epithelialtissue

Epithelial tissue is one of the four basic tissue types in animals. It forms continuous sheets that line surfaces and cavities, forms glands, and is tightly joined by intercellular junctions. It is avascular and relies on diffusion from underlying tissues. It has polarity with apical and basolateral surfaces, and rests on a basement membrane. It has high regenerative capacity through stem cells in the basal layer.

Classification in epithelial tissue is typically based on layering and cell shape. Simple epithelia have a single cell layer; stratified epithelia have multiple layers; pseudostratified epithelia appear layered but all cells contact the basement membrane. By shape, epithelia are described as squamous (flat), cuboidal (cube-like), or columnar (tall). Transitional epithelium can stretch. Some epithelia are specialized with cilia or microvilli, and goblet cells may secrete mucus.

Functions of epithelial tissue include protection against mechanical injury, dehydration, and pathogens; selective permeability for absorption and filtration; secretion of enzymes, hormones, and mucus; and sensory roles. Glandular epithelia form endocrine and exocrine glands and contribute to secretion of various fluids.

Locations and organization vary by type. Epithelial tissues line body surfaces and organ cavities and form glands. The skin epidermis is a keratinized stratified squamous epithelium; the lining of the intestinal tract is simple columnar epithelium; the respiratory tract often contains pseudostratified ciliated columnar epithelium; kidney tubules are simple cuboidal epithelium.

Clinical notes often emphasize that many cancers arise from epithelial cells, known as carcinomas. Epithelial tissue also demonstrates notable regenerative capacity, contributing to wound healing and tissue maintenance. Developmentally, epithelia originate from multiple germ layers and participate in a wide range of physiological functions.