Developing a Mouse Model to Study Mesenchymal Stem Cell Survival, Proliferation, and Migration in Lumbar Posterolateral Spinal Fusion

Abstract

Introduction While spinal fusion surgery has advanced over the past decade, a great deal is unknown regarding the underlying mechanisms that contribute to spinal fusion failure or pseudoarthrosis. Clinically, bone marrow aspirate has been successfully used alongside a variety of grafting materials to augment fusion (Neen et al. Spine (Phila Pa 1976). 2006;31(18):E636; Carter et al. Spine J. 2009;9(6):434). To date, studies evaluating these procedures have focused on radiographic monitoring and endpoint analysis. Little is known about the survival and proliferation of these transplanted mesenchymal cells or how much they contribute to bone formation compared with host cells from adjacent decorticated bone. Insights into these cell dynamics will give us a deeper understanding of the biological processes underlying spinal fusion and allow for evaluation and development of fusion therapies that reduce rates of pseudoarthrosis. Material and Methods A total of 40 FVB/NJ mice underwent posterolateral lumbar spinal fusion surgery and were divided into four groups based upon the fusion graft employed: syngeneic iliac crest bone graft ( n = 10), Vitoss bone-matrix alone ( n = 10), Vitoss bone-matrix with syngeneic bone marrow cells (2.5 × 106 cell/side) ( n = 10), and Vitoss bone-matrix with transgenic luciferase-expressing syngeneic bone marrow cells (2.5 × 106 cell/side) ( n = 10). Bone marrow cells were isolated from the long bones of either syngeneic FVB/NJ mice or syngeneic FVB mice which constitutively express the luciferase gene (FVB-Tg (CAG-luc,-GFP) L2G85Chco/J mice). The in vivo survival, distribution, and proliferation of the luciferase-expressing bone marrow-derived cells were monitored via bioluminescence imaging over a period of 8 weeks. Spinal fusion was assessed 4, 6, and 8 weeks postsurgery via micro-CT imaging, as well as via manual palpation at 8 weeks. Immunohistochemistry was performed to assess fusion bone growth as well as determine the number of donor versus host cells within the fusion mass. Results Bioluminescent imaging data demonstrated quantifiable cellular viability within the fusion mass as early as 24 hours following surgery, with increasing bioluminescent signal over the first 2 weeks postsurgery, thus indicating cellular proliferation. Bioluminescence decreased between weeks 4 and 8 suggesting a loss of transplanted mesenchymal progenitor cells within the fusion mass at later time points. Correlations with histology and micro-CT imaging of the developing fusion mass are currently under analysis. Conclusions We have demonstrated a novel murine model which enables noninvasive in vivo monitoring of implanted transgenic luciferase-expressing bone marrow cells following spinal fusion surgery. Future applications of this model will include the assessment of additional therapeutics for improving spinal fusion in translational research studies.