Bone formation and resorption of highly purified β-tricalcium phosphate in the rat femoral condyle

The aim of this study was to examine the chronological histology associated with highly purified β-tricalcium phosphate (β-TCP) implanted in the rat femoral condyle. Specimens were harvested on days 4, 7, 14, 28 and 56 after implantation, and were analyzed by tartrate-resistant acid phosphatase (TRAP) staining, immunohistochemistry of the ED1 protein as a marker of the phagocyte system, and in situ hybridization with digoxigenin-labeled α1 chain of type I procollagen (COL1A1), osteopontin and osteocalcin. β-TCP was resorbed in a chronological manner. Although new bone was not observed on day 4, fibroblast-like cells around β-TCP were positive for COL1A1 and osteopontin mRNA. New bone formation presented after day 7. In the double-staining for OPN and ED1 on day 7, most cells around β-TCP were positive for either osteopontin mRNA or ED1 protein. However, there were some doubly positive multinucleated cells, suggesting that they belonged to the mononuclear phagocyte system. After day 28, the implanted region was replaced with bone marrow. Multinucleated TRAP-positive and ED1-positive cells which adhered to β-TCP at all stages seemed to be osteoclasts and they continuously resorbed β-TCP. β-TCP has a good biocompatibility since both bioresorption and bone formation started at an early stage after implantation.