Subperiosteal lmplantation of octacalcium phosphate (OCP) stimulates both chondrogenesis and osteogenesis in the tibia, but only osteogenesis in the parietal bone of a rat

Background: It is not known whether long bones and calvaria have distinct biological characteristics. Octacalcium phosphate (OCP), which is a precursor phase of the hydroxyapatite, has been reported to stimulate bone formation if implanted in the subperiosteal region of mouse calvaria. The present study was designed to investigate how the long bone and the calvarium respond to OCP implantation and to compare their biological characteristics. Methods: The synthetic OCP was implanted into the subperiosteal region of rat tibiae and parietal bones being mixed with bovine type I collagen treated by pepsin (Atelocollagen). The biological response was examined histologically and immunohistochemically for collagen matrix phenotypes of types I and II to identify bone and cartilage formation. Results: Both chondrogenesis and osteogenesis were initiated in the tibia 1 week after implantation of OCP and most of the cartilage was replaced by bone at week 2. However, the parietal bone did not show osteogenesis responding to OCP implantation until week 3, and no cartilage formation was associated with the osteogenesis. Conclusions: The present study demonstrated the distinct characteristics of biological response to OCP implantation between the long bone and the calvarium in terms of whether or not cartilage formation is involved in the stimulated osteogenesis by OCP, and in terms of timing of the stimulated chondrogenesis and/or osteogenesis, i.e., the parietal bone takes more time to respond to OCP implantation than the tibia. © 1995 Wiley‐Liss, Inc.