]-bound form) face the lamella behind (Cramer, 1997). This creates [[treadmilling]] throughout the lamellipodium, which aids in the retrograde flow of particles throughout (ibid.). [[Arp2/3 complex]]es are present at microfilament-microfilament junctions in lamellipodia, and help create the actin meshwork. Arp 2/3 can only join onto previously existing microfilaments, but once bound it creates a site for the extension of ''new'' microfilaments, which creates branching (Weed, et al, 2000). Another molecule that is often found in polymerizing actin with Arp2/3 is [[cortactin]], which appears to link [[tyrosine kinase]] signalling to cytoskeletal reorganization in the lamellipodium and its associated structures (ibid.).
[[Rac(biology)|Rac]] and [[Cdc42]] are two [[Rho]]-family [[GTPase]]s which are normally [[cytosol]]ic but can also be found in the cell membrane under certain conditions (Small, et al, 2002). When Cdc42 is activated, it can interact with [[Wiskott-Aldrich syndrome]] protein ([[WASp]]) family receptors, in particular [[N-WASp]], which then activates Arp2/3. This stimulates actin branching and increases cell [[motility]] (Small, et al, 2002). Rac1 induces cortactin to localize to the cell membrane, where it simultaneously binds F-actin and Arp2/3. The result is a structural reorganization of the lamellipodium and ensuing cell motility (Weed, et al, 2000).
==References==
#Alberts, Bruce, ''et al.'' <u>Molecular Biology of the Cell</u>. Fourth Edition. Garland Science, Taylor & Francis Group. New York, 2002: pgs 908, 931, 973-975.
#Cramer, Louise P. ''Molecular Mechanism of Actin-Dependent Pretrograde Flow in Lamellipodia of Motile Cells.'' <u>Frontiers in Bioscience</u>, 2, d260-270, June 1, 1997.
#Small, Victor J., ''et al.'' ''The lamellipodium: where motility begins.'' <u>Trends in Cell Biology</u>, Vol. 12 No. 3, March 2002: pgs. 112-120.
#Weed, Scott A., ''et al.'' ''Cortactin Localization to Sites of Actin Assembly in Lamellipodia Requires Interactions with F-Actin and the Arp2/3 Complex''. <u>The Journal of Cell Biology</u>, Vol.151 No.1, October 2, 2000. pgs 29-40. [http://www.jcb.org/cgi/content/full/151/1/29 Available Online]
#[http://cellix.imolbio.oeaw.ac.at/Videotour/video_tour_5.html Lamellipodia at the Department of Cell Biology at IMB Salzburg]
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