epithelio

Epithelio, or epithelium, is a tissue that covers external surfaces and lines internal cavities, ducts, and glands. It consists of closely packed cells with minimal extracellular matrix, arranged in one or more layers and anchored to a basement membrane. Epithelial tissue is typically avascular and relies on diffusion from underlying tissues for nutrients; it is usually highly regenerative, renewing through mitosis to replace lost cells. Cells exhibit apical-basal polarity, with the apical surface facing a lumen or exterior and the basolateral surface interacting with underlying tissues. Specialized cell junctions hold adjacent cells together and regulate permeability, including tight junctions, adherens junctions, desmosomes, and gap junctions.

Epithelia are classified by layer number and cell shape. Layerwise, they can be simple (one cell layer), stratified (two or more layers), or pseudostratified (appearing multi-layered due to nuclei at different heights). By shape, they include squamous (flat), cuboidal (cube-like), columnar (tall), and transitional epithelia. Some epithelia are specialized, such as ciliated epithelia in the respiratory tract or keratinized stratified epithelia in the skin. Glandular epithelia form secretory units and are categorized as exocrine (releasing products onto surfaces or through ducts) or endocrine (releasing hormones into the bloodstream).

Common locations include the skin (epidermis) and mucosal linings of the digestive, respiratory, and urinary tracts, as well as ducts and glands. The epithelium also forms the mesothelium lining body cavities. Functions encompass protection, selective permeability, absorption, secretion, and sensation.

Clinical relevance includes the origin of many cancers (carcinomas) from epithelial tissues and the tissue’s central role in wound healing and barrier maintenance.