Collagen types. Molecular structure and tissue distribution.

The collagens are products of a superfamily of closely related genes. Currently, there are 13 described collagens encompassing at least 25 separate genes. The collagen molecules can be categorized into four classes. Class I consists of molecules that form the banded collagen fibers that are readily seen by routine electron microscopy. The banded fibers are heterogenous with respect to collagen type, containing at least two and often three collagen types in each fibril. This multiplicity is believed to effect the rate of fibril growth and the final fibril diameter. Class II contains collagens that adhere to the surface of the banded fibrils. The function of these molecules is not yet known. The third Class consists of molecules that form independent fiber systems. These include the basement membrane, beaded filaments, anchoring fibrils, and the network surrounding hypertrophic chondrocytes. The last class contains several collagens with unknown fiber forms, and whose functions are unclear. Tissues contain multiple fiber forms and therefore many individual collagen types. Bone is no different, and there are presently four known collagens in the bone cortex. This article summarizes knowledge of the structures and functions of the collagen superfamily.