|MadSci Network: Cell Biology|
Gap junctions, tight junctions, and desmosomes are all examples of structures involved in intercellular adhesion processes. Desmosomes and tight junctions appear to serve a more or less exclusively adhesive function, while gap junctions are intimately involved with intracellular communications. Desmosomes are primarily associated with epithelial cells. They have a characteristic appearance under the electron microscope -- the adjacent cell membranes show an area of $BEU(Jhickening$BG (J called adesmosomal plaque, about 300 nanometers in length, with the interior portion of the plaque having a sort of $BEG(Juzzy$BG(Jappearance. The desmosome is made up of several specialized proteins. Desmocollins, which are large proteins (molecular weight 130,000) embedded in the cell membrane, are the proteins responsible for the actual adhesion of the cells -- desmocollins from one cell reach out and attach to desmocollins in the adjacent cell. Other membrane-embedded proteins, desmogleins (molecular weight 165,000) and plakoglobins (molecular weight 83,000), extend into the cytoplasm. Another set of proteins, desmoplakins, are entirely cytoplasmic and connect the desmosome with intermediate filament proteins. These give the desmosome its $BEG(Juzzy$BGï(J appearance. Desmosomes form a tight, impenetrable barrier anchoring cells to each other or to a substrate. A disease known as pemphigus, which is a rather painful condition characterized by the formation of large blisters on the skin, is caused by a breakdown of desmosomal structures. Tight junctions are areas of cell contact where the cells come into such close contact that the adjacent membranes actually appear to fuse. This isn$BCU(J the case, however -- instead, the adhesion is mediated again by proteins embedded in the cell membrane. There are a number of proteins that appear to be associated with tight junctions, but only a few have been identified so far. Two proteins, called cingulin and ZO1, are found in the area of tight junctions, but it$BCT(J not known yet if they actually participate in the adhesion process. Tight junctions serve to form barriers between cells, helping to maintain tissue compartmentalization as well as direct the flow of nutrients and waste products through cells rather than around cells in the extracellular spaces. Gap junctions can be thought of as channels connecting cells to each other. The cell membranes come close together, but are separated by a $BEH(Jap$BG(Jof about 2-4 nanometers. These junctions again are mediated by proteins in the cell membrane. A protein called connexin appears to be the main gap junction component. Connexins come together in groups of 6 to form hexagonal ring-shaped channels called connexons in the cell membrane. These come together with connexons in the membranes of adjacent cells to form the gap junction. The ring-like structure of the gap junction allows for communication between cells. Ions and small molecules such as amino acids and nucleotides can pass freely between the cytoplasms of cells connected by gap junctions. This was demonstrated in the laboratory by injecting cells with fluorescent dyes and watching the dye spread from the cell into which it was injected to neighboring cells. By using dye molecules of increasing size, the size of the channels connecting the cells could be determined (about 2 nanometers in diameter). Gap junctions have been proposed to play important roles in intracellular communication and signaling during embryonic development. Antibodies that bind to gap junction proteins can be injected into cells to prevent transfer of molecules through the channels. When researchers have injected such antibodies into developing embryos (usually frog embryos because of their large size), the normal development of these embryos is disrupted, showing the importance of gap junction proteins in embryonic development.
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