Intervertebral Disc Regeneration by Use of Autologous Mesenchymal Stem Cells, an Experimental Model in Rabbits

• a - CRCG seed grant, HKU foundation seed grant

Introduction: Intervertebral disc degeneration (IVD) is manifested by a gradual loss of proteoglycans and water within the intervertebral disc. This is likely due to an inability of the aging disc cells to maintain its structural integrity. If new cells can be introduced, or cells within the intervertebral disc can be rejuvenated, IVD may be halted. One such approach is by the use of mesenchymal stem cell (MSC) therapy. MSCs are pluripotent adult stem cells found within the bone marrow and are capable of differentiating in osteoblasts and chondrocytes. Although there are many studies on stem cell therapy, the intervertebral disc may be a particularly challenging environment, as it is avascular and is subjected to large mechanical loads. Disc cells are also not the same as chondrocytes. This study explores the possibility of using MSC to regenerate the intervertebral disc.

Methods: Autologous MSCs were isolated from the bone marrow of 15 New Zealand white rabbits, cultured and expanded ex vivo. They were tagged by bromo-deoxyuridine (BrdU), seeded onto a gelatin matrix, and re-implanted by open surgery into the lumbar disc spaces. Four levels were operated on per rabbit, including, one control level where only the matrix was implanted; Two test levels where MSCs were implanted; And one sham level where the disc content was removed. Progress was assessed by bimonthly radiographs, and histologically after sacrifice at 3 weeks and 3 months post surgery.

Results: S: Of the 15 rabbits, 11 were operated on via a posterior approach (similar to a discectomy) and 4 via an anterior approach (with an annular flap). Histological examination of retrieved discs showed only one of the 11 posteriorly operated rabbits retained the MSCs, while 3 of the 4 anteriorly operated rabbits retained the cells. Radiographs of those with retained MSCs showed evidence of progressive degeneration, with end-plate sclerosis and marginal osteophytes, but disc height was maintained. Of the 4 rabbits that had MSCs retained within the disc space, BrdU-labeled cells could be detected within the nucleus pulposus, annulus fibrosis and cartilage end-plates at 3 weeks and 3 months post-operatively. Moreover, these BrdU-labeled cells are morphologically similar to adjacent endogenous cells, suggesting the cells have taken on a disc phenotype.

Discussion: Anterior surgical approach has a higher success rate of retaining cells probably because of the ability to create an annular window which can cover the defect and therefore prevent the high axial forces to push out the gelatin scaffold and cells. Histological sections at 3 months clearly demonstrated the survival and incorporation of these cells into the intervertebral disc. This study provided evidence that direct stem cell therapy is feasible. Future work will focus on the gene expression profile of these cells, whether the matrix is altered as a result of their introduction, and ways to optimize their behavior in vivo.

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