Cell junctions are structures that bind cells together, usually just below the plasma membrane of their apical surfaces. In epithelial tissues, with their abundance of cells, there are five types of cell junctions. These include tight junctions, adherens junctions, desmosomes , gap junctions, and hemidesmosomes: 1. Tight junctions exist between adjacent cells just below the apical surfaces of their plasma membranes. They control the passage of solutes ( ions and molecules) between cells. Tight junctions of small intestine epithelium monitor the absorption of nutrient solutes. Here they ensure that nutrients solutes cannot get to the basal extracelluar fluid (ECF), or to blood, without passing through the semipermeable plasma membrane. Adherens junctions also help to hold epithelial cells together. In this type of seal, cells are held together by circular bands or patches. These seals comprise combinations of cadherin, actin, and cantenin proteins. Adherens junctions strengthen tissue sheets and help resist tearing or separation of cells. Desmosomes: This type of junction is common in the epidermis. They comprise cadherin glyco- proteins that extend into intercellular spaces and attach to transcellular intermediate filaments of keratin. Desmosomes prevent cells from separating. Gap junctions are made up of molecules of the protein connexins. Connexins do not fuse cells together, rather, they form small passages ( connexons or gaps) between them. Ions, and small molecules diffuse through gap junctions between adjacent cells. However, large protein molecules cannot pass through. Gap junctions are found in all tissues, but they are most important in avascular tissues ( (lens and cornea, where they facilitate the transport of nutrients and wastes. Hemidesmosomes , attach epithelial tissue to their basement membranes.
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Tight junctions join the membranes of adjacent cells together to form a selectively permeable seals. The structures are made of proteins such as claudins, occludins and adhesion proteins . The adhesion proteins bind to cytoskeletal protein fibers within the cytoplasm of adjacent cells.. Tight junctions keep epidermal cells together resist cell separation, and help waterproof the skn. In the small intestine, they prevent leakage of digestive enzymes and nutrient solution into ECF and blood. Tight junctions are also called zonula occludens. 2. Adherens junctions. These are complex protein structures that join cells just below the position of tight junctions.. Actin microfilaments, cahedrins and catenin ( adhesion) proteins combine to form these structures which are also called zonula adherens. The main contribution of adherens junctions to epithelia is to enhance resistance to mechanical stress. Gap junctions are small passages that connect the cytoplasm of adjacent cells. In these structures, connxein proteins form connexons ( gaps or channels ) between adjacent cells.. Half of each connexon is associated with an adjacent cell; they halves meet together to form one channel with 12 connexin molecules. The gaps are not always open. When they are open, they provide passageways for small ions and some small molecules; molecules of large molecular weight are not permitted to pass through. Gap junctions serve a vital purpose in epithelia , but they are more important in cardiac muscle and in nerves.