Inflammatory mediators and the destruction of bone.

Bone is remodelled by the coordinated actions of osteoclasts and osteoblasts. Cellular remodelling occurs in discrete packets of bone, and is regulated by local cytokines produced in the environment of the remodelling cells. These cytokines are secreted by immune cells and by bone cells. In addition, some growth regulatory factors are incorporated into the noncollagenous bone matrix and are released in an active form when bone is stimulated to resorb. Complex interactions between these cytokines and their target cells are responsible for the normal delicate balance between bone resorption and bone formation, and disorders of bone loss are due to imbalances between the rates of resorption and formation.