Epithelial tissue

• Epithelia are specialized layers that line the internal and cover the external surfaces of the body.
• An epithelium consists of a sheet of cells lying close together with little intercellular space.
• These cells have distinct biochemical, functional, and structural domains that confer polarity, or sidedness, to epithelia.
• First-degree burns are lesions caused by heat, friction, or other agents.
• Damage in first-degree burns is limited to the superficial layers of the epithelium (usually the epidermis of the skin).
• Redness and edema occur, but blisters do not form in first-degree burns.
• Mitotically active cells remain viable in the deeper layers of the epidermis in first-degree burns.
• These cells divide and replace the damaged or destroyed cells in first-degree burns.
• The basement membrane separates the epithelium from underlying connective tissue and blood vessels.
• Epithelia are avascular and receive nourishment by diffusion of molecules through the basal lamina.
• Epithelia are classified into various types on the basis of the number of cell layers (one cell layer is simple; more than one is stratified) and the shape of the superficial cells.
• Pseudostratified epithelia appear to have multiple cell layers, but all cells are in contact with the basal lamina.
• Endocrine glands may be unicellular (e.g., individual endocrine cells in gastrointestinal and respiratory epithelia) or multicellular (e.g., adrenal gland), and they lack a duct system.
• Serous secretions are watery and often rich in enzymes.
• In multicellular glands, secretory material is released into fenestrated capillaries, which are abundant just outside the basal lamina of the glandular epithelium.
• Mixed secretions contain both mucous and serous components.
• Glands may have ducts between lobes (interlobar), within lobes (intralobar), between lobules (interlobular), or within lobules (intralobular), such as striated and intercalated ducts.
• Multicellular glands secrete various substances, including mucus, serous secretions, and mixed secretions.
• Mucus is a viscous material that usually protects or lubricates cell surfaces.
• In merocrine glands (e.g., parotid gland), the secretory cells release their contents by exocytosis.
• In holocrine glands (e.g., sebaceous gland), the entire secretory cell along with its contents is released.
• In apocrine glands (e.g., lactating mammary gland), part of the apical cytoplasm of the secretory cell is released along with the contents.
• Transcellular transport of molecules from one epithelial surface to another occurs by various processes, including diffusion of oxygen and carbon dioxide across the epithelial cells of lung alveoli and capillaries, carrier protein–mediated transport of amino acids and glucose across intestinal epithelia, vesicle-mediated transport of immunoglobulin A (IgA) and other molecules, absorption via endocytosis or pinocytosis in various organs (e.g., the proximal convoluted tubule of the kidney), and secretion of various molecules (e.g., hormones, mucus, proteins) by exocytosis.
• Adjacent plasma membranes are separated by gap junctions, which are communicating junctions where ions and small molecules are permitted to pass between adjoining cells.
• Zonulae occludentes are occluding junctions where the outer leaflets of the apposing cell membranes fuse with each other, preventing material from taking the paracellular route between the connective tissue and the lumen.
• The basal lamina is produced by the epithelium resting upon it and is composed mainly of type IV collagen, laminin, entactin, and proteoglycans (rich in heparan sulfate).
• The intermediate junction (belt desmosome; zonula adherens) is the zone that surrounds the entire perimeter of epithelial cells just basal to the tight junction.
• The basal lamina is an extracellular supportive structure 20 to 100 nm thick that is visible only by electron microscopy.
• Hemidesmosomes function in mediating the adherence of epithelial cells to the underlying basal lamina.
• E-cadherin is markedly dependent on calcium ions for promoting adhesion at this structurally supportive junction.
• The intermediate junction is characterized by a 10- to 20-nm separation between the adjacent plasma membranes, where the extracellular portions of cadherin molecules occupy the intercellular space.
• The intermediate filaments form a meshwork that is attached to the E-cadherins by the other molecules.
• A mat of actin filaments is located on each of its cytoplasmic surfaces.
• Plaque maculae adherentes are characterized by desmogleins and E-cadherins transmembrane glycoproteins, whose cytoplasmic ends are associated with a plaque composed of desmoplakins.
• The actin filaments are linked, via β-actinin and vinculin, to the transmembrane glycoprotein E-cadherin.
• Intermediate filaments, forming hairpin loops, enter and exit the plaque.
• The tight junction is analogous to the fascia occludens, a ribbonlike area of fusion between transmembrane proteins on adjacent endothelial cells lining capillaries.
• Selective permeability results from the presence of tight junctions between epithelial cells and permits fluids with different compositions and concentrations to exist on separate sides of an epithelial layer (e.g., intestinal epithelium).
• A connective tissue capsule may surround the gland, or septa of connective tissue may divide the gland into lobes and smaller lobules.
• The inner two triplets of the basal body give rise to the doublet microtubules of the cilium axoneme.