Cellular activity, matrix proteins, and aging bone.

The extracellular matrix of bone is composed of proteins from both local and exogenous sources. Many of these are known growth factors (e.g., transforming growth factors-beta; insulin-like growth factors; and fibroblast growth factors) which concentrate in mineralized bone and probably contribute to the ability of bone to regenerate itself on injury. With advancing age, human osteoblasts have reduced bone formative properties and in vitro, osteoblasts from fetuses generally have greatly increased proliferative and biosynthetic capacities compared to cells from adult donors. Aside from type I collagen, many noncollagen components are synthesized by osteoblasts and secreted to the bone matrix space. Several of these are cell attachment proteins (e.g., fibronectin, thrombospondin, osteopontin) which greatly influence cytodevelopment and differentiation. They are degraded to lower molecular weight fragments with advancing age, probably deactivating their true bioactivities. It is not know whether the age-related degradation of these proteins affects bone cell function in aged individuals. Several of the bone matrix proteins are also found in platelets and have been implicated in the wound-repair process. One of these, osteonectin, is found in a wide variety of nonbone cell systems, but only in periods of rapid growth and proliferation. Osteonectin production is the highest and is maintained the longest in bone compared to all other tissues of the body. Thus, reduced osteonectin production in aged bone cells may be an important parameter for further study.