Final answer:
The thin layer of endothelium overlying a thin layer of connective tissue is found in blood vessels.
Step-by-step explanation:
The thin layer of endothelium overlying a thin layer of connective tissue is found in blood vessels. The tunica intima, which is the innermost layer of arteries and veins, consists of a single layer of endothelial cells surrounded by a thin layer of connective tissue. This structure reduces friction between the blood and the inside of the blood vessel walls.