Cell adhesion 531064 212920141 2008-05-16T21:37:19Z 71.219.131.161 /* Cytoskeletal interactions */ {{Cleanup|date=May 2007}} [[Image:Cell_Adhesion.png|thumb|300px|Schematic of cell adhesion]] '''Cellular adhesion''' is the binding of a [[cell (biology)|cell]] to another cell or to a [[surface]] or [[extracellular matrix|matrix]]. Cellular adhesion is regulated by specific [[cell adhesion molecule]]s that interact with molecules on the opposing cell or surface. Such adhesion molecules are also termed "receptors" and the molecules they recognize are termed "[[ligand (biochemistry)|ligands]]" (and sometimes "counterreceptors"). Since [[Cell (biology)|cells]] are not often found in isolation, rather they tend to stick to other cells or non-cellular components of their environment, a fundamental question is: what makes cells sticky? Cell adhesion generally involves [[protein molecule]]s at the surface of cells, so the study of cell adhesion involves [[cell adhesion molecule|cell adhesion proteins]] and the molecules that they bind to. ==Cytoskeletal interactions== For a cell adhesion protein like the one shown in the diagram, the intracellular domain binds to protein components of the cell's [[cytoskeleton]]. This allows for very tight adhesion. Without attachment to the cytoskeleton, a cell adhesion protein that is tightly bound to a ligand would be in danger of being [[hydrolysis|hydrolyzed]] by extracellular [[hydrolase|hydrolytic enzymes]]. This will tear away the layer unneeded for molecular distibution of arion cells. Then from out of the adhesion protein from the fragile cell membrane. Often the connection between the cell adhesion proteins and the cytoskeleton is not as direct as shown in the diagram. For example, [[cadherin]] cell adhesion proteins are typically coupled to the cytoskeleton by way of special linking proteins called "[[catenins]]". ==Adhesion in Prokaryotes== [[Prokaryotes]] have adhesion molecules usually termed "[[adhesins]]". Adhesins may occur on [[pili]] (fimbriae), [[flagellae]], or the cell surface. Adhesion of bacteria is the first step in colonization and regulates [[tropism]] (tissue- or cell-specific interactions). ==Adhesion in Viruses== [[Viruses]] also have adhesion molecules required for viral binding to host cells. For example, [[influenza]] virus has a [[hemagglutinin]] on its surface that is required for recognition of the [[sugar]] [[sialic acid]] on host cell surface molecules. [[HIV]] has an adhesion molecule termed [[gp120]] that binds to its ligand [[CD4]], which is expressed on [[lymphocytes]]. ==Adhesion in Eukaryotes== Eukaryotic [[protozoans]] also express multiple adhesion molecules. An example of a pathogenic protozoan is the [[malaria|malarial]] parasite (''Plasmodium falciparum''), which uses one adhesion molecule called the circumsporozoite protein to bind to liver cells, and another adhesion molecule called the merozoite surface protein to bind [[red blood cells]]. In [[human]] cells, which have many different types of adhesion molecules, the major classes are named [[integrins]], [[Immunoglobulin superfamily|Ig superfamily members]], [[cadherins]], and [[selectins]]. Each of these adhesion molecules has a different function and recognizes different ligands. Defects in cell adhesion are usually attributable to defects in expression of adhesion molecules. ===Human Genetic Diseases=== There are the human [[Genetics|genetic]] diseases caused by inability to express a specific adhesion molecule. An example is [[leukocyte adhesion deficiency]]-I (LAD-I), where patients do not express the β2-integrin subunit precursor. This integrin is required for [[leukocytes]] to adhere to the blood vessel wall during [[inflammation]] in order to fight infection. The leukocytes from LAD-I patients fail to adhere and patients exhibit serious episodes of [[infection]] that can be life threatening. ==Differential Adhesion Hypothesis== The '''differential adhesion hypothesis''' (sometimes called the "thermodynamic hypothesis") is a theory of [[cell adhesion]] advanced by [[Malcolm Steinberg]] in 1964 to explain the mechanism by which [[heterotypic cells]] in [[mixed aggregates]] sort out into [[holotype|isotypic territories]].<ref>[http://www.biology-online.org/dictionary/Differential_adhesion Differential adhesion - definition from Biology-Online.org<!-- Bot generated title -->]</ref> The DAH postulates that tissues are viscoelastic liquids, and as such possess measurable tissue surface tensions. These surface tensions have been determined for a variety of tissues, including embryonic tissues and cell lines. The surface tensions correspond to the mutual sorting behavior: the tissue type with the higher surface tension will occupy an internal position relative to a tissue with a lower surface tension (if these tissues can interact with each other through their adhesion machinery). Quantitative differences in homo and heterotypic adhesion are supposed to be sufficient to account for the phenomenon without the need to postulate cell type specific adhesion systems: fairly generally accepted, although some [[biological tissue|tissue]] specific cell adhesion [[molecules]] are now known to exist. ==References== {{Unreferenced|date=February 2007}} <references /> ==External links== * [http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowSection&rid=cooper.section.2058 The Cell] by G. Cooper (online textbook) *[http://www.ncbi.nlm.nih.gov:80/books/bv.fcgi?call=bv.View..ShowSection&rid=mcb.chapter.6480 Molecular Cell Biology] by Lodish et al (online textbook) *[http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=adhesion+AND+cell%5Bbook%5D+AND+cell%5Bbook%5D+AND+cell%5Bbook%5D+AND+8192%5Buid%5D&rid=cell.section.5121 Molecular Biology of the Cell] by Alberts et al (online textbook) no anda *[http://www.biochemweb.org/adhesion_ecm.shtml Cell Adhesion and Extracellular Matrix - The Virtual Library of Biochemistry and Cell Biology] [[Category:Cell biology]] [[ar:التصاق الخلايا]] [[de:Zelladhäsion]] [[es:Adherencia celular]] [[ja:細胞接着]]