Tissue engineered nucleus pulposus tissue formed on a porous calcium polyphosphate substrate

Study Design. This study describes the formation of nucleus pulposus tissue using a novel tissue engineering approach. Objectives. To determine if a construct composed of nucleus pulposus tissue on the surface of a calcium polyphosphate substrate could be formed in vitro with properties similar to native nucleus pulposus tissue. Summary of Background Data. There is no optimal treatment for the persistent pain associated with intervertebral disc degeneration. Disc replacement using artificial intervertebral discs has met with some success, and biologic transplantation is limited by the availability of donor tissues. Methods. Nucleus pulposus cells were isolated from bovine caudal intervertebral discs. Cells were seeded at high density on the upper surface of a porous bone substitute material ( calcium polyphosphate) and maintained up to 6 weeks in culture. In vitro formed tissue was compared to native nucleus pulposus for histologic appearance, biochemical composition ( tissue cellularity, proteoglycan and collagen accumulation), and compressive mechanical properties. Results. When maintained on the surface of a three-dimensional substrate, nucleus pulposus cells formed a continuous layer of tissue with a proteoglycan content equivalent to the native tissue. Although collagen accumulation attained only 26% than that of the native tissue, there was no difference in tissue stiffness, viscosity, or weight-bearing capacity of the in vitro formed tissue when compared with the native tissue. Conclusion. Nucleus pulposus-like tissue formed in vitro on the surface of a calcium polyphosphate substrate resembles the native tissue in terms of proteoglycan content and compressive mechanical properties. These studies are the first step toward developing a functional spinal unit in vitro.