NORMAL AND THERAPEUTIC REGULATION OF BONE-RESORPTION

Excessive bone loss occurs in several common bone disorders. it may be local or diffuse, like the mechanisms that control bone resorption. N ormal bone remodelling is regulated systemically, by hormones to contr ol calcium homeostasis and locally, by cytokines in response to change s in stress and strain. The osteoclast is the cell that resorbs bone. Regulation of resorption may occur at each step in the osteoclast's de velopment from the pluripotent haematopoietic stem cell, through circu lating monocyte to recruitment, differentiation and fusion on the surf ace of bone. Once the active osteoclast is formed it may be subject to inhibition by calcitonin and down-regulation by inhibitors of prostag landin synthesis. The hormones, growth factors and cytokines known to be involved in these steps are discussed along with the mechanisms of important therapeutic agents.