Lining Epithelial Tissue PDF 2024

EPITHELIAL TISSUE Dr. Nuriye Ezgi BEKTUR AYKANAT Department of Histology and Embryology Faculty of Medicine ATILIM UNIVERSITY 1  Despite its complexity, the organs of the human body are composed of only four basic tissue ty...

EPITHELIAL TISSUE Dr. Nuriye Ezgi BEKTUR AYKANAT Department of Histology and Embryology Faculty of Medicine ATILIM UNIVERSITY 1  Despite its complexity, the organs of the human body are composed of only four basic tissue types: epithelial, connective, muscular, and nervous tissues. Each tissue is an assemblage of similarly specialized cells united in performing a specific function. The basic tissues, each containing extracellular matrix (ECM) as well as cells, associate with one another in the variable proportions and morphologies characteristic of each organ. 2  Connective tissue is characterized by cells producing very abundant ECM; muscle tissue is composed of elongated cells specialized for contraction and movement; and nervous tissue is composed of cells with long, fine processes specialized to receive, generate, and transmit nerve impulses. 3  Most organs can be divided into the parenchyma, which is composed of the cells responsible for the organ’s specialized functions, and the stroma, the cells of which have a supporting role in the organ. Except in the brain and spinal cord, the stroma is always connective tissue. 4  Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface. Epithelia (Gr. epi, upon + thele, nipple) line all external and internal surfaces of the body and all substances that enter or leave an organ must cross this type of tissue. 5  The principal functions of epithelial tissues include the following: Covering, lining, and protecting surfaces (eg, epidermis) Absorption (eg, the intestinal lining) Secretion (eg, parenchymal cells of salivary glands) 6  Specific cells of certain epithelia may be contractile (myoepithelial cells) or specialized sensory cells, such as those of taste buds or the olfactory epithelium. 7  Most epithelia are adjacent to connective tissue containing blood vessels from which the epithelial cells receive nutrients and O2. Even thick epithelia DO NOT themselves normally contain blood vessels. 8  Epithelial cells generally show polarity, with organelles and membrane proteins distributed unevenly within the cell. The region of the cell contacting the ECM and connective tissue is called the basal pole and the opposite end, usually facing a space, is the apical pole, with the two poles differing significantly in both structure and function. 9  Regions of cuboidal or columnar cells that adjoin neighboring cells comprise the cells’ lateral surfaces. 10 Basement Membranes The basal surface of all epithelia rests on a thin extracellular, a semi- permeable filter for substances reaching epithelial cells from below. Glycoproteins and other components in this structure can often be stained and visualized with the light microscope. 11  With the transmission electron microscope (TEM) two parts of the basement membrane may be resolved. Nearest the epithelial cells is the basal lamina, a thin, electron-dense, sheet-like layer of fine fibrils, and beneath this layer is a more diffuse and fibrous reticular lamina. The terms “basement membrane” and “basal lamina” are sometimes used interchangeably, but “basal lamina” usually denotes the fine extracellular layer seen ultrastructurally and “basement membrane” the entire structure beneath the epithelial cells visible with the light microscope. 12  The macromolecules of the basal lamina are secreted from the basal sides of the epithelial cells and form a sheet-like array. ECM components of the basal lamina characteristically include the following: Type IV collagen: Monomers of type IV collagen self-assemble into a two-dimensional network of evenly spaced subunits resembling the mesh of a window screen. Laminin: These are large glycoproteins that attach to transmembrane integrin proteins in the basal cell membrane and project through the mesh formed by the type IV collagen. Nidogen and perlecan: Respectively a short, rod-like protein and a proteoglycan, both of these cross-link laminins to the type IV collagen network, helping to provide the basal lamina’s three-dimensional structure, to bind the epithelium to that structure, and to determine its porosity and the size of molecules able to filter through it. 13  The more diffuse meshwork of the reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are produced by cells of the connective tissue. 14 1. Besides acting as filters, functions of basement membranes include helping to provide structural support for epithelial cells and attach epithelia to underlying connective tissue. 2. Basal lamina components help organize integrins and other proteins in the plasma membrane of epithelial cells, maintaining cell polarity and helping to localize endocytosis, signal transduction, and other activities. 3. Basement membrane proteins also mediate many cell-to-cell interactions involving epithelia and mark routes for certain cell migrations along epithelia. 4. Finally, the basement membrane also serves as a scaffold that allows rapid epithelial repair and regeneration. 15 Intercellular Adhesion & Other Junctions Several membrane-associated structures provide adhesion and communication between cells. Some are present in other tissues but all are particularly numerous and prominent in epithelia. Epithelial cells adhere strongly to neighboring cells and basal laminae, particularly in epithelia subject to friction or other mechanical forces. 16  Tight or occluding junctions form a seal between adjacent cells. Adherent or anchoring junctions are sites of strong cell adhesion. Gap junctions are channels for communication between adjacent cells. 17  Also known as tight junctions, zonulae occludentes (zonula occludens is the singular form) are located between adjacent plasma membranes and are the most apically located junction between the cells of the epithelia. They form a belt-like junction (zonula = latin for belt) that encircles the entire circumference of the cell. The term “zonula” indicates that the junction forms a band completely encircling each cell. The seal between the two cell membranes is due to tight interactions between the transmembrane proteins: claudin, occludin, nectins, and junctional adhesive molecules [JAMs]. 18  Occludin interacts with four major zonula occludin (ZO) proteins: ZO-1, ZO-2, ZO-3, and afadin. Nectins are connected to actin filaments through the protein afadin (AF6). Claudin (Latin claudere, to close), a family of 24 proteins forming linear fibrils in the tight junctions, form a water channel for paracellular absorption. Junctional adhesive molecules [JAMs] These four proteins participate in the formation of the zonula occludens and bind to the actin filaments of the cell’s cytoskeleton, thus providing stability to the tight junction. 19 Clinical Correlations- Food poisoning Zonula occludens proteins are targets for some bacteria. Enterotoxin, which causes food poisoning secreted by Clostridium perfringesns, binds to claudin molecules in intestinal cells, prevents these proteins from joining the tight junction structure and causes the loss of tissue fluid paracellularly along the 20 intestinal lumen and triggers cell death. Clinical Correlations- Gastric Ulcer In gastric ulcer, Helicobacter pylori binds to the extracellular regions of tight junction proteins in stomach cells and disrupts the connection by introducing a protein that targets ZO-1. 21  The second type of junction is the adherens junction or zonula adherens, which also encircles the epithelial cell, usually immediately below the tight junction. This is an adherent junction, firmly anchoring a cell to its neighbors. 22  Cell adhesion is mediated by E-cadherins, transmembrane glycoproteins of each cell that bind each other in the presence of Ca+2. At their cytoplasmic ends, cadherins bind catenins that link to actin filaments with actin- binding proteins. The actin filaments linked to the adherens junctions form part of the “terminal web,” a cytoskeletal feature at the apical pole in many epithelial cells. Together, the tight and adherent junctions encircling the apical ends of epithelial cells function like the plastic bands that hold a six- pack of canned drinks together. 23  Fascia adherens is similar to zonula adherens but does not go around the entire circumference of the cell. Cardiac muscle cells, for example, are attached to each other at their longitudinal terminals via the fascia adherens. 24 Desmosome Another anchoring junction is the desmosome (Gr., desmos, binding and soma, body) or macula adherens (L. macula, spot). As the name implies, this junction resembles a single “spot-weld” and does not form a belt around the cell. Desmosomes are disc-shaped structures at the surface of one cell that are matched with identical structures at an adjacent cell surface. 25  The extracellular space between the two cells is approximately 30 nm in width and is occupied by the extracellular regions of desmocollins and desmogleins, the cadherins (transmembrane proteins whose name comes from calcium-dependent adhesion) that project from both cell membranes into this space. In the presence of calcium ions, the extracellular portions of these cadherins contact each other and form bonds that attach these adjacent cells to one another. 26  The outer dense plaque is closely adhering to the cytoplasmic aspect of the plasmalemma. This plaque is composed of the glycoproteins plakoglobins and plakophilins held together by proteins known as desmoplakins. Desmoplakins in turn bind intermediate filament (desmin) proteins rather than actins. 27 Gap Junctions Gap junctions are widespread in epithelial tissues throughout the body as well as in cardiac muscle cells, smooth muscle cells, and neurons but not skeletal muscle cells. They differ from the occluding and anchoring junctions, in that they mediate intercellular communication by permitting the passage of various small molecules (