Abstract
Hydroxyapatite (HA), which is the major inorganic component of bones and teeth, has been suggested to be formed via precursors, such as octacalcium phosphate (OCP). Previous studies reported that synthetic OCP stimulated osteogenesis if implanted into the subperiosteal region of murine bone. The present study was designed to investigate how implantation of OCP influences osteoblastic and/or osteoblastic progenitor cell populations residing in bone marrow.
OCP or HA was implanted into the bone marrow in the tibia of 6 week-old male Wistar rats through a small hole made at the mid-shaft of the bone. Only the hole, without any implant, was made in the control group. The rats were killed 1, 2, 4, and 8 weeks after implantation of OCP or HA, and the tibia was removed. The specimens were decalcified in 10% EDTA and processed for haematoxylin-eosin staining, histochemical analysis of tartrateresistant acid phosphatase activity (TRAP), and transmission electron microscopy.
Bone formation was initiated on the OCP implant in one week, whereas bone formation was not observed until 2 weeks after implantation of HA. Both OCP and HA implants were surrounded by multinucleated giant cells (MNGC), some of which were positive for TRAP histochemically. Part of the organic matrices that accumulated on OCP were also stained with TRAP. Transmission electron microscopic observation demonstrated that MNGC on the OCP implant had structures such as a clear zone and a ruffled border, which are characteristic of osteoclasts, whereas MNGC on the HA implant had a clear-zone-like structure, but no ruffled border.
The present study suggested that OCP could be resorbed by osteoclast-like cells and could be replaced by new bone formed by stimulated osteogenesis coupled with bone resorption.
