HOW THE OSTEOCLAST DEGRADES BONE

Osteoclasts are multinucleated monocyte-macrophage derivatives that de grade bone. Their specialized role is central to a process that contin uously removes and replaces segments of the skeleton in the higher ver tebrates. Osteoclasts allow skeletal mineral to be used to manage extr acellular calcium activity, which is an important adaptation for life on land, and solid skeletal structure to be replaced by hollow archite cture that has a superior strength-to-weight ratio. Degrading bone als o allows periodic repair and remodeling for ordered growth and efficie nt response to mechanical loads. A fairly comprehensive view of osteoc lastic ontogeny and function is emerging from recent studies. Osteocla sts dissolve bone mineral by massive acid secretion and secrete specia lized proteinases that degrade the organic matrix, mainly type I colla gen, in this acidic milieu. The site of bone dissolution is a high-cal cium environment; removal of degradation products by transcytosis of m embrane vesicles allows the osteoclast to maintain a normal intracellu lar calcium. Osteoclastic differentiation is normally balanced with bo ne formation, although bone formation is the function of unrelated str omal cell-derived osteoblasts. Interactions between osteoclast precurs ors and bone-forming cells are believed to control osteoclast differen tiation under most circumstances, preserving bone architecture over ma ny cycles of bone replacement. BioEssays 20:837-846, 1998. (C) 1998 Jo hn Wiley & Sons, Inc.