Mononuclear phagocytes as precursors of osteoclastic cells in resorbing pathological conditions of bone
This review focuses on the role mononuclear phagocytes (MPs) play in pathological bone resorption. Monocytes and macrophages are known to be 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 focused on an additional mechanism whereby MPs may directly cause bone resorption: it has been shown that both monocytes and macrophages, including those isolated from extraskeletal neoplastic and inflammatory lesions,can be induced to differentiate into cells showing all the cytochemical and functional characteristics of osteoclasts, particularly that of extensive lacunar bone resorption; for this to occur the presence of specific bone stromal cells and 1,25 dihydroxy vitamin D, is essential. The osteolysis associated with various resorbing pathological lesions of bone, many of which contain a chronic inflammatory infiltrate, may thus in part be due to MPs providing a precursor cell population capable of differentiation into osteoclast-like lacunar bone resorbing cells.