CELLS OF THE MONONUCLEAR PHAGOCYTE SERIES DIFFERENTIATE INTO OSTEOCLASTIC LACUNAR BONE-RESORBING CELLS

Although the osteoclast shares several features with other cells of th e mononuclear phagocyte system (MPS), its precise cellular ontogeny is unknown, and its membership of the MPS is controversial. This study e xamined whether various cells of the MPS can be induced to differentia te into cells capable of the highly specialized osteoclastic function of lacunar bone resorption. We isolated mouse and rat monocytes, mouse (liver, peritoneal, alveolar, brain) tissue macrophages, and spleen a nd marrow haemopoietic cells, as well as foreign body macrophages and macrophage polykaryons derived from subcutaneous granulomas formed by implantation of latex beads and coverslips in mice. When these cells w ere incubated with UMR106 osteoblast-like cells on glass coverslips an d human cortical bone slices in the presence of 1,25-dihydroxy vitamin D-3 [1,25(OH)(2)D-3] for 7 and 14 days, numerous tartrate-resistant a cid phosphatase-positive cells formed in these co-cultures and scannin g electron microscopy revealed extensive lacunar resorption of the bon e surface. Bone resorption was seen as early as 4 days after monocytes were co-cultured with UMR106 cells. With the exception of bone marrow -derived cells, lacunar resorption was not seen in the absence of UMR1 06 cells. These findings show that a bone-derived stromal cell element is necessary for differentiation of monocytes and tissue and inflamma tory macrophages into osteoclast-like cells capable of extensive lacun ar bone resorption, and would argue in favour of osteoclast membership of the MPS.