Immediate Bone Forming Capability of Prefabricated Osteogenic Hydroxyapatite

We employed culture technology, which provides bone tissue in vitro, to expand and promote the osteogenic ability of marrow cells in porous hydroxyapatite (HA). Marrow cells were obtained from rat femur and cultured in a standard medium for 10 days, then trypsinized to make composites of HA and the cells. An additional 2-week culture (subculture) was done for the composite in a standard medium with or without the addition of dexamethasone (Dex). The 2-week subcultured composites were implanted into subcutaneous sites of syngeneic rats. These implants were harvested and prepared for the biochemical analysis of alkaline phosphatase activity and bone Gla protein content, as well as histological analysis of decalcified and undecalcified sections. In Dex-treated composites, high alkaline phosphatase activity could be detected 1 week after implantation and was maintained until 8 weeks after implantation. The bone Gla protein content could also be detected 1 week after implantation, followed by a steady increase with the passage of time until 8 weeks after implantation. The histological analysis showed active bone formation even 1 week after implantation. The bone formation was evidenced by active osteoblast lining and the appearance of calcein labeling following calcein injection 1 week after implantation. Thus, Dex-treated subcultured marrow cells in pore regions of HA showed a high osteogenic response immediately after transplantation. In contrast, Dex-untreated composite did not show bone formation and contained traces of these biochemical parameters. These results indicate that the inherent osteogenic ability of marrow stromal stem cells in pore regions of HA can be stimulated using tissue culture technology; and thus, formed osteogenic HA can show immediate osteoblastic activity in in vivo situations, suggesting the applicability of the HA in clinical situations.