Chapter 4 - Epithelial Histology PDF

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Summary

Chapter 4: Epithelial TissueEPITHELIAL TISSUE Composed of closely aggregated polyhedral cells adhering strongly and to one another and to a thin layer of ECM forming cellular sheets which lines the cavities of organs Functions: Covering, lining, and protecting surfaces Absorption Secretion Main func...

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Chapter 4: Epithelial Tissue EPITHELIAL TISSUE - Composed of closely aggregated polyhedral cells adhering strongly and to one another and to a thin layer of ECM forming cellular sheets which lines the cavities of organs -

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Functions: Covering, lining, and protecting surfaces Absorption Secretion Main function: lining of surface or body cavities; glandular secretion

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Specific cells of certain epithelia may be contractile or specialized sensory cells, such as those of taste buds or the olfactory epithelium

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Characteristic features:  Shapes  Columnar - Elongated nuclei, height > weight  Cuboidal / pyramidal - More spherical nuclei  Squamous - Flattened nuclei  Most epithelia are adjacent to a connective tissue containing blood vessel  Epithelial cells receive oxygen and nutrient from the connective tissue  Epithelium is avascular (lack of blood vessel) 

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Lamina Propria Connective tissue under the epithelial lining of the organs of the digestive, respiratory, and urinary Papillae Projection of the lamina propria into the epithelium Occur most frequently in in ET subject to friction (covering of skin or tongue) Epithelial cells show polarity, with organelles and membrane proteins distributed unevenly within the cell. Basal Pole Region in contact with the ECM and CT Apical pole Upper part of the epithelial cell, facing the space (lumen) Lateral Surface Side of the EC, region that adjoins neighboring cells

BASEMENT MEMBRANE - Thin extracellular felt like sheet of macromolecules - Semi-permeable filter for substances reaching epithelial cells from below - Where the basal surface of epithelium rests on - Function: Filter Helps provide structural support Attach epithelia to underlying CT - Two parts: Basal lamina - Nearest to the epithelial cell - Thin, electron-dense, sheet like layer of fine fibrils  Reticular lamina - Under the basal lamina - More diffused and fibrous - Basement membrane and basal lamina and are sometimes used interchangeably - Macromolecules of basement membrane  Secreted from basal sides of EC and form a sheet-like array - ECM components:  Type IV collagen - Monomers of type iv collagen self – assembled into a two-dimensional network of evenly spaced subunits resembling the mesh of a window screen - Most common  Laminin - Large glycoprotein that attach to transmembrane integrin protein  Nidogen and Perlecan - Helps provide the basal lamina’s three-dimensional structure - The more diffused meshwork of the reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen - Basal Lamina Components:  organize integrins and other proteins in the plasma membrane  helps localize endocytosis, signal transduction, etc. - Basement membrane protein: mediate many cell-tocell interactions involving epithelia and mark routes for certain migrations along the epithelia

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INTRACELLULAR ADHESION - Functions  Tight or occluding junctions Also called zonulae occlude ens Most apical Zonula meaning the junction forms a band encircling each cell The seal bet. 2 cell membranes is due to tight interaction between the transmembrane protein: claudin and occluding Molecules crossing goes through rather than in between Fences restricting actions  Adherent or anchoring: site for c. adhe Also called zonula adherens Encircles the epithelial cell below the tight junction Adherent & anchoring junction Cell adhesion is mediated by cadherins. Cadherins bins catenins that link to actin filaments with actin binding proteins. Desmosome or macula adherens, resembles a spot – weld and does not form a belt around the cell  Gap junction channel for communication between adjacent cells has transmembrane protein called: connexins mediate signal transduction - Hemidesmosome - Anchors - Attaches the epithelial cell to BL - Reassembles half of a desmosome - Transmembrane protein:  Integrin  Focal Adhesion - Basal anchoring junction found in cells that are moving during epithelial repair or regeneration - Focal adhesion kinase SPECIALIZATION OF THE APICAL CELL SURFACE MICROVILLI - smallest of the three - Seen in epithelia specialized for absorption - Uniform in length

Usually called bush border or started border (densely packed projecting to the lumen) - 1 um long ans 0.1 um wide STEREOCILIA - Less common type of apical process - Seen on the absorptive epithelial cells’ lining and male reproductive system - Increases the cell’s surface area, facilitating absorption CILIA - Long, highly motile, apical structures, larger than microvilli, and contains internal rays of microtubules  BASAL BODIES Apical cytoplasmic structures below the c.m.  Primary cilia Not motile, enriched with  Motile cilium 5 to 10 um long and 0.2 um in diameter Each cilium has a core structure consisting of nine peripheral microtubules doublets arrayed around two central microtubules. 9 + 2 assembly = axoneme TYPES OF EPITHELIA 1. Covering or lining 2. Secretory COVERING OR LINING - Organized into one or more layers that cover the surface or line the cavities of an organ - Epithelia may be classified according to:  Cell layers 1. Simple epithelia - Contain one layers 2. Stratified epithelia - Contain two or more layers  Cell shape 1. Squamous - Thin cells 2. Cuboidal - Width and thickness roughly similar 3. Columnar - Taller than they are wide  Outer layer 1. Keratinized - Packed with keratin filaments 2. Nonkeratinized - With relatively sparse keratin

Stratified squamous keratinized epithelium - Found mainly in the epidermis of the skin - Helps prevent dehydration from the tissue Stratified squamous nonkeratinized epithelium - lines moist internal cavities where water loss is not a problem - flattened cell of the surface retains their nuclei and most metabolic functions Transitional epithelium / urothelium - lines much of the urinary tract, extending from the kidney to the proximal part of the urethra - characterized by a superficial layer of large domelike cells sometimes called umbrella cells Pseudostratified columnar epithelium - tall, irregular cells all are attached to the basement membrane - their nuclei are at different levels and not all cell levels extend to the free surface, giving a stratified appearance SECRETORY EPITHELIA & GLANDS - secretory cells may synthesize, store, and release proteins, lipids, or complexes of carbohydrates and proteins. - Consist of cell that function to produce and secrete macromolecules Unicellular gland (goblet) - Consist of one cell scattered among epithelial tissue - Abundant in the lining of small intestine and respiratory tract Exocrine glands - Remain connected with the surface epithelium - Remain connected with the surface epithelium, the connection forming the tubular ducts lined with epithelium that deliver the secreted material where it is used - Continuous system of many small secretory portions and ducts - Structure:  Simple: ducts not branched  Compound: ducts with two or more branches  Tubular: short/long & coiled  Acinar: rounded or sac like

Endocrine glands - Lose their connection to the original epithelium and therefore lack ducts THREE BASIC MECHANISMS 1. Merocrine secretion Most common of protein and glycoprotein secretion Involve exocytosis from membrane – bound vesicles May be further categorized as either serous or mucous 2. Holocrine secretion Cells accumulate products continuously as they enlarge as undergo terminal differentiation 3. Apocrine secretion Product accumulates at the cell’s apical end Mucins - Heavily glycosylated proteins - When released from the cell, it becomes hydrated and turns inti mucus Seromucous glands - Salivary glands - Products of such glands is a mixture of digestive enzymes and watery mucus Myoepithelial cells - Contractile - Found at the basal end of the secretory cells TRANSPORT ACROSS EPITHELIA - Transcellular transport  Ex: transfer of ions and water across the epithelium - Apical tight junction prevents paracellular diffusion or backflow between cells - Transcytosis  Through endocytosis & exocytosis RENEWAL OF EPITHELIAL CELL - Epithelial tissue can renew regularly - Fast like intestinal epithelium which is replaced weekly - Normally capable of rapid repair and replacement apoptotic or damaged cell...