Epithelial Tissue/Epithelium PDF

CHAPTER 04. EPITHELIAL TISSUE/EPITHELIUM Dr. Jaycee Dela Chica, RMT, MD  Parenchyma the cells responsible for the organ’s specialized functions Stroma which have a supporting role in the organ except in the brain and spinal cord, the stroma is always connective tissue ...

CHAPTER 04. EPITHELIAL TISSUE/EPITHELIUM Dr. Jaycee Dela Chica, RMT, MD  Parenchyma the cells responsible for the organ’s specialized functions Stroma which have a supporting role in the organ except in the brain and spinal cord, the stroma is always connective tissue  EPITHELIAL TISSUES 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. line all external and internal surfaces of the body and all substances that enter or leave an organ  PRINCIPAL FUNCTIONS OF EPITHELIAL TISSUES Covering, lining, and protecting surfaces (eg, epidermis) Absorption (eg, the intestinal lining) Secretion (eg, parenchymal cells of glands) myoepithelial cells specialized sensory cells  CHARACTERISTIC FEATURES OF EPITHELIAL CELLS shapes and dimensions of epithelial cells are quite variable, ranging from tall columnar to cuboidal to low squamous cells Epithelial cell nuclei vary in shape and may be elliptic (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape Most epithelia are adjacent to connective tissue containing blood vessels Lamina propria: underlies the epithelia lining the organs of the digestive, respiratory, and urinary systems  CHARACTERISTIC FEATURES OF EPITHELIAL CELLS The area of contact between the two tissues may be increased by small evaginations called papillae covering of the skin or tongue Epithelial cells generally show polarity basal pole apical pole lateral surfaces  BASEMENT MEMBRANES basal surface of all epithelia rests on a thin extracellular, felt-like sheet of macromolecules a semipermeable filter for substances reaching epithelial cells from below 2 parts on TEM: Basal lamina Reticular lamina  ECM COMPONENTS OF BASAL LAMINA Type IV collagen Laminin: glycoprotein; attach to integrins Nidogen and perlecan: CHON and proteoglycan respectively;  ECM COMPONENTS OF RETICULAR LAMINA Type III collagen Type VII collagen  FUNCTIONS OF BASEMENT MEMBRANE Filter Structural support Organize proteins Maintaining cell polarity Cell-to-cell interactions Scaffold that allows rapid epithelial repair and regeneration INTERCELLULAR ADHESION & OTHER JUNCTIONS  TIGHT JUNCTION (ZONULA OCCLUDENS) term “zonula” indicates that the junction forms a band completely encircling each cell most apical of the junctions band of branching strands in the membrane around each cell’s apical end. The seal between the two cell membranes is due to tight interactions between the transmembrane proteins claudin and occludin serve as fences restricting movements of membrane lipids and proteins at the apical cell surface into the lateral and basal surfaces, and vice versa.  ADHERENT JUNCTION (ZONULA ADHERENS) also encircles the epithelial cell, usually immediately below the tight junction firmly anchoring a cell to its neighbors mediated by cadherins, catenins “terminal web”  DESMOSOME (MACULA ADHERENS) resembles a single “spot-weld” and does not form a belt around the cell disc-shaped structures Desmosomes contain larger members of the cadherin family called desmogleins and desmocollins. Provide firm cellular adhesion and strength throughout the epithelium  HEMIDESMOSOME On the basal epithelial surface, cells attach to the basal lamina by anchoring junctions structures resemble a half-desmosome ultrastructurally Integrins bind primarily to laminin molecules in the basal lamina  GAP JUNCTION (NEXUS) mediate intercellular communication 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 Permit intercellular exchange of molecules with small (< 1.5 nm) diameters Allowing cells in many tissues to act in a coordinated manner SPECIALIZATIONS OF THE APICAL CELL SURFACE  MICROVILLI Seen on apical cell surfaces of epithelia specialized for absorption “brush border or striated border” E.g. small intestine Increase total surface area by 20- or 30-fold Covered with glycocalyx Each microvillus contains bundled actin filaments  STEREOCILIA less common type of apical process absorptive epithelial cells lining the male reproductive system (e.g. epididymis) increase the cells’ surface area, facilitating absorption More specialized stereocilia with a motion-detecting function are important components of inner ear sensory cells contain arrays of microfilaments and actin-binding proteins typically much longer and less motile than microvilli, and may show branching distally  CILIA long, highly motile apical structures; containing internal arrays of microtubules Primary cilium Not motile for detection of light, odors, motion, and flow of liquid 9 + 0 patternof microtubules Motile cilia/typical cilia: abundant on cuboidal or columnar cells Axoneme: 9 + 2 assembly of microtubules Basal bodies: similar structure to centrioles exhibit rapid beating patterns Respiratory epithelium, sperm cell TYPES OF EPITHELIA  TYPES OF EPITHELIA Covering / Lining Epithelia Secretory / Glandular Epithelia  COVERING / LINING EPITHELIA organized into one or more layers that cover the surface or line the cavities of an organ According to the number of cell layers Simple Stratified According to cell shape Squamous Cuboidal Columnar  SECRETORY EPITHELIA & GLANDS function mainly to produce and secrete various macromolecules Secretory cells may synthesize, store, and release proteins (eg, in the pancreas), lipids (eg, adrenal, sebaceous glands), or complexes of carbohydrates and proteins (eg, salivary glands).  CLASSIFICATION Acc to number of cells Unicellular – scattered secretory cells Goblet cell Multicellular  CLASSIFICATION According to Manner of Secretion (Use of Ducts) Exocrine remain connected with the surface epithelium, the connection forming the tubular ducts Endocrine lose the connection to their original epithelium lack ducts  EXOCRINE GLANDS organized as a continuous system of many small secretory portions and ducts In both exocrine and endocrine glands the secretory units are supported by a stroma of connective tissue  CLASSIFICATION According to Morphology simple (ducts not branched) or compound (ducts with two or more branches) tubular (either short or long and coiled) or acinar (rounded and saclike) Compound glands can have branching ducts and can have multiple tubular, acinar, or tubuloacinar secretory portions.  CLASSIFICATION According to Fate of Secretion Merocrine - no destruction; release protein via exocytosis; sweat glands Holocrine – total destruction; e.g. sebaceous glands Apocrine – partial destruction; e.g. mammary gland  CLASSIFICATION According to Type of Secretion Serous glands - synthesize non-glycosylated protein; with rough endoplasmic reticulum and Golgi complex; heavily stained (e.g. acini of pancreas and parotid salivary glands), digestive enzymes Mucous glands - contain glycosylated proteins called mucins which are hydrophilic; lightly stained Mixed glands or Seromucous glands - The product of such glands is a mixture of digestive enzymes and watery mucus; submaxillary/mandibular glands  ENDOCRINE GLANDS Will be discussed on another chapter…  RENEWAL OF EPITHELIAL CELLS Epithelial tissues are relatively labile structures whose cells are renewed continuously by mitotic activity and stem cell populations. rate of renewal varies widely In stratified epithelial tissues, stem cells and mitosis occur only within the basal layer in contact with the basal lamina Epithelia are normally capable of rapid repair and replacement of apoptotic or damaged cells  REVIEW OF NAMING Step 1: By NUMBER OF CELL LAYER/S Simple (1 layer) Stratified (2 or more) Pseudostratified (false stratification) Transitional  NAMING Step 2 : By SHAPE (shape of the nucleus also corresponds roughly to cell shape) Squamous (thin cells, flat nucleus) Cuboidal (square/polygonal cell, spherical nucleus, centrally located) Columnar (cells are taller than they are wide, elongated nucleus located near the basement membrane) * Most stratified epithelia are classified according to the cell shape of the superficial outer layer(s).  NAMING Optional, if with special feature/s Microvilli Cilia Stereocilia Goblet Keratinized Non-keratinized END OF CHAPTER 4 Thank you for listening!