Epithelium in anatomy, is a layer of cells that serves as a protective covering over some surface, such as the outside of an organ or the lining of the wall of a cavity in the body. Epithelial cells generally have large nuclei, clear outlines, and a large amount of granular protoplasm. Some, called columnar, are long and narrow and may, as in the respiratory tract, have hairlike cilia extending from the outer surface. The skin contains several layers of epithelial cells, with columnar cells at the bottom, squamous cells at the top, and cuboidal cells in middle layers. Many of the secretory glands are composed of epithelial cells as are some of the endocrine glands.