Comparison of canine allogenic umbilical cord blood derived mesenchymal stem cells and their lysates mixed with beta-tricalcium phosphate in orthotopic implantation

The objective of this study is to evaluate the effect of mixtures of β-tricalcium phosphate(β-Ca₃(PO₄)₂, β-TCP) granules with canine allogenic umbilical cord blood derived mesenchymal stem cells(UCB-MSC) and their lysates on bone formation. The mixtures were implanted into cylindrical bone defects(7 mm diameter, 10 mm length) made bilaterally by using the trephine in the medial aspect of the distal femur and proximal tibia. Fluorochromes for bone labeling were administered for measuring the rate of mineral apposition at 4, 6 and 8 weeks after implantation. A radiographic evaluation of the radiopacity of the implant in the bone defect was made at 2 weeks intervals. At 10 weeks, postoperatively, bone regeneration and resorption of β-TCP were assessed by micro-computed tomography(CT) in the tibia and by fluorescence microscopy and histomorphological examination in the femur and tibia. In micro CT, cortical bony bridging in the UCB-MSC, cell lysate and control groups was observed in 2, 1 and 0 out of each of the 3 samples, respectively. Bone mineral densities within trabecular bone defects were significantly different among the different groups(UCB-MSC < cell lysate < control) but the mineral apposition rate per day from 4 weeks after implantation was not different. In histomorphology the rates of new bone formation and resorption of the β-TCP in the UCB-MSC group were significantly higher than in the control group of both femur and tibia than in the cell lysate group in the tibia(p<0.05). The rate of new bone formation in the cell lysate group was not different compared to that of the control group in both the femur and the tibia. However, the absorption rate of the β-TCP in the tibia was significantly higher than that of the control group. The UCB-MSC improved the absorption of β-TCP and bone formation without site-dependence; however their lysates showed site-dependent effects. These results suggested that the canine allogenic UCB-MSC mixed with β-TCP had a higher bone healing activity for bone defects than their lysates.