Epithelial tissue (epithelium) covers the outer surfaces of the body and lines internal surfaces such as blood vessels, airways and the gut. Epithelial cells are packed tightly together in sheets with very little material between them, and they sit on a basement membrane, a thin layer of protein and polysaccharide fibres that attaches them to the tissue below.Epithelia are classified by the shape of their cells and the number of layers.Squamous epithelium
Very thin, flat cells, forming a smooth layer only one cell thick. This gives a short diffusion distance, so it is found where rapid diffusion is needed: the walls of the alveoli in the lungs, and the walls of capillaries. In the alveoli, oxygen diffuses across the squamous epithelium of the alveolus and of the capillary into the blood.Cuboidal epithelium
Cube-shaped cells, found for example lining kidney tubules and some glands, where cells carry out secretion or absorption.Columnar epithelium
Tall, column-shaped cells. In the small intestine, columnar cells have microvilli that increase the surface area for absorption. In the trachea and bronchi, ciliated columnar epithelium is found with goblet cells. Goblet cells secrete mucus, which traps dust and microorganisms, and the cilia beat to sweep the mucus up and away from the lungs.
Epithelial cells are replaced often, because surfaces are worn away. They are therefore frequently dividing, which is also why many cancers start in epithelia.
Very thin, flat cells, forming a smooth layer only one cell thick. This gives a short diffusion distance, so it is found where rapid diffusion is needed: the walls of the alveoli in the lungs, and the walls of capillaries. In the alveoli, oxygen diffuses across the squamous epithelium of the alveolus and of the capillary into the blood.Cuboidal epithelium
Cube-shaped cells, found for example lining kidney tubules and some glands, where cells carry out secretion or absorption.Columnar epithelium
Tall, column-shaped cells. In the small intestine, columnar cells have microvilli that increase the surface area for absorption. In the trachea and bronchi, ciliated columnar epithelium is found with goblet cells. Goblet cells secrete mucus, which traps dust and microorganisms, and the cilia beat to sweep the mucus up and away from the lungs.
Epithelial cells are replaced often, because surfaces are worn away. They are therefore frequently dividing, which is also why many cancers start in epithelia.