Epitela

Epitela, or epithelium, is a tissue that lines surfaces and cavities of the body, forming a barrier between internal compartments and the external environment. It is composed of closely packed cells with minimal extracellular matrix and rests on a basement membrane. Epithelia protect, secrete, absorb, and sense stimuli, and often participate in selective transport and secretion.

Classification of epithelia is based on cell shape and arrangement. Cell shapes include squamous (flat), cuboidal (cube-like), and columnar (tylinder- or rectangle-shaped). Arrangements include simple epithelia with a single cell layer, stratified epithelia with multiple layers for protection, pseudostratified epithelia that appear layered but are not, and transitional epithelia that can stretch, as in the urinary tract.

Key features of epithelia include specialized surfaces such as microvilli for absorption and cilia for movement, as well as goblet cells that secrete mucus. Epithelia are connected by junctional complexes, including tight junctions that regulate paracellular transport, adherens and desmosomes that provide mechanical strength, and gap junctions that permit intercellular communication. Epithelia also exhibit apical-basal polarity and a distinct basement membrane that anchors them to underlying tissues.

Glandular epithelia form glands, including exocrine glands that secrete onto surfaces and endocrine glands that release hormones into the bloodstream.

Examples include the stratified squamous epithelium of the skin, the simple columnar epithelium of the intestinal lining with microvilli, the pseudostratified ciliated columnar epithelium of the trachea, and the transitional epithelium of the urinary bladder.

Epithelia arise from all three germ layers, show high regenerative capacity, and play a central role in protection and homeostasis. Epithelial cancers (carcinomas) are among the most common malignancies and reflect the central importance of this tissue in health and disease.