The human body is made up of four different types of tissues, these are epithelial, muscular, connective and nervous tissues. Epithelial tissues refers to the thin tissues, which covers all the exposed surfaces of the human body. They can be found in the following places: digestive tract, inner lining of the mouth, secretory glands, external skin, the lining of hollow part of the heart, etc. The cells of epithelial tissues are usually closely bound together and they lack blood vessels and nerves.