The cell. 3. Cell membrane.
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Cell junctions are large multiprotein associations making strong adhesion contacts. Tight junctions. They are strong junctions between adjoining cells, as a zipper, avoiding the intercellular movement of molecules.
Adherent junctions. They are found in the lateral membranes of epithelial cells, as a belt. They are important during morphogenetic processes.
Desmosomes. They are spot-like adhesions, as rivets, and can be found in many tissues, not just epithelium.
Hemidesmosomes. They are spot-like junctions between basal membranes of epithelial cells and the basal lamina.
Focal junctions. They are small groups of adhesion molecules that establish adhesions between cells and extracellular matrix.
Cells adhere to other cells and to the extracellular matrix by means of adhesion transmembrane proteins. Integrins, cadherins, selectins and immunoglobulins stand out. Sometimes many adhesion proteins join together to form large adhesion complexes, generally known as cell junctions. These molecular scaffolds are essential to keep the integrity of many tissues, such as epithelium, muscle, and nervous tissue.
Cell junctions are classified regarding their morphology, the type of adhesion molecules they contain, the structures they adhere to, and their interactions with the cytoskeleton. Cell junctions were first observed with electron microscopes and were named according to their morphology, but it was molecular biology that showed their complex molecular organization.
Organization of epithelial tight junctions of intestine. The molecular organization is similar in the different epithelia, as well as in the cardiac muscle (modified from Niessen, 2007).
Tight junctions, also known as zonula occludens, are found in the more apical part of the the lateral membranes of epithelial cells, and in the cardiac muscle. They make so tight contacts between adjoining cells that the intercellular space is occluded. In the epithelial cells, tight junctions function as a zipper around the cell perimeter. Apart from the adhesion role, tight junctions carry out other functions. For example, in the intestine eiphelium, tight junctions avoid molecules to enter from the lumen through the intercellular spaces between enterocytes, and molecules are forced to cross through the enterocyte, a much more selective pathway. Furthermore, tight junctions maintain the epithelial cell polarity because they form a physical barrier, so that they prevent the lateral diffusion of membrane molecules. Thus, an apical plasma membrane domain is established, which is different from the latero-basal domain. This is essential for the cell since the proteins to enter molecules such as glucose must face the intestine lumen, and those proteins exporting glucose from the interior of epithelial cells toward blood vessels must be in the basolateral membrane domain.
Tight junctions are composed of occludins and some members of a family of proteins known as claudins. Claudins are transmembrane proteins in charge of making the cell-cell adhesion, and between the adhesion points there is a very narrow space (around 1 nm) that lets ions to travel extracellularly between the cells. There are twenty types of claudins that let the extracellular passages have different sizes. Cells may change the expression of claudin types, thus being able to regulate the permeability of the intercellular space. The intracellular domain of these proteins is associated with other molecules known as ZO, which in turn are connected to actin filaments and other cytosolic proteins. Depending on the adhesion state of the cell, these intracellular molecular interactions may trigger signaling pathways that affect the cell physiology.
Adherens junctions (zonula occludens) are cell junctions between epithelial cells. They are located in the apical part of the cell, just below than tight junctions. During development, adherens junctions are the first cell junctions to appear in epithelia, where they are thought to be involved in morphogenetic processes. Like tight junctions, adherens junctions form a belt-like structure all around the cell. E-Cadherins make the cell-cell contacts through their extracellular domains, whereas the intracellular domains are attached to actin filaments. This intracellular connection is facilitated by other molecules, such as β-catenin. β-catenin may be linked to cadherin-actin complexes or be free in the cytosol. Furthermore, β-catenin may enter the nucleus and change gene expresion. In this way, adhesion and cell behavior may be influenced reciprocally.
Desmosomes (macula adherens) make spot-like cell-cell adhesions, as rivets. They are very abundant in epithelial tissues, but can also be found almost in every tissue, for example in nervous tissue. Cadherins (desmoglein and desmocollin) are the adhesion proteins in desmosomes. The intracellular domain of desmosome cadherins are associated to intermediate filaments through intermediary proteins.
Hemidesmosomes and focal adhesions make adhesion contacts between cells and the extracellular matrix. Both contain integrins as the adhesion molecule. Hemidesmosomes are junctions between epitelial cells and the basal lamina. Intracellularly, the cystolic domain of these integrins are associated to intermediate filaments. Although hemidesmosomes look like one half of a desmosome, they are molecularly different. Focal adhesions are smaller in size and make junctions between cells and different types of extracellular matrix by means of integrins, which are attached intracellularly to actin filaments.
Organization of a hemisdesmosome (modified from Hahn 2001)
It is common to include gap junctions within the cell junctions group. Gap junctions are molecular structures that allow the direct communication between the cytoplasm of adjoining cells. For that, cells synthesize transmembrane proteins known as connexins. 6 connexins form a connexon, which contains a very small channel spanning the plama membrane. Connexons of contiguous cells connect their channels and make possible the diffusion of small molecules between both cytoplasms. That is why gap junctions should be regarded as involved in cell-cell communication instead of in cell-cell adhesion.
Hahn B-S, Labouesse M. Tissue integrity: Hemidesmosomes and resistance to stress.2001. Current biology 11:R858-R861.
Huber O. Structure and function of desmosomal proteins and their role in development and disease. 2003. Cell and molecular life science. 60:1872-1890.
Niessen CM. Tight junctions/adherens junctions: basic structure and function. 2007. Journal of investigative dermatology. 127:2525-2532.
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Updated: 28-04-2016. 17:40
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo