Histochemical examination of ectopic bone formation induced in rat bone marrow

We examined the rapid formation and subsequent resorption of woven bone induced by partial ablation of rat bone marrow. On the 1st day after ablation, masses of clots occupied the region from which marrow was eliminated. On the 3rd day, alkaline phosphatase- (ALPase-) positive osteoblastic cells appeared in the vicinity of the marrow-eliminated region, forming woven bone. Other ectopic woven bone extended from the endosteal surface toward the bone marrow. Therefore, the newly formed bone originated in two different sites, the endosteal bone surface and the marrow tissues near the marrow-eliminated region. On the 7th day, numerous tartrate-resistant acid phosphatase- (TRAPase-) positive osteoclasts and ALPase-positive osteoblasts expressing the osteonectin gene indicated high activity in both formation and resorption of ectopic woven bone. On the 10th day, the ectopic bone had been markedly resorbed and replaced by bone marrow tissue as the ectopically formed woven bone had not been dynamically maintained, probably because of reduced bone formation activity. Immunoreactivity for basic fibroblast growth factor (bFGF) was indistinctly observed on osteoblastic and preosteoblastic cells on the 1st day after ablation. The fibroblastic cells in the marrow-eliminated region on the 3rd day, and both osteoblasts and preosteoblasts in the woven bone on the 7th day, showed strong immunoreactivity for bFGF. Unlike fractured cortical bone, no chondrogenesis was observed. This model appears to provide convenient material and an important clue for investigation of imbalanced bone formation and subsequent resorption.