epiteelkude

Epiteelkude, or epithelial tissue, is one of the four basic tissue groups in animals. It forms the coverings and linings of external surfaces, internal cavities and organs, and it also makes up glandular tissue that secretes substances such as enzymes, mucus and hormones.

Key features of epithelial tissue include a densely packed cellular organization with minimal extracellular matrix, apical surfaces that face a lumen or the external environment and basal surfaces anchored to a basement membrane, and a tendency to be avascular. Epithelial cells are typically well supplied with nerves and rely on diffusion from underlying connective tissue for nutrients. They exhibit high regenerative capacity and can repair themselves after injury.

Classification commonly uses two axes. By cell shape: squamous (flat), cuboidal (dice-like), and columnar (tall). By number of cell layers: simple (one layer), stratified (two or more layers), and pseudostratified (appears multilayered but each cell contacts the basement membrane). Specialized forms include transitional epithelium (urothelium), which lines the urinary tract and accommodates stretching; ciliated epithelium, often with goblet cells to produce mucus; and glandular epithelium, which forms exocrine glands (ducts) and endocrine glands (ductless).

Functions of epithelial tissue are diverse: protection (skin and mucosal surfaces), secretion (glandular epithelia), absorption (intestinal lining), filtration and selective permeability (kidney tubules), gas exchange surfaces (alveolar epithelium), and sensory roles in specialized epithelia such as taste and smell. In many organs, epithelia also participate in movement of mucus and other materials via ciliary action.

Developmentally, epithelial tissue originates from all three embryonic germ layers. It is typically involved in carcinomas, cancers arising from epithelial cells, underscoring its clinical relevance.