BASIC SCIENCE AND PATHOLOGY Collagens Synthesized by Healing Fractures

Several extracellular matrices are formed by healing fractures, and the collagens within these matrices have been identified in rabbit and rat fracture models. Type 111 collagen is the major collagen of the fibrous matrix that forms along the periosteal surface. Type I collagen is secreted in large amounts as trabeculae of bone develop within the fibrous tissue. Type V collagen is found in both the fibrous tissue and bone; it is particularly associated with blood vessels. Type 11 collagen is the last of the major collagens to be synthesized; its synthesis is dependent on the mechanical conditions under which the fractures are healing. A large area of cartilage, and hence of Type II collagen, is formed only if the fracture is mechanically unstable. Of the minor collagens, only Types IX and X have been identified to date. Type IX collagen is present throughout the large areas of cartilage, and Type X is present only in calcified regions. The matrices are compared with those produced during embryonic limb development. Although cartilage in the embryo provides a rapidly growing model of the future bone, in healing fractures cartilage is produced only where the cellular environment precludes the differentiation of os-teoblasts. The effects of mechanical stability on the matrices support and illustrate this conclusion.