Simple (epithelial classification) (SIM-pul): A classification term describing an epithelium composed of a single layer of cells, where all cells rest directly on the basement membrane. Its function is typically related to absorption, secretion, or filtration.
Squamous (SKWAY-mus): A classification term describing epithelial cells that are flat, thin, and scale-like (having a greater width than height); when applied to simple epithelia, it facilitates rapid diffusion (e.g., in the lung alveoli) or providing a smooth, frictionless surface (e.g., mesothelium).
Cuboidal (kyoo-BOY-dul): A classification term describing epithelial cells that are approximately square or cube-shaped (having roughly equal height and width); typical of surfaces involved in secretion and absorption, such as the lining of small ducts and kidney tubules.
Columnar (kuh-LUM-ner): A classification term describing epithelial cells that are tall and cylindrical (having a greater height than width); typical of surfaces involved in absorption (e.g., small intestine) or high-volume secretion.
Basement membrane (BAYS-munt MEM-brayn): A thin, acellular layer of specialized extracellular matrix (collagen and glycoproteins) that anchors the basal surface of all epithelial cells to the underlying connective tissue; it acts as a selective barrier. It is composed of a basal lamina (secreted by epithelial cells) and reticular lamina (secreted by fibroblasts). It is difficult to see under normal H&E staining conditions, however, can be visualized with PAS or reticular staining.
Pseudostratified (soo-doh-STRAT-uh-fyde): A classification term describing an epithelium that appears to have multiple layers due to the varying heights of the cells and the staggered position of their nuclei, but is actually simple because all cells rest on the basement membrane; a classic example is respiratory epithelium.
Microvilli (brush border) (my-kroh-VIL-eye): Numerous, extremely short, finger-like extensions of the apical plasma membrane of certain epithelial cells (especially simple columnar); they significantly increase the surface area available for absorption (e.g., in the small intestine and proximal convoluted tubule). Acidophilic staining due to their content of actin microfilaments and typical abundance of enzymes in the cell membrane (e.g., brush border enzymes).
Cytoplasm (SY-toh-plaz-um): The material within the cell membrane of an epithelial cell (or any cell), excluding the nucleus; its composition often provides clues to the cell's function (e.g., basophilic if rich in RER for protein secretion, eosinophilic if rich in mitochondria).
Stratified (STRAT-uh-fyde): A classification term describing an epithelium composed of two or more layers of cells, where only the basal layer rests on the basement membrane; its main function is protection (e.g., skin, esophagus).
Non-keratinized (Non KER-uh-tuh-nyzed): A descriptive term for stratified squamous epithelium where the apical surface cells retain their nuclei and are metabolically active; this provides a protective, moist lining (e.g., lining many of the bodies openings - the oral cavity, esophagus, anal canal, vagina).
Keratinized (KER-uh-tuh-nyzed): A descriptive term for stratified squamous epithelium where the apical surface cells are dead, lack nuclei, and are filled with the tough, protective protein keratin; this provides a highly durable, water-resistant surface (e.g., the epidermis of the skin).
Umbrella cells (UM-brel-uh selz): Large, specialized surface cells of the transitional epithelium/urothelium; they possess a thick, impermeable apical membrane that protects the underlying cells from hypertonic urine. Under resting conditions, these cells appear domed, but when stretched (e.g., in the urinary bladder), membranous plaques allow them to expand to almost squamous in appearance.
Transitional epithelium (Urothelium) (tran-ZISH-un-ul ep-uh-THEE-lee-um/yoor-oh-THEE-lee-um): A specialized stratified epithelium lining the urinary system (e.g., ureters and bladder); it is characterized by its dome-shaped umbrella cells and its ability to stretch significantly without damage.
Duct (duckt): A tube-like structure lined by epithelium (ductal cells) that conveys the secretion of an exocrine gland away from the secretory acini and onto an epithelial surface or into a lumen.
Ductal cell (DUK-tul sell): The epithelial cells that form the walls of a duct; these cells vary in shape (often cuboidal or columnar) depending on the size of the duct and can modify the ionic content of the glandular secretion.
Exocrine gland (EK-suh-krin gland): A gland that secretes its product onto an external or internal epithelial surface via a duct system (e.g., salivary glands, sweat glands, pancreas exocrine portion).
Endocrine gland (EN-duh-krin gland): A ductless gland that secretes its product (hormones) directly into the bloodstream or surrounding tissue fluid; these glands are highly vascularized with fenestrated capillaries (e.g., pituitary, thyroid, adrenal).
Capillary (KAP-ih-lair-ee): The smallest blood vessels, composed of a single layer of endothelium; these are highly permeable and are essential for providing nutrients and oxygen to the underlying connective tissue and for carrying away the hormones secreted by endocrine glands.
Surface cell (Glandular Tissue) (SUR-fis sell): The epithelial cell that lines the lumen of a secretory gland (acinus or tubule); this cell type (e.g., chief cell, luminal cell) synthesizes and releases the glandular product.