Gene expression ground biomaterials implanted into bone

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Tissue response around β-tricalciumphosphate (TCP, absorbable material) or poltmethylmethacrylate (PMMA, non-bioactive material) particles implanted into rat tibiae was analyzed by in stu hybridization with type I collagen (COL), osteonectin (ON), osteocalcin (OC) and osteopontin (OPN) RNA probes. Specimens were collected at 3, 5, 7, 10 days after operation. Holes without implantation were used as control. In all groups, new bone was formed centripetally and all four kinds of mRNA were expressed in activated osteoblasts with a similar pattern. 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. These active osteoblasts were often observed on the surface of the TCP but not on the PMMA particles. The activities of osteoblasts in the control and the TCP groups were similar, whereas that in the PMMA group was weaker. On the other hand, OPN-positive and COL-negative cells were observed on the surface of the PMMA particles strongly for a long time. They were speculated to be macrophages and osteoclasts because they showed acid phosphatase activity. These cells were rarely observed on the TCP particles and were not observed in the control. These findings suggest that the early cell responses and gene expressions reflect the bioactivity or biocompatibility of the implanted biomaterials.