Marrow-derived progenitor cell injections enhance new bone formation during distraction

Bilateral femoral distraction was performed in rats to investigate whether injections of marrow-derived mesenchymal progenitor cells could be used to facilitate new bone formation. The cells were isolated from whole marrow of 2-6-month-old Sprague-Dawley rats. One-year-old recipient Sprague-Dawley r ats were divided into five experimental groups. Rats in groups I, TI, and I II received injections of mesenchymal progenitor cells on days 6 (beginning ), 12 (middle), and 18 (end of distraction) after surgery, respectively Tho se in group IV received injections of serum and carrier gel alone, and thos e in group V received no injections. Distraction zones were harvested at 36 days and analyzed for new bone volume within the distraction gap by three- dimensional microcomputed tomography. Significant increases in new bone vol ume were observed for femora injected with marrow-derived progenitor cells compared with contralateral femora and controls (no injection). The timing of the cell injections appeared to have no effect on the experimental outco me. Histologic analyses demonstrated active formation of new trabecular bon e with marked osteoblastic activity and osteoid production. No qualitative differences in histologic appearances of new hone among rats in any of the five groups were seen. The results of in vitro lysis assays indicated that donor and recipient rats were not completely syngenic, leaving some doubt a s to the reasons for observed increases in new bone formation. Future work will focus on attempting to repeat these experiments in a fully syngenic ra t model. This rat distraction model can be used to explore the molecular an d cellular behavior of these progenitor cells in a clinically relevant ir? vivo environment.