Figure 2.

Osteoclasts and precursors are secretory cells. In inflammatory bone disease, systemic elevated TNF stimulates the generation of OCPs in the bone marrow and also enhances their egress into the bloodstream, from where they can alight in increased numbers at sites of inflammation. These OCPs differentiate to osteoclasts and increase their production of many factors in response to TNF and RANKL and thereby induce an auto-amplifying autocrine vicious cycle to increase osteoclast numbers and interact with immune and other cells to affect bone volume and turnover. OCP, osteoclast precursor; RANKL, receptor activator of nuclear factor-κB ligand; TNF, tumor necrosis factor.