Cell recognition and adhesion
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Gap junctions allow direct communication between adjacent cells by forming channels between the cells' cytoplasm. The structure of a gap junction consists of connexons - cylinders of six transmembrane protein subunits called connexins - arranged back-to-back between the plasma membranes of adjacent cells. These connexons join to form channels about 1.5-2.0 nm in diameter that connect the cytoplasm of the two cells and allow small molecules and ions to pass directly between cells, enabling both electrical and metabolic cooperation.
Cell recognition and adhesion
- 5. CELL RECOGNITION •The carbohydrate is responsible for cell specificity and is therefore fundamental to the specific antigenic properties of the cell membrane. •These antigenic properties are linked in some manner to the body’s immune system and the ability of that system to distinguish between cells that should be present in organism (native cells) and foreign cells (bacteria, other microorganisms, transplanted tissue, transfused blood). •Foreign cells may be recognized as foreign because their membrane glycoprotein contain different carbohydrate markers than those present in individuals own tissue. •Such a situation triggers the immune response. •In contrast, an individual’s cell membrane carbohydrate organization is recognized as being native (referred to as recognition of self) and does not normally trigger an immunological responses.
- 6. CELL ADHESION Commonly, there is a uniform intercellular space or gap of 200-300 A separating closely adjacent cells. Sometimes however, many cells in tissues are linked to each other and to the extracellular matrix at many specialised contact sites called cell junctions that permit or restrict the passage of ions and macromolecules between cells. Of all the social interactions between cells in multicellular organism, the most fundamental are those that hold the cells together.Many cells in tissues are linked to one another & to Extracellular Matrix at specialized contact sites called “CELL JUNCTIONS.” Cell junctions are very minute structures and are not visible by light microscopy. Classification of Cell Junctions I. Cell-cell Junctions Tight junctions, Gap junctions, Desmosomes Adhesion belt / zonula adherens II. Cell-matrix junctionsFocal contacts / adhesion plaques Hemidesmosomes Classification as in Review of Medical Physiology by Ganong I. Junctions that fasten cells to one another& to surrounding tissues Tight junctions, Adhesion belt,Desmosomes, HemidesmosomesFocal adhesions II. Junctions that permit transferof ions & other molecules fromone cell to another--Gap junctions. Classification as in AlbertsMolecular Biology of Cell I. Occluding Junctions :Tight Junctions II. Anchoring Junctions :Desmosomes, Hemidesmosomes, Focal adhesions & Zonula adherens III. Communicating Junctions :Gap Junctions
- 7. PLASMODESMATA Except few types of specialised cells, every living cell in a higher plant is interconnected to its living neighbours by fine cytoplasmic channels—(Each of which is called plasmodesma ; plural- plasmodesmata)that passes through the intervening cell walls. OCCURANCE: Depending on the plant type, the density of plasmodesmata varies from 1 to 10 per /µm2, the smallest meristematic cells have more than 1,000 interconnections with their neighbours. Nectar secreating glands(trichomes of Abutilon nectaris) have 15 or more plasmodesmata per square micrometer of wall surface and oher have 1 per square micrometer of wall surface . STRUCTURE: •Electron micrographs of plasmodesmata show that it is a roughly cylindrical, membrane-lined channel with a diameter of 20 to 60 nm and it traverses cell walls up to 90 nm thick. • Running from cell to cell through the centre of most plasmodesmata is a narrower cylindrical structure which is called desmotubule. •Electron micrographs of desmotubule show that is continuous with elements of endoplasmic reticulum membrane of each of the connected cells. •Between the outside of the desmotubule and the inner face of the cylindrical plasma membrane is an annulus of cytosol. It often appears to be constricted at each end of the plasmodesmata. These constrictions may regulate the flux of molecules through the annuals that joins the two cytosols.
- 8. FUNCTION: Like gap junctions, plasmodesmata provide intercellular channels for molecules of about 1,000 molecular weight, including a variety of metabolic and signalling compounds.a Many evidences indicate that plasmodesmata are, in fact, needed in cell-cell communication. Fluorescent water-soluble dye microinjected into plant cells spread to the cytoplasm of adjacent cells but not into the cell wall. Similarly, if pulses of electrical current are injected, through an electrode into one cell, a measuring electrode in an adjacent cell will detect the same pulses. Many normal metabolic products such as sucrose, spread from cell to cell. As with gap junctions, movement of molecules through plasmodesmata is reversibly inhibited by an increase in cytosolic Ca2+. Certain plant viruses and viriods can enlarge plasmodesmata in order to use this route to pass from cell to cell. •P30 in TMV nulifies the normal regulatory maechanism of plasmodesmata.
- 11. STRUCTURE: •Electron micrographs of plasmodesmata show that it is a roughly cylindrical, membrane-lined channel with a diameter of 20 to 60 nm and it traverses cell walls up to 90 nm thick. • Running from cell to cell through the centre of most plasmodesmata is a narrower cylindrical structure which is called desmotubule. •Electron micrographs of desmotubule show that is continuous with elements of endoplasmic reticulum membrane of each of the connected cells. •Between the outside of the desmotubule and the inner face of the cylindrical plasma membrane is an annulus of cytosol. It often appears to be constricted at each end of the plasmodesmata. These constrictions may regulate the flux of molecules through the annuals that joins the two cytosols.
- 13. Cell Junctions There are 3 functional groups of cell junctions:- Occluding junctions – Seal cells together in an epithelium in a way that prevents leakage of even small molecules from one cell to another. Anchoring junctions – Mechanically attach cells (precisely, their cytoskeleton) to their neighbours or to ECM (extracellular matrix). Communicating junctions – Allow passage of chemicals or electrical signals from one cell to another
- 14. Cell junctions fall into three function groups: 1. Occluding or tight junction; 2. Anchoring junction; 3. Communicating junction.
- 23. TIGHT JUNCTION /OCCLUDING/ZONULA OCCLUDENS OCCURANCE: In epithelial cells of the small intestine, tight junctions are usually located just below the apical microvillar surface. STRUCTURE: Tight junctions are composed of thin bands of anastomosing sealing fine standards that completely encircles the cell. Tight junctions are formed through fusion of plasma membranes of two adjacent cells at a series of points of contact without leaving any intercellular space. When thin sections through a tight junction are seen in the electron microscope, the plasma membranes of the adjacent cells appear to touch each other at intervals and even’ to fuse. Tight junctions alternate with a region where plasma membranes are separated by intercellular space. But Freeze-fracture electron microscope gives a different view of the tight junction. The tight junction appears to consist, of an interlocking network of ridges on the cytoplasmic face of the plasma membrane of each of the two contacting cells. These ridges are made of trans membrane protein particles 3-4 nm in diameter. FUNCTIONS: 1. they act as selective permeability barriers that inhibit even small molecules or fluid from leaking from one side to other side. This function is successfully well maintained due to presence of many tight junctions also called the zonulae occludens. •2. The epithelial lining of the small intestine absorbs all nutrients from lumen of gut and does not allow to flow back the same again into the lumen due to presence of the tight junctions. 3.They release the absorbed nutrients from the other side into the blood via extracellular fluid. 4.Another function attributed to these junctions is their role in maintaining cell polarity by affording a physical barrier to the movement of integral proteins laterally.
- 24. STRUCTURE: Tight junctions are composed of thin bands of anastomosing sealing fine standards that completely encircles the cell. Tight junctions are formed through fusion of plasma membranes of two adjacent cells at a series of points of contact without leaving any intercellular space. the plasma membranes of the adjacent cells appear to touch each other at intervals and even’ to fuse. Tight junctions alternate with a region where plasma membranes are separated by intercellular space. The tight junction appears to consist, of an interlocking network of ridges on the cytoplasmic face of the plasma membrane of each of the two contacting cells. These ridges are made of trans membrane protein particles 3-4 nm in diameter.
- 27. Anchoring Junctions Anchoring junction is a type of cell junction by which a group of cells are joined together into strong structural units by connecting elements of their cytoskeleton. This type of junction is situated below the zone of tight junction and widely distributed in different tissues. They are most abundant in tissues that are subjected to severe mechanical stress, such as skin epithelium, the neck of the uterus, cardiac muscle etc. •Desmosomes are button-like points of intercellular contact that rivet cells together. •They are connected with intermediate filaments (a type of cytoplasmic filament 8-12 nm in diameter). The particular type of intermediate filaments joined to the desmosome depends on the cell type. •The structure of desmosome is very complex. • On the cytoplasmic surface of each interacting plasma membrane there is a dense disc-shaped plaque (0.5 µm. in diameter) composed of mixture of intracellular attachment proteins called desmoplakins. •Each plaque is connected with a thick network of intermediate filaments which pass along the surface of the plaque. • Trans membrane linker glycoproteins called desmogleins bind to the plaque and interact through their extracellular part to hold the adjacent membrane.
- 28. ZONULA ADHERENS (intermediate junction, belt desmosomes) is basal to the zonula ocludens. The adjacent plasma membranes are separated by a gap of 15-20 nm, filled with an electron dense plaque containing a glycoprotein localized only in the membrane, (adherens junction-specific celladhesion molecule or A-CAM or E-cadherin). Myosin, tropom yosin, α-actinin, andvinculin, actin- containing microfilaments insert into the plaque to stabilize the junction between epithelialcells, fibroblast, smooth muscle cells and at intercalated discs. MACULA ADHERENS/SPOT DESMOSOME plasma membrane is seperated by 30 to 50 nm Localised circular areas of contact about 0.5 um in diameter. Under each facing plasma membrane, there is an discoidal intracellular plaque,15-20nm thick having non glycosylated proteins such as desmopakins 1,2, and 3. Numerous 10 nm thick intermediate filaments of keratin proteins called tono filaments ,covers towards plaque form a loop in a wide arc and couse back into cytoplasm. Provide intarcellular mechanical support. Thinner filaments that arise from each dense plaque form transmembrane linkers in the intercellular space. HEMIDESMOSOMES These are half desmosome resemble spot desmosome but join the basal surface of an epithelial cell to a basal lamina. They anchor extracellular proteins such as collagen and other proteins to the cell.
- 29. plasma membrane is seperated by 30 to 50 nm Localised circular areas of contact about 0.5 um in diameter. Under each facing plasma membrane, there is an discoidal intracellular plaque,15-20nm thick having non glycosylated proteins such as desmopakins 1,2, and 3. Numerous 10 nm thick intermediate filaments of keratin proteins called tono filaments ,covers towards plaque form a loop in a wide arc and couse back into cytoplasm. Provide intarcellular mechanical support. Thinner filaments that arise from each dense plaque form transmembrane linkers in the intercellular space.
- 32. GAP JUNCTION/NEXUS OCCURANCE: found abundant in embryonic cells ;In adult tissue –epithelia, cardiac cells and liver cells. STRUCTURE: •Plasma membrane of adjacent cells is seperated by only 2-4 nm. •Electron microscopic and X-ray diffraction observation reveal that both membranes contain cylinders of six dumbbell-shaped connection sub-units which are constructed from transmembrane protein. Consists of hollow channels around which a series of six protein subunits are located called connexon (each is connexin). •Two such cylinders join in the gap between the cells to form a channel about 1.5-2.0 nm in diameter that connects the cytoplasm of the two cells. •Model of a gap junction derived from electron-micrographic analysis shows that one rotation of the six connexion sub-units about a central axis mediates the transition from an open to closed state. •The channels close in presence of Ca2+ ion when its concentration rises markedly in the cytosol. Even slight increases in the level of cytosol Ca2+ ions or decreases in cytosolic pH can decrease the permeability of gap junctions. FUNCTION : Free flow of electrical current carried by ions.-----ELECTRICAL COUPLING. Gap junctions allow many small molecules to pass from one cell to other cell. For example, AMP, ADP or ATP, inorganic ions, sugar, amino acids, nucleotides and vitamins can pass through gap junction, but not their macromolecules protein, nucleic acid and polysaccharide etc. ------- METABOLIC COUPLING. Another important compound passed from cell to cell through gap junction is cyclic AMP which acts as an intracellular messenger and regulates a number of metabolic activities.
- 33. •Plasma membrane of adjacent cells is seperated by only 2-4 nm. •Electron microscopic and X-ray diffraction observation reveal that both membranes contain cylinders of six dumbbell-shaped connection sub-units which are constructed from transmembrane protein. Consists of hollow channels around which a series of six protein subunits are located called connexon (each is connexin). •Two such cylinders join in the gap between the cells to form a channel about 1.5-2.0 nm in diameter that connects the cytoplasm of the two cells.
- 34. •Model of a gap junction derived from electron-micrographic analysis shows that one rotation of the six connexion sub-units about a central axis mediates the transition from an open to closed state.