FUNCTIONS AND TYPES OF EPITHELIAL TISSUE
Epithelial tissue performs several important functions in the body:
Covers and lines body surfaces: It covers the external surface of the body and lines the internal surfaces of many organs. It helps protect these surfaces from mechanical damage, chemical agents, and dehydration.
Forms barriers: Epithelial tissue separates different body compartments and controls the movement of substances between them.
Produces and secretes substances: Many epithelial cells synthesize and secrete molecules. Glandular cells, for example, may produce substances such as enzymes, hormones, mucus, or sweat. In some glands, such as sweat glands, the secreted material consists largely of substances transported from the blood, with little or no new synthesis by the glandular cells.
Transports substances: Epithelial cells can transport molecules, ions, water, and particles between a body cavity or lumen and the underlying connective tissue. Examples include the epithelial cells lining blood and lymphatic vessels, the digestive tract, and the renal tubules.
Classification of epithelial tissue
Based mainly on the organization and functions of their cells, epithelial tissues are traditionally divided into two main types:
Covering or lining epithelium
Secretory or glandular epithelium
This classification is useful but not absolute. Many covering epithelia contain cells that also perform secretory functions. In some cases, therefore, the same epithelium can both line a surface and secrete substances