OSTEOBLASTIC PHENOTYPE EXPRESSION ON THE SURFACE OF HYDROXYAPATITE CERAMICS

To analyze the bone-bonding property of hydroxyapatite ceramics (HA), composites of rat marrow cells and porous HA were implanted subcutaneo usly and harvested at 3 to 4 weeks postimplantation. De novo bone form ation was observed primarily on the HA surface without fibrous tissue interposition. The HA/tissue interface was analyzed by the observation s of thin undecalcified histological sections and fractured surfaces o f the implants. The observations were done with a light microscope and a scanning electron microscope (SEM) connected to an energy dispersiv e spectrometer. The interfacial analyses showed the appearance of oste oblastic cells on the HA surface and that the cells had initiated part ially mineralized bone (osteoid) formation directly onto the surface. The osteoid matured into fully mineralized bone, resulting in firm bon e bonding to the HA surface. Characterization of osteoblastic cells on the surface was done by determining levels of protein and gene expres sion of bone Gla protein (BGP, a.k.a. Osteocalcin), i.e., immunohistoc hemistry and in situ hybridization, respectively. The existence of BGP and mRNA in the cytoplasmic area of the cells confirmed that active o steoblast apposition fabricated primary bone on the HA surface. All of these results indicate the importance of the HA surface in supporting osteoblastic differentiation of marrow stromal stem cells, which lead s to firm bone bonding. (C) 1997 John Wiley & Sons, Inc.