epithelial

Epithelial tissue, or epithelium, forms the covering of body surfaces, lines body cavities and ducts, and constitutes the secretory units of glands. It consists of tightly packed cells with little extracellular matrix, and it is avascular, obtaining nutrients through diffusion from underlying connective tissue. Epithelia exhibit polarity, with an apical (facing the lumen or exterior) surface, a basal surface attached to an underlying basement membrane, and lateral borders that contain specialized intercellular junctions.

Classification of epithelia typically uses two criteria: the number of cell layers and cell shape. Simple epithelia have a single cell layer, while stratified epithelia have multiple layers. Shapes include squamous (flattened), cuboidal (cube-shaped), and columnar (tall). Transitional epithelia can stretch, as seen in the urinary tract. Specialized epithelia include pseudostratified ciliated columnar epithelium, often with goblet cells, found in parts of the respiratory tract.

Common examples illustrate function and location: stratified squamous epithelium forms the epidermis, providing protection; simple squamous epithelium lines air sacs in the lungs and the glomeruli of the kidneys, aiding in diffusion and filtration; simple cuboidal epithelium lines many glands and ducts and kidney tubules, supporting secretion and absorption; simple columnar epithelium lines much of the intestine and often has microvilli for absorption. Glandular epithelia include endocrine glands (secreting into blood) and exocrine glands (secreting onto surfaces or ducts).

Functions of epithelium include protection, secretion, absorption, filtration, and sensation. It has a high turnover rate and plays essential roles in organ systems and homeostasis. Embryologically, epithelia arise from all three germ layers.