The key difference between epithelium and endothelium is that the epithelium is the tissue that lines the outer surfaces of organs and blood vessels, as well as the inner surfaces of cavities in many internal organs while the endothelium is the tissue that lines the interior surface of blood vessels and lymphatic vessels.
A tissue is a group of physically linked cells and associated intercellular substances that are specialized for particular functions. On the basis of their structure and functions, there are four fundamental types of tissues in the animal body such as epithelial tissue, connective tissue, muscular tissue and nervous tissue. The epithelial tissue is the covering of all external and internal body surfaces. Therefore, it lines the whole external surface of the skin, internal cavities and lumens as well as the external and internal surfaces of the vessels.
Moreover, they also help the exocrine function by the formation of glands. The external epithelium is called the exothelium; it is the epithelium that covers the skin and organ lining. There are also two subtypes of the epithelium: mesoderm that lines internal cavities and lumens and the mesothelium that covers vessels and the heart chambers. Hence, essentially the endothelium is part of the epithelial tissue that helps the body protect itself from damages.
