Tissue engineering for intervertebral disc repair

Degenerative disc disease, an irreversible process, is the leading cause of back pain. Nonoperative treatment is symptomatic and does not restore the disc. Current surgical treatment is limited to spinal fusion with varied results. Even if the fusion is successful, it eliminates joint motion and leads to accelerated disc degeneration at adjacent segments. Engineering replacement disc tissue in the long term is likely to supercede fusion therapy. Herein, we describe a novel approach to the treatment of disc disease using tissue-engineering principles. We investigate the use of nucleus pulposus cells as well as biodegradable materials (bioactive glass and polylactide-co-glycolide PLGA) to stimulate nucleus pulposus regeneration. In this thesis, we focus mostly on the biologic aspects of the investigation, since without a source of these cells, tissue engineering of the disc would be impossible. Success in isolating and expanding the nucleus pulposus cell population allows us to then proceed with engineering hybrids that can be used for disc replacement therapy. The first objective was to characterize the nucleus pulposus cells in situ. Then, a method was described for isolating and maintaining the cells in culture. Using a micromass culture system, it was shown that the cells exhibited the morphological and phenotypical characteristics of the nucleus pulposus in situ. Based on these findings, the capability of bioactive glass and the biodegradable polymer PLGA, to serve as substrates for nucleus pulposus cells, was investigated. Our data showed that the nucleus pulposus cells could be isolated and maintained in culture in a manner which preserved their phenotype. The monolayer and micromass culture systems provided techniques which allowed the cells to proliferate and express critical characteristics of the nucleus pulposus in situ prior to implantation. An investigation into the bioactive glass-cell hybrid suggested that nucleus pulposus cell proliferation might be an anchorage dependent event, and that the calcium phosphate-rich layer facilitates cell adhesion, and subsequent proliferation. However, unlike bioactive glass which actively induced cellular activity, PLGA only provided a surface for cell attachment and proliferation. Consequently, cell attachment to the polymer was limited. However, those that did attach, proliferated and maintained their phenotype on the substrate. All of these findings point to the advantages of bioactive glass as well as PLGA as carriers for nucleus pulposus cells. The use of the substrate-cell hybrid construct provides a new approach for treatment of degenerative disc disease