New roles for osteoclasts in bone.

Osteoclasts have long been recognized as the cells that resorb bone in normal bone remodeling and in pathologic conditions in which bone resorption is increased. They are derived from precursors in the mononuclear phagocyte lineage, which arise in the bone marrow and fuse with one another to form the multinucleated cells that resorb calcified matrixes under the influence of osteoblastic cells in bone marrow. There is growing evidence that osteoclast precursors (OCPs) and osteoclasts have functions in and around bone other than bone resorption. For example, they modulate the differentiation of other cells, including osteoblastic cells; they regulate hematopoietic stem cell movement from the bone marrow to the bloodstream; and they are secretory cells that participate in immune responses. In this article, we review these findings, which support new roles for osteoclasts and OCPs in the growing field of osteoimmunology and in common pathologic conditions affecting bones and joints.