Endothelial Cells: The cells that form the innermost lining of blood vessels

Epithelial Cells: The cells that cover any surface of the body that contacts the outside world

More generally, you can think of -thelium here as meaning “covering” or “wall” and Endo- meaning “inside”and Epi- meaning "over” or “on”.

The epithelium covers all the areas of the body that can contact the outside world, including your skin, eyes, mouth, stomach, intestines, etc. All of these things touch are in contact with the outside world and the epithelium controls what can get in or go out of your body.

The endothelium forms all the innermost wall of the blood vessels. They form the barrier that keeps your blood in the circulatory system and controls movement of substances into and out of all your organs.

Mesothelium (meso- = middle) forms the coverings of everything in the middle of the body. It is tissuethat surrounds the organs of the chest, abdominal cavity, and pelvis that serves to protect internal structures and aid in movement.