epithelien

Epithelien is the German term for epithelia, the tissues that line body surfaces and cavities and form many glands. Epithelial tissue covers exposed surfaces, lines internal passages such as the gut and airways, and forms the secretory portions of many glands. It is characterized by closely packed cells with minimal extracellular matrix, resting on a basement membrane, and it shows distinct polarity with an apical surface facing a lumen or external environment and a basal surface anchored to underlying tissue. In many epithelia, the apical surface bears microvilli or cilia, enhancing absorption or movement of substances.

Epithelien are generally avascular, receiving nutrients by diffusion from the underlying connective tissue. They renew rapidly through stem cells located in the basal layer, allowing quick repair after injury. Developmentally, epithelial tissues originate from all three germ layers (ectoderm, endoderm, and mesoderm), with glandular epithelia formed by invagination or proliferation of epithelial cells.

Classification commonly distinguishes by number of layers—simple (one layer), stratified (multiple layers), pseudostratified, and transitional (urothelium)—and by cell shape—squamous, cuboidal, and columnar, with transitional epithelia capable of changing shape. Typical regional examples include simple squamous epithelium lining blood vessels and alveoli; simple cuboidal and columnar epithelia in glandular and digestive systems; pseudostratified ciliated columnar epithelium in much of the respiratory tract; stratified squamous epithelium in the skin; and transitional epithelium in the bladder.

Functions of epithelial tissue include protection, selective absorption and secretion, filtration, sensation, and barrier formation. Epithelien play a central role in physiology and pathology, including cancer (carcinomas) arising from epithelial cells.