epithelium

Epithelium is a tissue that covers body surfaces, lines cavities and ducts, and forms the secretory units of glands. It sits on a basement membrane and consists of tightly packed cells with little extracellular matrix between them. Epithelial tissue provides protection, serves as a barrier to control permeability, and participates in absorption, secretion, filtration, and sensation. Many epithelia are avascular and receive nutrients by diffusion from underlying tissues.

Epithelia are classified by cell layers and by cell shape. Simple epithelia have a single cell layer, while stratified epithelia have multiple layers. Pseudostratified epithelia appear stratified but are a single layer of irregularly situated cells. Cell shapes include squamous (flat), cuboidal (cube-like), and columnar (taller than wide). Epithelia can be keratinized (as in the skin) or non-keratinized (such as mucosal linings). Transitional epithelium is a specialized stratified type found in the urinary tract. Glandular epithelium forms glands and can be endocrine (hormone-secreting, without ducts) or exocrine (duct-bearing, releasing onto surfaces).

Common locations include the skin (stratified squamous keratinized), the lining of hollow organs and vessels (endothelium and various simple epithelia), the digestive tract (simple columnar with microvilli), and the respiratory tract (pseudostratified ciliated columnar with goblet cells). The epithelium also lines body cavities (mesothelium) and forms the functional tissue of many glands.

Key features include cell polarity with apical, lateral, and basal surfaces, specialized junctions (tight junctions, adherens junctions, desmosomes), and surface modifications such as microvilli or cilia. Epithelial tissue renews rapidly from stem cells in the basal layer, and abnormalities can lead to conditions including metaplasia and carcinomas.