Epiteelium

Epiteelium, commonly referred to as epithelium, is a sheet of tightly packed cells that lines body surfaces, internal cavities, ducts, and structures such as glands. It provides a protective barrier, mediates selective absorption and secretion, and contributes to sensation and immune defense. Epithelial tissue is avascular, relying on nearby tissues for nourishment, and rests on a basement membrane that anchors it to the underlying connective tissue.

Epithelial cells are arranged with polarity: an apical surface facing a lumen or the external environment, a basolateral surface contacting neighboring cells, and a basal region attached to the basement membrane. The apical surface may bear microvilli or cilia, enhancing absorption or movement of substances. Lateral membranes contain junctional complexes—tight junctions, adherens junctions, and desmosomes—that regulate permeability and maintain tissue integrity.

Classification can follow two schemes. By cell layers: simple (one cell layer), stratified (two or more layers), pseudostratified (nuclei appear at different levels but all cells touch the basement membrane), and transitional (specialized for distension, as in the urinary tract). By cell shape: squamous, cuboidal, or columnar; keratinized or non-keratinized forms describe protective surfaces such as skin versus mucosal linings.

Functions include protection, filtration and diffusion, secretion (glands), absorption, and sensory roles. Epithelia line the skin, gastrointestinal tract, respiratory tract, urinary tract, blood vessels (endothelium), and body cavities (mesothelium), and they form secretory glands and ducts.

Development and maintenance involve contributions from all germ layers, with rapid turnover in many epithelia through resident stem cells, enabling repair after injury.