Epithelial tissue is tissue that covers the whole surface of the body. It consists of closely packed cells in one or more layers. Epithelial tissue also lines the internal and external body surfaces. For example it lines the outer and inner surface of the vessels in the human body. Epithelial tissue that lines interior organs is called endothelium, and epithelial tissue only one cell thick is called simple epithelium. If it two or more layers thick such as the surface of the skin it is called stratified epithelium. The epithelial tissue in the human body serves many purposes such as protection, sensation, secretion, absorption, excretion, and diffusion.