collagen, any of a group of proteins that are components of whitish fibers of great tensile strength, such as those present in tendons and ligaments and in the dermis (the connective tissue layer of the skin), in the dentin of the teeth, and in cartilage. Collagen constitutes roughly 30 percent of total protein mass in mammals, making it one of the most abundant proteins in mammalian species.
Collagen is classified as a scleroprotein, being one of a family of proteins marked by low solubility in water. Its fibers occur in bundles up to several hundred micrometers in width. The individual fibers can be separated into fine fibrils, which consist of even finer filaments with a periodic banded structure. Collagen is especially rich in the amino acid glycine, and it is the only protein known to contain a substantial proportion of hydroxyproline. Upon exposure to boiling water, it is converted to gelatin.
Collagen is a major component of the extracellular matrix, which provides structural scaffolding to tissues and organs. In bony tissue, it supports the framework for mineral deposition and thereby lends strength to the tissue. In the skin, it helps to maintain elasticity and firmness. Collagen fibers are highly resistant to stretching; thus, in tendons and ligaments, they enable tissue to withstand tension imparted by force and movement. Collagen also has important roles in tissue repair and the maintenance of tissue integrity.
There are various types of collagen, the most common of which are designated types I through V. Type I collagen occurs in bone, ligaments, skin, and tendons, where it offers tensile strength. Type II collagen is found in cartilage and is associated with maintenance of joint health. Type III collagen, located in blood vessels, organs, and the skin, contributes to tissue flexibility. Type IV collagen is a component of basement membranes, where it serves various functions, such as providing mechanical stability. Type V collagen occurs in hair and cell surfaces, where it lends structural support.