SUBPERIOSTEAL IMPLANTATION OF OCTACALCIUM PHOSPHATE (OCP) STIMULATES BATH 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 disti nct biological characteristics. Octacalcium phosphate (OCP), which is a precursor phase of the hydroxyapatite, has been reported to stimulat e bone formation if implanted in the subperiosteal region of mouse cal varia. The present study was designed to investigate how the long bone and the calvarium respond to OCP implantation and to compare their bi ological characteristics. Methods: The synthetic OCP was implanted int o the subperiosteal region of rat tibiae and parietal bones being mixe d with bovine type I collagen treated by pepsin (Atelocollagen), The b iological response was examined histologically and immunohistochemical ly for collagen matrix phenotypes of types I and II to identify bone a nd 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 w eek 3, and no cartilage formation was associated with the osteogenesis . Conclusions: The present study demonstrated the distinct characteris tics of biological response to OCP implantation between the long bone and the calvarium in terms of whether or not cartilage formation is in volved in the stimulated osteogenesis by OCP, and in terms of timing o f the stimulated chondrogenesis and/or osteogenesis, i,e., the parieta l bone takes more time to respond to OCP implantation than the tibia. (C) 1995 wiley-Liss, Inc.