Multinucleated giant cells recruited by implantation of octacalcium phosphate (OCP) in rat bone marrow share ultrastructural characteristics with osteoclasts.

The ultrastructural characteristics of multinucleated giant cells (MNGCs) on octacalcium phosphate (OCP) and hydroxyapatite (HA) were investigated in comparison with those of osteoclasts, when the synthetic OCP and HA were implanted in rat bone marrow. The morphological difference in the MNGCs were shown between OCP and HA implants in 2 weeks after implantation. The MNGC on the implanted OCP (i-OCP) developed the ruffled border-like structure and the clear zone-like structure. The i-OCP was frayed where it was in contact with the ruffled border-like structure. The MNGC on the implanted HA (i-HA) developed the clear zone-like structure, whereas no ruffled border was seen. The surface of i-HA associated with the MNGC was smooth and not frayed. Bone formed directly on the OCP or HA implants. The interface between the i-OCP and bone matrix interdigitated, whereas that between the i-HA and bone matrix was comparatively smooth. The present study suggested that the i-OCP could be resorbed by the MNGCs which share some ultrastructural characteristics with osteoclasts.