Collagen

Collagen is a group of naturally occurring proteins. In nature, it is found exclusively in animals, especially in the flesh and connective tissues of mammals.
It is the main component of connective tissue, and is the most abundant protein in mammals. It makes up about 25% to 35% of the whole-body protein content.
Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendon, ligament and skin, and is also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral discs.
Collagen makes up 1% to 2% of muscle tissue, and accounts for 6% of the weight of strong, tendinous muscles.
Gelatin, which is used in the food industry, is collagen that has been irreversibly hydrolyzed.