epithelium (ĕpˌəthēˈlēəm) [key], sheet of tissue that covers or lines the external and internal body surfaces. The epithelium is closely packed, has little intercellular material, and is lacking in blood vessels. There are three characteristic types of epithelial cells: squamous, cuboidal, and columnar. Squamous epithelial cells are flat and often overlapping; they compose the outer layer of skin (epidermis) and line certain internal cavities, e.g., the mouth. Cuboidal epithelial cells are rounded and elastic and line such structures as the urinary bladder, where, by stretching and becoming flatter, they increase the organ's capacity to hold fluid. The cells of the columnar epithelium are long and thin; they are found as a single layer of secretory and absorptive cells in the gastrointestinal tract, and they form the ciliated lining of the respiratory tract. Embryologically, epithelium may be derived from any of the three germ layers, i.e., ectoderm, mesoderm, and endoderm, and may be classified accordingly as epithelium proper, mesothelium, and endothelium.