Experimental study on the haematogenous origin of multinucleate osteoclasts in the rat.

When the resorption zones of the distal femoral and proximal tibial epiphyseal plates of rats are examined with the electron microscope, numerous macrophages can be seen surrounding degenerated chondrocytes. Macrophages are also found in the neighbourhood of the invading sinusoids and in close proximity to multinucleate osteoclasts. Cell fusion, however, could not be observed. Repeated transfusions of female leukocytes to isohistogenic growing male rats caused an increase in the number of nuclei containing a Barr body in the osteoclasts of the males. The result suggests that leukocytes--of which only the monocytes come in question--are the source of multinucleate osteoclasts in endochondral ossification.