Human osteoclast ontogeny and pathological bone resorption

Monocytes and macrophages are capable of degrading both the mineral and organic components of bone and are known to secrete local factors which stimulate host osteoclastic bone resorption. Recent studies have shown that monocytes and macrophages, including those isolated from neoplastic and inflammatory lesions, can also be induced to differentiate into cells that show all the cytochemical and functional characteristics of mature osteoclasts, including lacunar bone resorption. Monocyte/macrophage-osteoclast differentiation occurs in the presence of osteoblasts/bone stromal cells (which express osteoclast differentiation factor) and macrophage-colony stimulating factor and is inhibited by osteoprotegerin. Various systemic hormones and local factors (e.g. cytokines, growth factors, prostaglandins) modulate osteoclast formation by controlling these cellular and humoral elements. Various pathological lesions of bone and joint (e.g. carcinomatous metastases, arthritis, aseptic loosening) are associated with osteolysis. These lesions generally contain a chronic inflammatory infiltrate in which macrophages form a significant fraction. One cellular mechanism whereby pathological bone resorption may be effected is through generation of increased numbers of bone-resorbing osteoclasts from macrophages. Production of humoral factors which stimulate mononuclear phagocyte-osteoclast differentiation and osteoclast activity is also likely to influence the extent of pathological bone resorption.