Bone formation on synthetic precursors of hydroxyapatite

The aim of this study was to investigate the reaction of skeletal tissue to various synthetic calcium phosphate (Ca-P) compounds in vivo. Five synthetic Ca-P compounds were implanted into the subperiosteal area of the calvaria of 7-week-old BALB/c mice for one to 15 weeks. Synthetic compounds were dicalcium phosphate (DCP), octacalcium phosphate (OCP), amorphous calcium phosphate (ACP), Ca-deficient hydroxyapatite and hydroxyapatile (HA). Implanted DCP, OCP and ACP were found to be converted to apatitic phase by x-ray microdiffraction analysis using undecalcified specimens. Structure of bone was found out on all of Ca-P compounds eventually at late stage under the light microscope, but the rate of bone formation calculated from a number of experiments varied on respective synthetic Ca-P compound. It was high as 80% for DCP, OCP and ACP, but was low as 5.6% for Ca-deficient HA, and no reaction was found for HA at the stage of 3 weeks. Fine filaments and granular materials in the newly formed bone matrix were detected at 7 days around the remnants of OCP particles which already converted to apatitic phase by ultrastructural study of decalcified specimens. These structures were very similar to the components of bone nodules seen in intramembranous osteogenesis. It is postulated that the precursors of HA have an important role in intramembranous osteogenesis.