Organ culture bioreactors--platforms to study human intervertebral disc degeneration and regenerative therapy

Abstract

In recent decades the application of bioreactors has revolutionized the concept of culturing tissues and organs that require mechanical loading. In intervertebral disc (IVD) research, collaborative efforts of biomedical engineering, biology and mechatronics have led to the innovation of new loading devices that can maintain viable IVD organ explants from large animals and human cadavers in precisely defined nutritional and mechanical environments over extended culture periods. Particularly in spine and IVD research, these organ culture models offer appealing alternatives, as large bipedal animal models with naturally occurring IVD degeneration and a genetic background similar to the human condition do not exist. Latest research has demonstrated important concepts including the potential of homing of mesenchymal stem cells to nutritionally or mechanically stressed IVDs, and the regenerative potential of "smart" biomaterials for nucleus pulposus or annulus fibrosus repair. In this review, we summarize the current knowledge about cell therapy, injection of cytokines and short peptides to rescue the degenerating IVD. We further stress that most bioreactor systems simplify the real in vivo conditions providing a useful proof of concept. Limitations are that certain aspects of the immune host response and pain assessments cannot be addressed with ex vivo systems. Coccygeal animal disc models are commonly used because of their availability and similarity to human IVDs. Although in vitro loading environments are not identical to the human in vivo situation, 3D ex vivo organ culture models of large animal coccygeal and human lumbar IVDs should be seen as valid alternatives for screening and feasibility testing to augment existing small animal, large animal, and human clinical trial experiments.

Figure 1

General set-up of a loading chamber and overview of current main questions addressed for degeneration and regenerative approaches. The material of the containment is ideally glass or cytocompatible biomaterial that can be cleaned and sterilized easily. A media refreshment pump and media tubing can help to maintain cell viability longer. Loading of the disc can be achieved by application of uni-axial compression or complex multiple degree of freedom loading.

Figure 2. The evolution of intervertebral disc organ culture systems. From relatively simple designs with “free-swelling static” to dynamically loaded systems including media refreshment

A. free-swelling organ culture in a simple plastic beaker. B. Organ culture using porous platens and uni-axial compression. Possibility to apply diurnal loading by using defined weights on top of the culture system. C. Cyclic compression system allowing for frequency control . D. Cyclic compression and media refreshment system using a peristaltic pump. E. Inlet: Adjusted platens to adapt the concave shape of the CEP in the case of the coccygeal bovine IVD. F. Culture chamber fitting the special porous platens.

Figure 3. Five examples of Bioreactors recently published by different research teams

A. Polycarbonate chamber with gas-permeable silicon membrane [36,37]. Key feature: media refreshment system with tubing; culture of IVD with BEP, a steel ball in the axis to ensure parallel and even force transition. 1. Force cell, 2. Coupling with steel ball, 3. Polycarbonate culture chamber, 4. Pneumatic actuator “fluidic muscle”. B. Polycarbonate culture system to allow IVD culture with BEP; key feature: perfusion of culture media through large medium containers [31]. C. Key feature: high force hydraulically actuated bioreactor, Instrumented with load cells and inductive way sensors to allow measurements of IVD height and mechanics throughout testing and culture, chambers are made from Ultem® [29]. D. Glass and Polyoxymethylen (POM) press fit design with uniaxial compression. key feature: specialized adjustments to allow bovine and human IVD culture [33]. E. Glass and POM bioreactor [32]; key feature: presence of serrated titanium plates to allow torsion and compression, special release mechanism to hook and un-hook the bioreactor stations.