Epithelial Tissue: Structure and Function

• Organs consist of four basic tissue types: epithelial, connective, muscular, and nervous.
• Each tissue assembles from similarly specialized cells performing specific functions.
• Tissues contain cells and the extracellular matrix (ECM), with variable proportions and morphologies specific to each organ.
• Main characteristics of the four basic tissue types are summarized in Table 4-1.
• Epithelial tissues include: covering/lining and protection of surfaces (e.g., epidermis), absorption (e.g., the intestinal lining), secretion (e.g., parenchymal cells of glands)
• Epithelial tissues consist of aggregated polyhedral cells adhering strongly to each other and to a thin ECM layer.
• These tissues form cellular sheets lining organ cavities and covering body surfaces.
• Epithelia line all external and internal body surfaces
• Substances entering or leaving an organ must cross this tissue.
• Specialized epithelial cells may be contractile (myoepithelial cells) or sensory cells, such as taste buds or olfactory epithelium.

Characteristic Features of Epithelial Cells

• Cells range in shape from tall columnar to cuboidal to low squamous.
• Cell size and morphology are generally dictated by function
• Nuclei shape can be elliptic, spherical, or flattened, corresponding to cell shape.
• Columnar cells tend to have elongated nuclei, squamous cells have flattened nuclei, and cuboidal/pyramidal cells have more spherical nuclei.
• The number and shape of stained nuclei indicate cell shape and density since epithelial cell membranes are often indistinguishable by light microscopy.
• Nuclei also determine the number of cell layers in an epithelium, used for classification.
• Most epithelia are adjacent to connective tissue containing blood vessels, supplying nutrients and O2 and waste products are passed between them using the lamina propria.
• Underlying the epithelia lining the digestive, respiratory, and urinary systems, this area of contact is increased by papillae projecting from the connective tissue into the epithelium.
• Papillae are most common in epithelial tissues subject to friction, like skin or tongue coverings.
• Epithelial cells exhibit polarity, with uneven distribution of organelles and membrane proteins, the basal pole contacts ECM, and the apical pole faces a space.
• Lateral surfaces between cuboidal/columnar cells have folds increasing the cells' area and functional capacity.
• Basement Membrane
• It's a thin, extracellular, felt-like sheet of macromolecules where the basal surface of all epithelia rests.
• Acts as a semipermeable filter for substances reaching epithelial cells from below.
• Glycoproteins and other components in this structure can be stained and visualized with a light microscope
• TEM resolves two parts of the basement membrane:
  • The basal lamina is a thin, electron-dense, sheetlike layer of fine fibrils closest to epithelial cells.
  • The reticular lamina is a more diffuse and fibrous layer beneath the basal lamina.
• Macromolecules of the basal lamina are secreted from the basal sides of epithelial cells and form a sheetlike array and includes components such as:
  • Type IV collagen: Monomers self-assemble into a mesh-like network.
  • Laminin: Glycoproteins attach to transmembrane integrin proteins in the basal cell membrane and project through the type IV collagen mesh.
  • Nidogen and perlecan: Cross-link laminins to the type IV collagen network, helping with structure, binding, and porosity.
• Basal laminae (external laminae) are often called external laminae serve as barriers regulating macromolecular exchange surrounding muscle cells, nerves, and fat-storing cells.
• Reticular lamina contains type III collagen and is bound to the basal lamina by anchoring fibrils composed of type VII collagen.
• Basement membrane functions:
  • Acting as filters with structural support and attachment for epithelial cells to connective tissue.
  • Organizing integrins and other proteins in the epithelial cell plasma membrane.
  • Maintaining cell polarity, localizing endocytosis, signal transduction, and other activities.
  • Serving as a scaffold for rapid epithelial repair and regeneration.

Intercellular Adhesion & Other Junctions

• Structures provide adhesion and communication between cells, numerous in epithelia.
• Epithelial cells adhere strongly to each other and basal laminae, especially to friction or mechanical forces.
• Epithelial cells' lateral surfaces have specialized intercellular junctions with different functions:
  • Tight junctions: Form a seal between cells.
  • Adherens/anchoring junctions: Strong cell adhesion sites.
  • Gap junctions: Channels for communication between adjacent cells.
• Tight junctions (zonulae occludens) are most apical and their function is to prevent molecules from passing through passive flow between the cells.
• Tight junctions prevent movement of membrane lipids and proteins between apical and basolateral cell surfaces, to maintain distinct membrane domains.
• Adherens junctions/zonula adherens: Encircle the cell, anchoring it to neighbors (adherent junction), Cell adhesion mediated by cadherins (transmembrane glycoproteins that bind each other with Ca2+)
• Cadherins bind catenins that link to actin filaments with actin-binding proteins at their cytoplasmic ends that make the "terminal web" function together,
• Desmosome/macula adherens: Anchoring junction that does not form a belt around the cell, and resembles 'spot-welds'.
• Desmosomes contain cadherin family members, desmogleins, and desmocollins.
• Desmoplakins (electron-dense plaque) in turn bind intermediate filament proteins.
• Connected to cable-like filaments of cytokeratin used for strong cellular adhesion throughout epithelium.
• Gap junctions: mediate intercellular communication instead of adhesion or occlusion.
• Gap junctions are abundant and are also functionally important in nearly all mammalian tissues.
• Cryofracture preparations: show these consist of aggregated transmembrane protein complexes forming circular patches.
• Transmembrane gap junction proteins (connexins) form hexameric complexes which allow for the exchange of molucules from one cell to another.
• Molecules include cyclic nucleotides and ions, that allow tissues to act coordinated instead of independent units.
• Hemidesmosomes: anchoring junctions to the basal lamina where transmembrane proteins (integrins) indirectly link to cytoskeletal components (instead of cadherins).
• Focal adhesion/contact: junctions are numerous and consist of integrins linked to bundled actin filaments, which function is signalling for cell adhesion, mobility, by integrin binding to specific ECM proteins

Specializations of the Apical Cell Surface

• Apical ends of columnar and cuboidal epithelial cells have specialized projecting structures.
• Function to increase apical surface area for absorption or move substances along the surface.
• Microvilli
  • Cells have cytoplasmic projections: temporaries and variable reflecting actin filament movements.
  • Epithelia specialized for absorption: have projecting microvilli increasing the cells lumen in uniform length:
  • Visible a brush or striated border (lining the small intestine), average microvillus is 1 µm long/0.1 µm wide, function by absorption due to high surface area.
• Covering microvilli contains bundled actin filaments capped and that's bound to the surrounding plasma membrane by actin-binding proteins.
• Microfilament arrays are dynamic and undergo myosin-based movements, maintaining optimal conditions for absorption
  • Actin filaments insert into the terminal web of cortical microfilaments at the base of microvilli.

Stereocilia

• Less common type of apical process, and increases cells' surface area, facilitating absorption (male reproductive system).
• Also important components inner ear sensory cells (motion-detection).
• Consist of similar diameters, arrangements of microfilaments and actin-binding proteins, and connections to the cell's terminal web.
• They are typically much longer and less motile than microvilli, and may branch distally.

Cilia

• Motile apical structures containing internal arrays of microtubules.
• Primary cilium: short, nonmotile projection enriched with receptors and signal transduction complexes (detection of light, odors, motion, liquid flow).
• Motile cilia are abundant on cuboidal or columnar cells of many epithelia.
• Typical cilia are 5-10 µm long and 0.2 µm in diameter.
• Consist of nine peripheral microtubule doublets arranged around two central microtubules (axoneme).
• Ciliary motion occurs through changes in axoneme conformation, with accessory proteins making each cilium relatively stiff but elastic, and through the protein dynein.

Types of Epithelia

• Divided into two main groups: covering (lining) epithelia (cover the surface or lines the cavities of an organ) and secretory (glandular) epithelia.
• Organized into one or more layers that cover the surface or line the organ cavities (according to number of cell layers and cell morphology);
• Simple epithelia: contain one cell layer and are classified based on cell shape (Simple squamous (thin cells), cuboidal (width=thickness), or columnar (taller wide).
• Stratified epithelia contain two or more layers Classified by the cell shape of the superficial outer layers.
• The very thin surface cells can be:
  • "keratinized" (packed with keratin filaments, water loss prevention)
  • "nonkeratinized" (relatively sparse keratin, internal cavities use.)
• Stratified squamous keratinized is usually the epidermis, where water loss is prevents by cuboidal cell differentiation towards thin metabolically packets of keratin
• Unique transitional epithelium/urothelium: lines much of the urinary tract, extending from kidneys to urethra and protects from hypertonic effects or urine with dome-like umbrella cells .
• Pseudostratified Columnar Epithelium: All cells are attached to basement membrane unevenly causing a stratified looking appearance.

Secretory Epithelia & Glands

• Main function is to produce and secrete macromolecules; may occur in epithelia with other functions or comprise glands.
• Secretory cells may synthesize, store, and release: proteins (pancreas), lipids (adrenal/sebaceous glands), protein and carbohydrates complexes (salivary glands.); Mammary glands: all three
• Glands are both exocrine glands, that exocrine glands that release their product through tubular ducts that are lined with epithelium and endocrine glands that release hormones.
• In the glands the secretory units are supported by stroma connective tissue.
• Key points of exocrine gland structures include:
  • Glands: Simple (ducts not branched) or compound (branched ducts)
  • Secretory: Tubular or acinar/rounded(if branched then branched)
  • Compound: duct branches w tubular, acinar and tubuloacinar.
• Three basic mechanisms for product release:
  • Merocrine secretion: (Protein/glycoprotein) Exocytosis from membrane-bound vesicles/secretory granules
  • Holocrine secretion: (cells enlarge) product accumulates and undergoes terminal differentiation, cells completely disrupt release products/debris.
  • Apocrine secretion: Apical ends of cells accumulates in portions which are then pinched by cell membranes and by then are releases.
• Glands with merocrine: further categorized by their secretory products, with serous cells being (protein that's not glycosylated) or mucous (heavily glycosylated).
• some glands are mixed seromucous glands (digestive enzymes and mucus). Additional notes: Exocrine gland (sweat, lacrymal, salivary, mammary) contain Myoepithelal cells at the basal ends that help strong contractions via filaments aid propelling acini secretion products to ducts

Transport Across Epithelia

• Cells actively transport ions against concentration and electrical potential gradients (e.g., Na+/K+-ATPase).
• Allows regulation for the transfer of ions (pumps) and water (via the membrane channels called aquaporins)in direction through the epithelia (transcellular transport). Apical tight junctions prevent paracellular diffusion or backflow between the cells. Epithelia or kdney aid in water and ion transfer that maintain the salt composition for the body.
• All cells: internalize extracellular molecules/fluids for membrane/vesicle forming through cytoplasm (endocytosis). The thin cells have vesicles: released by exocytosis The Transcytosis: Occurs between the apical and basolateral.

Renewal of Epithelial Cells

• Continuously are renewed via Stem Cell populations with the highest rates being (Intestines is replaced weekly) and low (Large Glands).
• Cancer is linked for both benign and malignant with malignant being carcinomas (glandular epithelial are adenocarcinomas).
• Epithelia that under go abnormal growth and dys-genesis become cancerous . Reversible with out the addition of early neoplastic .
• Epithelial which undergo transformation = metaplasia( ciliated pseudo straterfied turns into stratified squamous).

Explore the critical roles of stained nuclei in identifying epithelial tissue. Understand the significance of papillae in friction-prone areas. Discover how the basement membrane facilitates interaction with connective tissue, and the importance of cell polarity.