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Epithelium is a type of tissue that covers the surfaces of the body, both external and internal. It forms linings for organs, cavities, and canals, as well as forming glands. Epithelial tissue is characterized by cells that are closely packed together with very little extracellular space. These cells are arranged in layers or a single layer, and they have distinct apical (top) and basal (bottom) surfaces. The apical surface often faces a lumen or the external environment, while the basal surface rests on a basement membrane, a thin layer of connective tissue.

Epithelial tissues serve a variety of functions, including protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception. The specific function of an epithelium is closely related to its structure. For example, stratified squamous epithelium, with multiple layers of flattened cells, provides a robust protective barrier for the skin. Simple columnar epithelium, with a single layer of tall, rectangular cells, is often specialized for absorption and secretion, as seen in the lining of the digestive tract. Glandular epithelia are responsible for producing and releasing substances like hormones, mucus, and enzymes. Epithelial cells have a high rate of regeneration, which is crucial for repairing damage and replacing worn-out cells.