A tissue in vertebrates consisting of closely packed cells in a sheet with little intercellular material. It covers the outer surfaces of the body and walls of the internal cavities (coeloms) and is often underlain by a basement membrane. It also forms glands and parts of sense organs. Its functions are protective, absorptive, secretory, and sensory. The types of cell vary, giving rise to squamous epithelium; ciliated epithelium; cuboidal epithelium, with cube-shaped cells; and columnar epithelium, with rectangular cells (see also stratified epithelium). Epithelium is derived from ectoderm and endoderm. Compare endothelium; mesothelium.