Epithelia

Epithelia are sheets of closely packed cells that line the surfaces and cavities of the body and form the lining of glands. They cover body surfaces, line hollow organs and vessels, and provide a selective barrier between internal and external environments. Epithelial tissues rest on a basement membrane, display apical and basolateral polarity, and are generally avascular, relying on underlying tissues for nutrients. They have a high capacity for renewal through stem or progenitor cells at or near the basal layer.

Epithelia are classified by two criteria: cell layers and cell shape. Layerwise, they are described as simple (one cell layer thick), stratified (two or more layers), or pseudostratified (appearing layered but all cells contact the basement membrane). By shape, they are named as squamous (flat), cuboidal (cube-shaped), columnar (taller than wide), and transitional (specialized for stretch, as in the urinary bladder). Transitional epithelia can change shape between cuboidal and squamous as they stretch. Common combinations include simple squamous, simple cuboidal, simple columnar, stratified squamous, and pseudostratified columnar.

Functions of epithelia include protection, selective absorption and secretion, filtration, and sensation. Specialized epithelia form glands (endocrine and exocrine) and, in many regions, ciliated or microvilliated surfaces to move mucus or increase surface area. Junctional complexes such as tight junctions, desmosomes, and gap junctions help maintain integrity and communication. Epithelial cells regenerate from basal stem cells, enabling repair after injury.

Key locations include skin (stratified squamous keratinized), the lining of the gut (simple columnar with microvilli and goblet cells), alveolar sacs (simple squamous), renal tubules (simple cuboidal), and the respiratory tract (pseudostratified ciliated columnar with goblet cells).