Epithelium is one of the four tissue types and covers every exposed surface of the body. It also lines the internal structures of the body, such as the digestive and urinary tracts. Epithelium is avascular, and is forced to obtain nutrients solely through diffusion. The base of epithelium is attached to a basal lamina for support. Epithelium provides four main functions: Protection, control of permeability, sensation, and glandular secretions. Cell junctions hold epithelial cells together and allow communication between cells. The epidermis of the skin shown above is an example of stratified squamous epithelium.