tissuesepithelial

Epithelial tissue, or epithelium, is one of the four basic tissue types in the body. It forms continuous sheets that cover body surfaces, line cavities and hollow organs, and compose the secretory portions of glands. Epithelial cells are closely packed with little extracellular matrix, and the tissue as a whole is typically avascular, receiving nutrients by diffusion from underlying connective tissue. Epithelia have an apical surface facing the exterior or a body cavity, a basal surface anchored to a basement membrane, and lateral surfaces with numerous cell junctions.

Classification and organization follow two main schemes. By cell shape, epithelia are described as squamous (flat), cuboidal (cube-shaped), or columnar (taller than wide). By layering, they are simple (one cell layer), stratified (two or more layers), pseudostratified (appears layered but all cells contact the basement membrane), or transitional (multilayered and stretchable, found in the urinary tract). Glandular epithelia form the secretory portions of glands and can be further categorized as exocrine (ducts present) or endocrine (ductless, secreting into the bloodstream).

Functions of epithelial tissue are diverse and essential. It provides protection against mechanical and chemical injury, forms selective barriers that regulate permeability, and participates in absorption, secretion, and sensation. Specialized features include microvilli to increase surface area for absorption (as in the small intestine), cilia to move substances along surfaces (as in the respiratory tract), and goblet cells that secrete mucus. Keratinized epithelia, such as the skin, gain added protection through keratin, while non-keratinized mucosal epithelia remain moist.

Glands arise from epithelial tissue and contribute to endocrine and exocrine secretions. Epithelial tissues regenerate readily from resident stem cells, enabling repair after injury. Malignant transformation of epithelia can lead to carcinomas, which are among the most common cancer types.