INTEGRATING BIOMECHANICS AND CELL PHYSIOLOGY TO UNDERSTANDING IVD NUTRITION AND CELL HOMEOSTASIS

Back pain associated with degeneration of the intervertebral disc (IVD) is a major public health problem in Western industrialized societies. Degeneration of the IVD changes the osmotic and nutrient environment in the extracellular matrix (ECM) which affects cell behaviors, including: cell proliferation, cell energy metabolism, and matrix synthesis. In addition, a thin layer of hyaline cartilaginous end-plate (CEP) at the superior/inferior disc-vertebral interface was found to play an important role in nutrient supply as well as load distribution in the IVD. Therefore, our general hypothesis is that the CEP regulates the ECM osmotic and nutrient environment which further affects IVD cell energy metabolism and homeostasis. First, based on the triphasic theory, we developed a multiphasic model that considered the IVD tissue as a mixture with four phases: solid phase with fixed charges, interstitial water phase, ion phase with two monovalent species (e.g., Na+ and Cl‾), and an uncharged nutrient solute phase. Our numerical results showed calcification of the CEP significantly reduced the nutrient levels in the human IVD. In cell based therapies for IVD regeneration, excessive amounts of injected cells may cause further deterioration of the nutrient environment in the degenerated disc. To address the lack of experimental data on CEP tissue, the regional biomechanical and biochemical characterization of the bovine CEP was conducted. We found that the lateral endplate was much stiffer than the central endplate and might share a greater portion of loading. Our results also indicated that the CEP could block rapid solute convection and allowed pressurization of the interstitial fluid in response to loading. The energy metabolism properties of human IVD cells in different extracellular nutrient environments were also outlined. We found that human IVD cells prefer a more prevalent glycolytic pathway for energy needs under harsh nutrient environmental conditions and may switch towards oxidative phosphorylation once the glucose and oxygen levels increase. In order to further analyze the effect of the extracellular environment on cell homeostasis, IVD cells were defined as a fluid-filled membrane using mixture theory. The active ion transport process, which imparts momentum to solutes or solvent, was also incorporated in a supply term as it appears in the conservation of linear momentum. Meanwhile, the trans-membrane transport parameters (i.e hydraulic permeability and ion conductance) were experimentally determined from the measurements of passive cell volume response and trans-membrane ion transport using the differential interference contrast (DIC) and patch clamp techniques. This novel single cell model could help to further illuminate the mechanisms affecting IVD cell homeostasis. The objective of this project was to develop a multi-scale analytical model by incorporating experimentally determined IVD tissue and cell properties to predict the ECM environment and further analyzing its effect on cell energy metabolism and homeostasis. This work provided new insights into IVD degeneration mechanisms and cell based IVD regeneration therapies for low back pain