Collagen is the most abundant protein in the human body, found in the bones, muscles, skin, and tendons. It is the substance that holds the body together. Collagen forms a scaffold to provide strength and structure. Endogenous collagen is natural collagen, synthesized by the body. Exogenous collagen is synthetic. It comes from an outside source, such as supplements. Endogenous collagen has a number of important functions. Breakdown and depletion is linked to a number of health problems. Exogenous collagen is used for medical and cosmetic purposes, including the repair of body tissues.