Intervertebral disc degeneration is a contributing cause for many spine-related disabilities, and is a significant health concern in the United States. Currently available treatments aim to gain symptomatic relief, and do not treat the underlying biological problem. Cell therapy aims to target the cause of disc degeneration by repopulating the disc with cells capable of synthesizing appropriate matrix, restoring its biomechanical properties. The goal of this research was to investigate the use of mesenchymal stem cells (MSCs) for nucleus pulposus regeneration.

An in vivo injection model was used to characterize the regeneration potential of undifferentiated MSCs, juvenile chondrocytes, and fibrin carrier in rat coccygeal discs. Juvenile chondrocyte-injected discs displayed significantly improved MRI index over fibrin-injected discs, and histology indicated that injecting a more mature cell type resulted in maintenance of normal disc morphology.

To advance our understanding of the phenotypic changes that might arise in MSCs after transplantation to the disc environment, a novel bioreactor system was developed that mimics they hypoxia and hydrostatic pressure found in the intervertebral disc. Culturing MSCs in this system demonstrated that the physiochemical environment of the disc is sufficient to induce differentiation of MSCs, as evidenced by GAG production and increase in chondrogenic genes such as aggrecan, Sox9, and collagen II.

Pre-treatment of MSCs with TGF-!3 prior to bioreactor culture indicated that differentiated cells may function more effectively in disc-mimetic conditions, as evidenced by increased cell viability, GAG production, and persistence of TGF-!3- induced gene expression. More importantly, environment-dependent differences in gene expression highlighted the necessity of mimicking several aspects of the disc environment in concert when evaluating the fate of cells post-implantation. To better mimic the degenerated disc environment, inflammation was incorporated into the bioreactor. Expression levels of aggrecan, collagen II, and collagen X suggest that when MSCs are appropriately pre-differentiated and evaluated in a disc-mimetic environment, inflammation may not be as detrimental as seen in previous in vitro experiments.

This research highlights the importance of representing the salient features of the anticipated environment when evaluating cells for in situ disc regeneration, and demonstrates that MSCs may be optimized for nucleus pulposus regeneration.