MC3T3-E1 osteoprogenitor cells systemically migrate to a bone defect and enhance bone healing

Abstract

Although iliac crest autologous bone graft remains the gold standard for treatment of bone defects, delayed- and nonunions, and arthrodeses, several alternative strategies have been attempted, including the use of mesenchymal stem cells. Whether cells from the osteoblast lineage demonstrate systemic recruitment to an acute bone defect or fracture, and whether these cells directly participate in bone healing is controversial. This study tests two hypotheses: (1) that exogenous murine MC3T3-E1 osteoprogenitor cells with a high propensity for osteoblast differentiation are able to systemically migrate to a bone defect and (2) that the migrated MC3T3-E1 cells enhance bone healing. Two groups of nude mice were used; a bone defect was drilled in the left femoral shaft in both groups. MC3T3-E1 were used as reporter cells and injected in the left ventricle of the heart, to avoid sequestration in the lungs. Injection of saline served as a control. We used bioluminescence and microCT to assay cell recruitment and bone mineral density (BMD). Immunohistochemical staining was used to confirm the migration of reporter cells. MC3T3-E1 cells were found to systemically migrate to the bone defect. Further, BMD at the defect was significantly increased when cells were injected. Systemic cell therapy using osteoprogenitor cells may be a potential strategy to enhance bone healing.

FIG. 1.

MC3T3 cells injected into the left ventricle have systemically migrated to a bone defect drilled in the left femur. Ratio of bioluminescence signal from operated femora divided by nonoperated femora from day 0 to 14. *p<0.05.

FIG. 2.

Bone mineral density analysis. Bone density of the callus is significantly increased when mice were injected with MC3T3-E1 cells, compared with control injection of saline. Total bone mineral density (post−pre values). *p<0.05.

FIG. 3.

Coronal view of reconstructed left femora. CT performed 1 day before cell injection and at day 14. The size of the callus (red arrows) is increased when mice where injected with reporter cells as opposed to saline alone. Color images available online at www.liebertonline.com/tea

FIG. 4.

Hematoxylin and eosin-stained sections from frozen, decalcified femora from Group 1 (A) with intracardiac osteoprogenitor cell injection, and Group 2 (B) without cell injection. Note the abundant callus (red arrows) over the cortical defect in Group 1. Plump osteoblasts surround the new woven bone. Original magnification: ×50. Color images available online at www.liebertonline.com/tea

FIG. 5.

Immunohistochemistry for Group 1 (left column) and Group 2 (right column). The bone defect is marked with arrows. Note that MC3T3-E1 reporter cells (yellow stain) have systemically migrated to the bone defect and produced a large amount of callus. Original magnification: ×50 (upper line) and ×200 (lower line). Color images available online at www.liebertonline.com/tea