Complete Epithelial Tissue Notes

Epithelial Tissue: Key Features and Functions

Characteristic Features of Epithelial Cells

  • Epithelial tissues consist of closely packed polyhedral cells strongly adhering to each other and a thin extracellular matrix (ECM).
  • They form cellular sheets lining organ cavities and covering body surfaces.
  • Epithelia line all external and internal body surfaces; all substances entering or leaving an organ must cross this tissue type.
  • Functions:
    • Covering, lining, and protecting surfaces (e.g., epidermis).
    • Absorption (e.g., intestinal lining).
    • Secretion (e.g., gland parenchymal cells).
  • Specific epithelial cells may be contractile (myoepithelial cells) or specialized sensory cells (e.g., taste buds, olfactory epithelium).
  • Epithelial cells vary in shape (columnar, cuboidal, squamous), with their morphology dictated by function.
    • Columnar cells: elongated nuclei.
    • Squamous cells: flattened nuclei.
    • Cuboidal/pyramidal cells: spherical nuclei.
  • Epithelia are typically adjacent to connective tissue containing blood vessels for nutrient and O2O_2 supply; epithelia themselves usually lack blood vessels.
  • The connective tissue underlying epithelia in digestive, respiratory, and urinary systems is called the lamina propria.
  • The contact area between epithelium and connective tissue may be increased by papillae (evaginations from connective tissue into the epithelium).
  • Epithelial cells exhibit polarity, with uneven distribution of organelles and membrane proteins.
    • Basal pole: contacts ECM and connective tissue.
    • Apical pole: faces a space.
    • Lateral surfaces: adjoin neighboring cells, often with folds to increase surface area.

Basement Membranes

  • The basal surface of epithelia rests on a thin, felt-like sheet of macromolecules called the basement membrane, which acts as a semipermeable filter.
  • Two Parts (TEM):
    • Basal lamina: A thin, electron-dense layer of fine fibrils nearest the epithelial cells.
    • Reticular lamina: A more diffuse, fibrous layer beneath the basal lamina.
  • Terms "basement membrane" and "basal lamina" are sometimes used interchangeably.
  • Macromolecules of Basal Lamina:
    • Type IV collagen: forms a two-dimensional network.
    • Laminin: large glycoproteins attaching to transmembrane integrins.
    • Nidogen and perlecan: cross-link laminins to the type IV collagen network.
  • Basal laminae (external laminae) also exist as thin sleeves around muscle cells and nerves, serving as semipermeable barriers.
  • The reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils of type VII collagen, both produced by connective tissue cells.
  • Functions:
    • Filters.
    • Structural support.
    • Attachment to connective tissue.
    • Organization of integrins and other proteins.
    • Cell-to-cell interactions.
    • Scaffold for epithelial repair and regeneration.

Intercellular Adhesion & Other Junctions

  • Epithelial cells adhere strongly to neighboring cells and basal laminae, especially in tissues subject to friction or mechanical forces.
  • Lateral Cell Surfaces Have Specialized Intercellular junctions:
    • Tight (occluding) junctions: Form a seal between adjacent cells.
    • Adherent (anchoring) junctions: Sites of strong cell adhesion.
    • Gap junctions: Channels for communication between adjacent cells.
  • Tight Junctions (Zonulae Occludens):
    • Most apical junctions, encircling each cell.
    • Adjacent membranes appear fused due to interactions between transmembrane proteins (claudin and occludin).
    • Ensure molecules cross the epithelium via a transcellular rather than paracellular path.
    • Serve as fences, restricting movement of membrane lipids and proteins between apical and basolateral surfaces.
    • Maintain distinct membrane domains with different functions.
  • Medical Application:
    • Clostridium perfringens enterotoxin binds claudin molecules, causing fluid loss into the intestinal lumen.
    • Helicobacter pylori binds tight-junction proteins, disrupting signaling and causing gastric ulcers.
  • Adherens Junctions (Zonula Adherens):
    • Encircle the epithelial cell immediately below the tight junction.
    • Anchor cells to neighbors.
    • Cell adhesion is mediated by cadherins (transmembrane glycoproteins).
    • Cadherins bind catenins (linked to actin filaments).
    • Actin filaments form part of the "terminal web".
  • Desmosomes (Macula Adherens):
    • Disc-shaped structures resembling "spot-welds."
    • Contain desmogleins and desmocollins (cadherin family members).
    • Cytoplasmic ends bind plakoglobins (catenin-like proteins) linked to desmoplakins (in an electron-dense plaque).
    • Desmoplakins bind intermediate filament proteins (cytokeratin/tonofilaments).
    • Provide firm cellular adhesion and strength.
  • Medical Application:
    • Pemphigus vulgaris: Autoimmune reactions against desmogleins, reducing cell-to-cell adhesion and causing blistering.
  • Gap Junctions:
    • Mediate intercellular communication.
    • Consist of aggregated transmembrane protein complexes (connexins).
    • Connexins form hexameric complexes (connexons) with a central hydrophilic pore (1.5 nm diameter).
    • Allow intercellular exchange of small molecules.
    • Some molecules (cyclic nucleotides, ions) move rapidly, coordinating cell action (e.g., heart and visceral muscles).
  • Hemidesmosomes:
    • Anchor cells to the basal lamina.
    • Transmembrane proteins are integrins rather than cadherins.
    • Integrins bind laminin molecules in the basal lamina.
  • Focal Adhesions:
    • Found in cells moving during epithelial repair or reorganization.
    • Smaller, more numerous than hemidesmosomes.
    • Composed of integrins linked indirectly to bundled actin filaments.
    • Integrins are linked to focal adhesion kinase (FAK), affecting cell adhesion, mobility, and gene expression.

Specializations of the Apical Cell Surface

  • Apical ends of columnar and cuboidal epithelial cells have specialized projections.
Microvilli
  • Cytoplasmic projections filled with array of projecting microvilli (L. villus, tuft), usually of uniform length.
  • In cells of the small intestine, microvilli are densely packed as a brush or striated border.
  • Averagemicrovillus:1 μmAverage microvillus: 1 \, \mu m long, 0.1 μm0.1 \, \mu m wide.
  • Increase surface area by 20- or 30-fold.
  • The glycocalyx covers microvilli and includes membrane-bound enzymes for digestion.
  • Each microvillus contains bundled actin filaments capped and bound to the plasma membrane via actin-binding protein.
  • Actin filaments undergo dynamic myosin-based movements.
  • Insert into the terminal web of cortical microfilaments.
  • Medical Application:
    • Celiac disease (gluten-sensitive enteropathy/sprue): Loss of microvilli brush border due to immune reaction against gluten, leading to enteritis and malabsorption.
Stereocilia
  • Less common apical process, found on epithelial cells of the male reproductive system.
  • Increase cell surface area.
  • Motion-detecting function in inner ear sensory cells.
  • Resemble microvilli in containing microfilaments and actin-binding proteins.
  • Typically longer and less motile than microvilli.
Cilia
  • Long, motile apical structures containing microtubules (not microfilaments).
  • Most cell types have at least one non-motile primary cilium with receptors and signal transduction complexes.
  • Motile cilia on cuboidal or columnar cells of many epithelia; for example, respiratory tract.
  • Typical Cilia are 5-10 μm long and 0.2 μm in diameter, longer and wider than microvilli.
  • Axoneme is internal core structure.
  • 9 + 2 arrangement of microtubules (axoneme).
  • Microtubules continuous with basal bodies (apical cytoplasmic structures).
  • Basal bodies similar to centrioles (microtubule triplets).
  • Cilia beat rapidly, moving fluid and suspended matter in one direction along the epithelium.
  • Complexes with axonemal dynein use ATP to slide adjacent microtubule doublets, bending the axoneme and producing motion.
  • Flagellum of sperm cells has a similar axonemal structure.
  • Medical Applications:
    • Immotile cilia syndrome (Kartagener syndrome): Mutations in cilia and flagella proteins, leading to respiratory infections and male infertility.

Types of Epithelia

  • Two main groups: covering (lining) and secretory (glandular) epithelia.
  • Covering epithelia are classified by the number of cell layers and the cell morphology in the outer layer.
  • Simple epithelia: one cell layer.
  • Stratified epithelia: two or more layers.
  • Based on cell shape, simple epithelia are classified as squamous, cuboidal, or columnar.
  • Most stratified epithelia are classified according to the cell shape of the superficial outer layer(s): squamous, cuboidal, or columnar.
  • Stratified squamous epithelia can be keratinized (packed with keratin filaments) or nonkeratinized (sparse keratin).
  • Stratified squamous keratinized epithelium is in the epidermis, preventing dehydration.
  • Stratified squamous nonkeratinized epithelium lines moist internal cavities (e.g., mouth, esophagus, vagina).
  • Stratified cuboidal and stratified columnar epithelia are relatively rare.
  • Transitional epithelium (urothelium) lines much of the urinary tract, characterized by dome-like umbrella cells.
  • Medical Application:
    • Vitamin A deficiency: Epithelial tissues of bronchi and urinary bladder may be replaced by stratified squamous epithelium.
  • Pseudostratified columnar epithelium: All cells attach to the basement membrane, but nuclei are at different levels, giving a stratified appearance; for example, the upper respiratory tract.
Secretory Epithelia & Glands
  • Epithelial cells produce and secrete macromolecules; they may occur in other epithelia or comprise specialized organs called glands.
  • Medical Application:
    • Chronic bronchitis: Goblet cells increase in airways, leading to excessive mucus production.
    • Ciliated pseudostratified epithelium can be transformed into stratified squamous epithelium by metaplasia.
  • Secretory cells synthesize, store, and release proteins, lipids, or complexes of carbohydrates and proteins.
  • Some gland cells secrete water and electrolytes.
  • Scattered secretory cells (unicellular glands) are common in simple cuboidal, simple columnar, and pseudostratified epithelia; for example, goblet cells.
  • Glands develop from covering epithelia by cell proliferation and growth into underlying connective tissue.
  • Exocrine glands remain connected to the surface epithelium via ducts.
  • Endocrine glands lose connection to the epithelium and lack ducts; their hormone products are absorbed by nearby blood vessels.
  • Epithelia of exocrine glands are organized as a continuous system of secretory portions and ducts.
  • Both exocrine and endocrine glands are supported by a stroma of connective tissue.
  • Classification of Exocrine Glands:
    • Simple (ducts not branched) or compound (ducts with two or more branches).
    • Tubular (short or long and coiled) or acinar (rounded and saclike) secretory portions.
    • Compound glands have branching ducts and tubular, acinar, or tubuloacinar secretory portions.
  • Three basic mechanisms of releasing product:
    1. Merocrine secretion: Exocytosis from membrane-bound vesicles.
    2. Holocrine secretion: Accumulation of product culminating in complete cell disruption.
    3. Apocrine secretion: Accumulation of product at the cell's apical end, portions of which are then extruded.
  • Exocrine glands with merocrine secretion are categorized as serous or mucous.
  • Serous cells synthesize mostly non-glycosylated proteins (e.g., digestive enzymes) and stain intensely.
  • Mucous cells contain heavily glycosylated proteins (mucins) which become hydrated to form mucus and stain poorly with eosin.
  • Some salivary glands are mixed seromucous glands.
  • Medical Application:
    Acne vulgaris: Excessive holocrine secretion of sebum and keratin triggered by testosterone, often leads to blocked ducts that become inflamed by bacterial activity.
Other Features of Glands
  • In addition to secretory cells, exocrine gland epithelia contain contractile myoepithelial cells that propel secretory products from acini into the duct system.
  • Endocrine glands specialize in protein or steroid hormone synthesis.
  • Endocrine signaling involves hormone transport in the blood to target cells.
  • Paracrine and autocrine signaling occur when hormones act on nearby cells or the secreting cell itself, respectively.
  • Important endocrine or paracrine cells also occur singly or in small groups in epithelia of the digestive, respiratory, and other organ systems.

Transport Across Epithelia

  • Many cells actively transport ions against concentration and electrical potential gradients, powered by Sodium/Potassium pump.
  • Epithelial cells transfer ions and water across the epithelium (transcellular transport).
  • Tight junctions prevent paracellular diffusion or backflow between the cells.
  • Epithelia of kidney tubules are key sites for ion and water transport.
  • Proximal renal tubule cells are structurally specialized for transcellular transport.
  • All cells internalize extracellular molecules and fluid using endocytosis and transcytosis.

Renewal of Epithelial Cells

  • Epithelial tissues are renewed continuously by mitotic activity and stem cell populations.
  • Renewal rates vary widely.
  • In stratified epithelial tissues, stem cells and mitosis occur only within the basal layer.
  • Some epithelia have stem cells located in restricted niches some distance from transit amplifying cells and differentiating cells.
  • Medical Application:
    • Both benign and malignant tumors can arise from most types of epithelial cells.
    • Malignant tumors of epithelial origin are called carcinomas; those from glandular epithelial tissue are called adenocarcinomas, the most common tumors in adults after age 45.
  • Some epithelia are prone to abnormal growth or dysplasia which can progress to precancerous neoplasia.
  • Under certain abnormal conditions, one type of epithelial tissue may undergo transformation into another (metaplasia); for example, in smokers, the ciliated pseudostratified epithelium of the bronchi can be transformed into stratified squamous epithelium.