epitelium

Epitelium is a tissue that lines exposed surfaces and body cavities, forms glands, and covers the exterior of the body. It consists of closely packed cells with little extracellular matrix, arranged in continuous sheets atop a basement membrane. Epitelium is avascular and relies on diffusion from underlying tissues for nutrients and oxygen. It exhibits polarity, with an apical surface facing a lumen or the outside and a basolateral surface connected to neighboring cells by junctions. It serves as a selective barrier and protective covering, participates in absorption and secretion, and has a high regenerative capacity through stem or progenitor cells in its basal layer.

Epitelium is classified by arrangement as simple (one cell layer), stratified (two or more layers), or pseudostratified (appearing layered but all cells contact the basement membrane). By shape, it is described as squamous (flat), cuboidal (cube-shaped), or columnar (tall). Specialized forms include glandular epithelium, which forms secretory units and ducts, as well as ciliated or microvillar epithelia. Some epithelia are adapted for sensation or specific functions, such as ciliated epithelium in airways and absorptive epithelium in the intestine.

Functions of epitelium include protection (as in the skin and mucous membranes), selective permeability for absorption and filtration (such as in the intestines and kidney tubules), secretion (glands and mucous membranes), and sensation (neuroepithelia). It lines many organs and surfaces, including the skin (stratified squamous), the lining of the digestive and respiratory tracts (simple or pseudostratified epithelia), and the ducts and secretory units of glands. Dysfunctions of epitelium can accompany pathologies such as metaplasia, dysplasia, and carcinomas arising from epithelial tissues.