Epithelial Tissue I PDF

Epithelial tissue I Dragana Nikitovic Professor of Histology-Embryology Medical School, University of Crete, Heraklion, Greece  Organization of the body Οrganization of a multicelluar organism: The diagram shows five levels of...

Epithelial tissue I Dragana Nikitovic Professor of Histology-Embryology Medical School, University of Crete, Heraklion, Greece  Organization of the body Οrganization of a multicelluar organism: The diagram shows five levels of organization in a multicellular organism. 1. Cell is the basic unit 2. Groups of similar cells form tissues; 3. Groups f different tissues make up organs 4. Groups of organs form organ systems 5. Cells, tissues, organs, and organ systems combine to form a multicellular organism. The human body consists of trillions of cells, each capable of growth, metabolism, response to stimuli, and, with some exceptions, reproduction. Although there are some 200 different types of cells in the body, these can be grouped into four basic classes. These four basic cell types, together with their extracellular materials, form the fundamental tissues of the human body 2  Introduction- the types of tissues  Epithelial  Connective  Muscle  Nerve The basic tissues co-exist and associate with each other in the variable proportions and morphologies characteristic of each organ Therefore the co-organization of basic tissues forms all the organs in the body 3   All the tissues consist of cells and components of the surrounding extracellular matrix (ECM)  In the space between the cells and the components of the ECM resides the intercellular fluid or the interstitial fluid in which the cells of a multicellular organisms survive 4 5  ΕΙΣΑΓΩΓΗ- ΤΥΠΟΙ ΙΣΤΩΝ Organs Parenchyma: Stroma: Composed of the cells Composed of cells and their responsible for the organ’s excreted products that have specialized functions a supporting role (except in the brain and spinal cord, the stroma is always connective tissue. 6  Tissues combine to form the various organs of the body 7  Epithelial tissues Epithelial tissues:  Consist of tightly packed polyhedral cells that adhere strongly to each other and to a thin layer of extracellular matrix (ECM), in this case denotated as basement membrane, forming cellular sheets that cover the body surface and line the cavities of organs  Epithelial cells are present on all external and internal surfaces of the body, and any substance entering or leaving an organ must pass through this tissue. 8  Functions of epithelial tissues  The main functions of epithelial tissues include:  Covering, lining, and protecting surfaces (e.g., the epidermis)  Absorption (e.g., the intestinal lining)  Secretion (e.g., the parenchymal cells of glands) 9  Specialized functions_ sensory and contractile epithelial cells  Specific cells of epithelia may be contractile (myoepithelial cells) or  Specialized sensory cells, such as those of taste buds or the olfactory epithelium. 10 CHARACTERISTIC FEATURES OF EPITHELIAL CELLS Epithelial cells generally show polarity! Organelles and membrane proteins are distributed unevenly within the cell. Τhe cells present:  Apical pole : typically the free surface facing a space.  The sides of cuboidal or columnar cells that are adjacent to neighboring cells make up the lateral surfaces, where the cell membranes often exhibit numerous folds that enhance the surface area and functional capacity.  Basal pole: The area of the cell that contacts the extracellular matrix (ECM) and connective tissue (Α) Επιθηλιακά κύτταρα πολικά (Β) Μη πολικά κύτταρα του συνδετικού  Apical and basal poles differ markedly ιστού in both structure and function 11 http://www.nature.com/aps/journal/v32/n5/fig_tab/aps201120 f1.html  Morphology of the epithelial cell width The size and shape of cells are typically determined height by their function. Shape and size vary:  Columnar  Cuboidal  Squamous cells 12  Columnar cells Columnar epithelium of the gall bladder Columnar emed.med.uoa.gr Large height– small width 13  Squamous epithelial cells Squamous cells (endothelial cells) Small height – large width-surface in contact with the basement membrane 14  Cuboidal cells Cuboidal cells Cuboidal cells of a collecting renal duct emed.med.uoa.gr Similar height - width 15  Take away!! Since the lipid-rich membranes of epithelial cells are often indistinguishable under light microscopy, the number and shape of their stained nuclei serve as key indicators of cell shape and density. The nuclei also help in determining the number of cell layers within an epithelium, which is a crucial morphological criterion for classifying different types of epithelia. The nuclei of epithelial cells generally reflect the cell’s overall shape. E.g, Columnar cells usually have elongated nuclei, squamous cells have flattened nuclei, and cuboidal or pyramidal cells tend to 16 have more spherical nuclei   Most epithelia are situated next to the connective tissue that contains blood vessels, which supply the epithelial cells with nutrients and oxygen  The connective tissue beneath the epithelia lining the organs of the digestive, respiratory, and urinary systems is known as the lamina propria.  The contact area between the two tissues can be increased by small projections called papillae that extend from the connective tissue into the epithelium and by the epithelial ridges  Papillae are most commonly found in epithelial tissues that experience friction, such as those covering the skin or tongue. 17  Basement membranes All epithelial cells at the points of contact of their basal surface with the underlying connective tissue, display a surface of a specialized extracellular substance (laminin, collagen type IV, nidogen, perlecan and deeper collagen III and VII)) called the basement membrane (BM)  a semipermeable 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 19  Basement membranes perform many functions:  Structural support  Create polarity  Attachment of epithelia to the underlying connective tissue (dystrophic epidermolysis bullosa)  Regulate the entry of substances from the underlying tissues to the epithelia  Serve as a scaffold that allows rapid epithelial repair and regeneration  Mediate many cell-to-cell interactions BMs consist of the basal (1) and reticular lamina (2) 19  The ultrastructural components of the basement membrane are revealed by TEM. EC The dense basal lamina (BL), 20-100 nm thick, may appear with thin clear zones on each side and is anchored to a thicker, more diffuse reticular lamina (RL) BL containing collagen III fibers. Hemidesmosomes (H) bind the basal surface of the epithelial cell (C) to the RL basal lamina. 20  Basal lamina: macromolecules of the basal lamina are secreted from the basal sides of the epithelial cells and form a sheetlike array Reticular lamina: contains type III collagen whici is bound to the basal lamina by anchoring fibrils of type VII collagen, both of which are produced by cells of the connective tissue 21  Tracheal section showing a well-developed basement membrane Section of a kidney, showing the well- stained basement membrane within a large, spherical renal corpuscle and surrounding tubules In the vascular glomeruli, the basement membrane, in addition to a supporting role, also has a functional role, acting as a filter that is a key point of kidney function 22  Tracheal section showing a well-developed basement membrane 23  Basal laminae (external laminae)  Basal laminae (external laminae) with similar composition also exist as thin sleeves surrounding muscle cells, nerves, and fat-storing cells  They serve as semipermeable barriers regulating macromolecular exchange between the enclosed cells and connective tissue Laminin, a major glycoprotein within basal laminae, is shown here by immunohistochemistry and identifies the basement membranes of the stratified epithelium (E) and the simple epithelium lining a small blood vessel (V). Laminin also occurs in the external laminae surrounding nerves (N) and muscle (M) fibers, seen here in cross-section. 24 25  SPECIALIZATIONS OF THE APICAL CELL SURFACE (1) Microvilli  A series of cytoplasmic projections, on the apical surface of cells  Abundant in specialized, absorptive epithelial cells  Contain actin bundles  Immobile 26  Microvilli Brush border  Microvilli in small intestine cells form a brush or striated border  Each microvillus is ~1 μm long and 0.1 μm wide  Hundreds to thousands of microvilli per cell increase surface area 20- to 30-fold  The glycocalyx on microvilli contains membrane-bound proteins and digestive enzymes  They facilitate absorption 27   Microvilli contain bundled actin filaments, stabilized by actin-binding proteins  Actin microfilaments are dynamic, enabling myosin- based movements  These movements optimize absorption through channels and receptors in the plasma membrane  Actin filaments connect to the terminal web of cortical microfilaments at the base of the microvilli. 28  Stereocilia  Rare apical processes found mainly in the male reproductive system  They increase surface area for absorption, similar to microvilli  Specialized stereocilia in the inner ear detect motion  Stereocilia contain microfilaments, actin-binding proteins, and connect to the Numerous very long stereocilia, at the apical ends of terminal web the tall epithelial cells lining organs such as the  They are longer, less motile, epididymis (shown here), increase the surface area and may have distal branching available for absorption. compared to microvilli. 29  Cilia  Cilia are long, highly motile apical structures, larger than microvilli, with internal microtubule arrays.  Most cells also have a non-motile primary cilium, rich in receptors and signal transduction complexes that detect light, odors, motion, and liquid flow  Motile cilia are common on cuboidal or columnar epithelial cells The cells of the respiratory epithelium  Typical cilia are 5-10 μm long and have cilia 0.2 μm in diameter, much larger than microvilli  Epithelial cell cilia exhibit movement to propel fluids and foreign substances in one direction across the epithelium 30   Each cilium has a core structure called an axoneme, composed of nine peripheral microtubule doublets around two central microtubules (9 + 2 arrangement)  Kinesin and dynein motors move along the peripheral microtubules, transporting molecular components (use of ATP)  Axoneme microtubules connect to basal bodies, which are located just below the cell membrane  Basal bodies resemble centrioles, with microtubule triplets and tubulin protofilaments anchoring them to the cytoskeleton. 31   Cilia exhibit rapid beating patterns that move a current of fluid and suspended matter in one direction along the epithelium  Ciliary motion occurs through successive changes in the conformation of the axoneme, in which various accessory proteins make each cilium relatively stiff, but elastic https://www.britannica.com/video/22035/cells-human-respiratory-tract 32  Microvilli Cilia 33  Lateral Surfaces Intercellular Adhesion & Junctions  Various membrane-associated structures facilitate adhesion and communication between cells  Especially abundant and prominent in epithelial tissues.  Epithelial cells are strongly connected to neighboring cells and to the basal lamina, especially in epithelia that are exposed to friction or other mechanical stresses 34  Intercellular Adhesion & Junctions  Structures in cell membranes that ensure cohesion & ability for cells to communicate  Epithelia display various cell junctions, which serve different functions: 1. Tight or Obstructive junctions: block the space between adjacent cells (claudin/occludin) and create two membrane compartments 2. Adherent or anchoring junctions: adhesion sites (also desmosomes) between cells (cadherins) strong cell adhesion. 3. Gap junctions: communication channels between cells (connexins) 35  In many epithelia, these junctions are present in a definite order at the apical end of the cells. 1. Tight junctions, also called zonulae occludens (form a band) are the most apical of the junctions, appear fused or very tightly apposed  Due to tight interactions between the transmembrane proteins: claudin and occludin.  Ensures that molecules crossing an epithelium do so by going through the cells (a transcellular path) rather than between them (the paracellular pathway) 2. Adherens junction or zonula adherens, usually immediately below the tight junction (encircles the cell).  Firmly anchors a cell to its neighbors  Cell adhesion is mediated by cadherins, the transmembrane glycoproteins  Desmosomes, are point-type adherent junctions connecting with intermediate filaments (desmosomes contain larger members of the cadherin family called desmogleins and desmocollin). 3. Gap junctions, mediate intercellular communication: 36  3. Gap junctions, mediate intercellular communication:  consist of aggregated transmembrane protein complexes that form circular patches in the plasma membrane  The transmembrane gap junction proteins, connexins, form hexameric complexes called connexons, each of which has a central hydrophilic pore about 1.5 nm in diameter  When two cells attach, connexins in the adjacent cell membranes move laterally and align to produce connexons between the two cells  Gap junctions permit intercellular exchange of molecules with small (