Analysis of osteoblast activity around beta-TCP particles implanted into bone: Expression of bone matrix protein mRNAs

Osteoblast activity around beta-tricalcium phosphate (beta -TCP) particles (150-300 mum in diameter) implanted into rat tibiae was analyzed by in situ hybridization with digoxygenin-labeled procollagen al(I) (COL), osteonectin (ON), osteocalcin (OC) and osteopontin (OPN) RNA probes. Holes without implantation were used as a control. Specimens were collected at 3, 5, 7 and 10 days after the operation. New bone was formed centripetally in both the beta -TCP implanted and control groups, and all four kinds of mRNA were expressed in activated osteoblasts. A COL signal was expressed most strongly and widely, and was detected at the peripheral region of the hole at day 3. The other three mRNAs were also expressed in bone forming osteoblasts by day 7. However, in the earlier cell reaction stage, OPN expression in the beta -TCP implanted group was different from that in the control group: OPN mRNA was seen exclusively in the cells on the particles, and an OPN signal was detected not only in COL-positive cells, but also in COL-negative cells. The former cells may be osteoblasts and reflect the early process of bone formation on biomaterials. The latter cells may be macrophages and reflect foreign body reactions. Expression of these OPN mRNAs induced by implantation of beta -TCP may play a role in bone formation on the materials and in determining the their biocompatibility.