épithélium

Epithelium, or épithélium, is a tissue that lines the surfaces and cavities of the body and forms glands. It consists of closely packed cells bound by junctions, arranged in sheets with an underlying basement membrane. Epithelium is avascular and receives nutrients by diffusion from adjacent connective tissue, and it tends to be highly regenerative due to resident stem cells in the basal layer. The tissue provides protection, absorption, secretion, filtration, and, in many regions, sensation or immune defense.

Classification of epithelium is traditionally based on (a) the number of cell layers: simple (one layer), stratified (multiple layers), and pseudostratified (appears layered but is not); and (b) cell shape: squamous (flat), cuboidal, and columnar. Specialized variants include keratinized stratified squamous epithelium (as in skin), non-keratinized varieties (such as oral mucosa), simple or pseudostratified ciliated columnar epithelium (commonly in the respiratory tract), and transitional epithelium (urothelium) found in the bladder and ureters that accommodates distension.

Epithelium also gives rise to glandular tissue, forming endocrine glands (ductless) and exocrine glands (with ducts). Epithelial cells are joined by tight junctions, adherens junctions, desmosomes, and gap junctions, and they exhibit apical-basal polarity with distinct membrane domains. The epithelium plays a central role in selective permeability, secretion, and barrier formation across many organ systems. In pathology, epithelial tissues can undergo metaplasia or dysplasia and can give rise to carcinomas.