Isolation and Implantation of Bone Marrow-Derived Mesenchymal Stem Cells with Fibrin Micro Beads to Repair a Critical-Size Bone Defect in Mice

Fibrin microbeads (FMBs) made using thermal treatment of fibrin drops in oil can efficiently isolate mesenchymal stem cells (MSCs) from bone marrow (BM) and other similar sources and culture them continuously in suspension culture. The pure mesenchymal profile of MSCs isolated using FMBs and their differentiation potency to different mesenchymal lineages were previously described in detail. In the current study, MSCs were isolated from the BM of (GFP+) C57/bl mice using FMBs. Addition of pro-osteogenic medium with 10 mM of beta-glycerolphosphate, 50 mu g/mL of ascorbic acid, and 10(-8) M of dexamethasone for 1 month resulted in ossified bone-like solid cellular structures, as seen using fluorescence and scanning electron microscopy (SEM). Such spontaneously formed structures were implanted in full-depth approximately 5-mm-diameter drilled defects in the skulls of wild-type c57/bl mice. Two months later, the excised upper parts of the skulls with the defects were viewed using fluorescence microscopy for green fluorescence protein of the cells in the defect and using SEM. They were also scanned using micro-computed tomography to visualize the formation of new hard tissue. Then the samples were processed and sectioned for hematoxylin and eosin staining and immunohistochemistry. Implanted FMBs loaded with (GFP+) MSCs formed partially mature, dense bone-like tissue using a residual moderate inflammatory process containing remnants of FMBs and neo-angiogenesis. The filled defect with bone-like tissue had a Ca/P ratio similar to that of native bone. Limited merging of the implant with the skull indicated that the induced bone regeneration derived from the MSCs that were delivered with the implant. No repair was seen in the control animals without implants or where the defect was filled with FMBs only. Repair scoring (on a 0-5 scale) was found to be 3.38 +/- 0.35 in the experimental arm, relative to 0 in the controls (p<0.001).