Monocytes from circulating blood fuse in vitro with purified osteoclasts in primary culture

The origin of osteoclasts from mononuclear phagocytes and the addition of new nuclei to already differentiated osteoclasts have already been documented by several authors, but the factors controlling these events have not yet been elucidated. With the aim of investigating this problem, monocytes, isolated from circulating blood of laying hens and cultured previously for 5 days, were added to osteoclasts isolated from the medullary bone of the same hen and cultured at low density. The cultures were either fixed and observed under phase contrast at 24-h intervals for 5 days or filmed by time-lapse cinemicrography. With both techniques the formation of extensive areas of membrane contacts, generally followed by fusion between some monocytes and osteoclasts, was observed. The absence of added resorbing factors and the possible mechanisms by which osteoclast precursors are recruited in vivo are discussed.