Epithelial tissue, sometimes referred to as epithelium, consists of a sheet of closely adhering cells, one or more cells thick, with the upper surface usually exposed to the environment or to an internal space in the body. Epithelium covers the body surface, lines body cavities, forms the external and internal linings of many organs, and constitutes most gland tissue. Epithelium is classified by the shape of the cells and also by the whether there is 1, known as simple, or more than 1, known as stratified, layer of cells. Refer to your reading assignment and the notes for more detailed information regarding epithelium. Click each thumbnail below to get a better look at each type of epithelium.