Molecular mechanisms in coupling of bone formation to resorption.

Bone remodeling is the process of removal and replacement of bone, taking place at many sites throughout the skeleton and regulated mainly by locally generated factors. Its purposes are to repair damaged bone, remove old bone, and facilitate skeletal responses to changes in loading requirements. Cells of the osteoblast lineage control the formation and activity of osteoclasts, which are responsible for initiation and execution of resorption at remodeling sites. The bone resorbed by osteoclasts is replaced through the differentiation and activity of osteoblasts. The consequent formation must match closely the amount of bone that is resorbed at each site. This coupling of the two processes is essential for bone balance. Both resorption products and osteoclast-derived factors contribute to the coupling of bone formation to resorption in bone remodeling. This review considers the molecular mechanisms and intercellular communication processes involved in remodeling and coupling.