Based on shape, simple epithelial cells can be classified as (1) Simple squamous: the cells are flat, thin, fish-scale shaped and have elliptical nuclei. They form the outer layers of the skin and the lining of cavities, such as the mouth, blood vessels, heart, and lungs. (2) Simple cuboidal: the cells are square or cuboidal, with spherical nuclei. They are found in the ducts of glands, lining of the kidney tubules and also form the germinal epithelium, which produces the eggs and sperms. (3) Simple columnar: the cells are columnar, with elongated nuclei and form the lining of the digestive tract. Simple columnar ciliated epithelium has fine, hair-like outgrowths on its’ free surface. Cilia are capable of rapid, rhythmic movement of mucus in the nose and the movement of ovum towards the uterus. (4) Simple pseudo-stratified columnar: the columns are all of the varying heights, giving a false impression of multiple layers. They are found in the nasal cavity, larynx, and trachea.