Autoradiographic demonstration of in vitro fusion of blood monocytes with osteoclasts.

The origin of osteoclasts from mononuclear phagocytes and the addition of new nuclei to already differentiated osteoclasts have been documented by several authors although the cellular cascade through which this is affected is not fully clear. In order to investigate this problem tritiated thymidine labeled monocytes, harvested from the blood of a donor chicken, were added to isolated osteoclasts cultured in vitro on coverslips. Autoradiographs of coverslips fixed at 24-hour intervals for 4 days showed that labeled nuclei were incorporated into the cluster of osteoclasts in a time-dependent manner.