In this paper, researchers report on progress towards the manufacture of intervertebral discs suitable for transplantation. These tissue structures sit between the bones of the spine, the vertebrae. A sizable proportion of the population suffers at least some degree of degenerative disc disease even quite early in later life. It is one of the first serious consequences of the underlying damage that accumulates to cause aging, as well as one of the most widespread, and so there is a large potential market for practical tissue engineering or regenerative medicine solutions in this part of the field. That said, anything involving surgery and the spine isn't going to be cheap, and this is one of many areas in which therapies that can restore and repair existing tissue structures would be vastly preferable.
The intervertebral disc (IVD) is located between the vertebral bodies and is responsible for distributing forces experienced by the spinal column. It is composed of nucleus pulposus (NP) surrounded by annulus fibrosus (AF). The NP is compression resistant and rich in type II collagen and proteoglycans. The AF is comprised of multiple lamellae of angle-ply and aligned bundles of collagen fibrils, which confer the stability for spinal motion by resisting tensile forces. Adding to the complexity of the AF structure, the extracellular matrix (ECM) composition of the AF varies from the inner zone adjacent to the NP, where it is rich in proteoglycans and both type II and I collagen, to the outer zone, which is rich in type I collagen.
Replacement of the damaged disc with an in vitro formed IVD that has the functionality of a healthy disc is a reparative approach that is currently being investigated. However, recapitulating the unique architecture of the disc has been a limitation to developing this approach for clinical use. Previous studies showed that NP tissues with compressive strength can be formed scaffold free and integrated to the top surface of a porous bone substitute material such as calcium polyphosphate (CPP). The bone substitute will help anchor the implanted tissue as bone ingrowth will fix it into the bone.
AF tissue has been generated using biodegradable electrospun-aligned nanofibrous polycarbonate urethane (PU). This scaffold has the tensile strength of a native AF lamella and fiber diameters similar to the native collagen fibrils, allowing seeded AF cells to accumulate collagen aligned parallel to the scaffold. However, the successful integration of in vitro generated NP and AF tissues is crucial for it to be mechanically functional in vivo and for the longevity of the engineered IVD replacement as the NP and AF function together to resist reduction in disc height and extraneous deformation. Defective integration between in vitro engineered AF and NP tissues would resemble a fissure within a disc and thus could result in the failure of disc replacement.
Thus, the goal of engineering of an IVD replacement should be focused toward generation of a disc that can serve as a functional motion segment that recapitulates the complex architecture of the disc, exhibits the capability to withstand complex forces in vivo, and shows integration between the different tissue types in the engineered disc and with the host tissues that will be maintained postimplantation. In this study, we report the development of a two-step process to form an in vitro integrated IVD model composed of preformed multilamellated AF tissue utilizing nanofibrous aligned PU scaffolds and NP tissues formed on a bone substitute material. This tissue was characterized histologically, by immunohistochemical staining, and biomechanically. To assess integration and adherence to the bone substitute in vivo, short-term evaluation of this construct in a bovine model was performed.
This study shows that it is possible to form a model of the IVD in vitro by combining preformed AF and NP tissues. These tissues integrate and have mechanical stability. This is the first report, to the best of our knowledge, describing integration of in vitro formed AF and NP tissues and evaluation of the interfacial shear strength. The mechanism that led to this integration is unknown. A previous study that examined bioengineered cartilage-cartilage integration suggested that the matrix between the two tissues intermingle as studies showed thin collagen fibers that were produced by the bioengineered cartilage admixed with the mature collagen fibers of the native cartilage across the interface with the host tissue. The presence of both type I and II collagens at the AF-NP interface suggests that this may be occurring in this situation as well.
Interestingly, it has been proposed that the integration of distinct tissue types requires an intervening region that serves as a gradual and continuous transition in ECM properties and/or mechanical properties. In summary, this study demonstrates that it is possible to generate a model of an IVD by combining the individual tissue components and forming various interfaces with sufficient mechanical strength to be handled. The construct was present 1 month after implantation and the AF tissue was intact. Further studies are required to optimize implant fixation and scale up the disc size to evaluate its suitability as a disc replacement in an animal model.