any of a group of proteins that are components of whitish, rather inelastic fibres of great tensile strength present in tendon and ligament and in the connective tissue layer of the skin—dermis—and in dentin and cartilage. Collagenous fibres occur in bundles up to several hundred microns wide, and the individual fibres can be separated into fine fibrils; the fibrils, furthermore, consist of even finer filaments with a periodic banded structure.

Collagen is a scleroprotein (q.v.), being one of a family of proteins marked by low solubility in water. 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, collagen is converted to gelatin.