Role of Monocytes in Bone Resorption

Abstract

Bone remodeling occurs in Haversian systems and on endosteal bone surfaces throughout life. The classic or traditional concept of bone remodeling holds that the initial event in bone turnover is osteoclastic bone resorption. Osteoclastic bone resorption causes degradation of the bone matrix and release of bone mineral and the formation of resorption bays or Howship’s lacunae. This phase of osteoclastic bone resorption is followed by a phase of bone formation by osteoblasts. The attraction of osteoblasts to the site of the resorption defect and the activation of the osteoblasts to lay down new bone leads to repair of the resorption bay. Although this coupling of bone formation to bone resorption occurs under physiological and most pathological circumstances, the mechanisms which are responsible for linking bone formation to bone resorption are unknown. Recently some workers have described a putative coupling factor found in bone which leads to increased bone formation (Howard et al., 1980). We have found a chemotactic factor released by resorbing bone which causes unidirectional migration of cells with the characteristics of osteoblasts (Mundy et al., 1980). These factors could explain the coupling or linkage which exists between bone resorption and bone formation.