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. Fast facts on collagen Here are some key points about collagen. More detail is in the main article. Collagen occurs throughout the body, but especially in the skin, bones, and connective tissues. Some types of collagen fibrils, gram-for-gram, are stronger than steel. Collagen production declines with age and exposure to factors such as smoking and UV light. Collagen can be used in collagen dressings, to attract new skin cells to wound sites. Cosmetic lotions that claim to increase collagen levels are unlikely to do so, as collagen molecules are too large to be absorbed through the skin