The epithelial tissue, or epithelium, is membranous tissue that covers the surface of the body and lines internal organ cavities, glands, and blood vessels. Externally, it forms the skin. When it lines internal cavities and the lumen of arteries, it is called endothelium. The underside of the epithelium is attached to connective tissue by a thin layer of basement membrane. Although epithelial tissue lacks blood vessels, nutrients diffuse to the epithelium from underlying connective tissues, which do have blood vessels.
The epithelial tissue is classified by the number of layers of cells and by their shapes. Thus, when it consists of a single layer of cells, it is called 'simple'. When it consists of two or more layers, it is called 'stratified'. When it is made up of flattened epithelial cell, it is 'squamous'. When it is constituted by cube-like cells, it is 'cuboidal'; and when it is formed by elongated cells, it is 'columnar'.
Simple squamous epithelium lines the lung alveoli, the heart cavities, and the lymphatic and blood vessels; this type allows materials to seep through by diffusion. Stratified squamous epithelial tissue lines the esophagus, mouth, and vagina.
Simple cuboidal epithelium lines ducts, kidney tubules, the ovaries, and secretory portion of small glands. Stratified cuboidal epithelial tissue, on the other hand, covers the inner walls of mammary glands, sweat glands, pancreas, and salivary glands.
Simple columnar epithelium lines the passages of respiratory system, such as the bronchi, as well as the trachea. Whereas stratified columnar epithelial tissue covers inner walls of male urethra and the ducts of some glands.
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