Mechanical loading of the intervertebral disc: from the macroscopic to the cellular level.

PURPOSE: Mechanical loading represents an integral part of intervertebral disc (IVD) homeostasis. This review aims to summarise recent knowledge on the effects of mechanical loads on the IVD and the disc cells, taking into consideration the changes that IVDs undergo during ageing and degeneration, from the macroscopic to the cellular and subcellular level. METHODS: Non-systematic literature review. RESULTS: Several scientific papers investigated the external loads that act on the spine and the resulting stresses inside the IVD, which contribute to estimate the mechanical stimuli that influence the cells that are embedded within the disc matrix. As disc cell responses are also influenced by their biochemical environment, recent papers addressed the role that degradation pathways play in the regulation of (1) cell viability, proliferation and differentiation and (2) matrix production and turnover. Special emphasis was put on the intracellular-signalling pathways, as mechanotransduction pathways play an important role in the maintenance of normal disc metabolism and in disc degenerative pathways. CONCLUSIONS: Disc cells are exposed to a wide range of mechanical loads, and the biochemical environment influences their responses. Degeneration-associated alterations of the disc matrix change the biochemical environment of disc cells and also the mechanical properties of the disc matrix. Recent studies indicate that these factors interact and regulate disc matrix turnover.

Effect of intervertebral disc degeneration on disc cell viability: a numerical investigation.

Degeneration of the intervertebral disc may be initiated and supported by impairment of the nutrition processes of the disc cells. The effects of degenerative changes on cell nutrition are, however, only partially understood. In this work, a finite volume model was used to investigate the effect of endplate calcification, water loss, reduction of disc height and cyclic mechanical loading on the sustainability of the disc cell population. Oxygen, lactate and glucose diffusion, production and consumption were modelled with non-linear coupled partial differential equations. Oxygen and glucose consumption and lactate production were expressed as a function of local oxygen concentration, pH and cell density. The cell viability criteria were based on local glucose concentration and pH. Considering a disc with normal water content, cell death was initiated in the centre of the nucleus for oxygen, glucose, and lactate diffusivities in the cartilaginous endplate below 20% of the physiological values. The initial cell population could not be sustained even in the non-calcified endplates when a reduction of diffusion inside the disc due to water loss was modelled. Alterations in the disc shape such as height loss, which shortens the transport route between the nutrient sources and the cells, and cyclic mechanical loads, could enhance cell nutrition processes.

Evaluation of platelet-rich plasma and hydrostatic pressure regarding cell differentiation in nucleus pulposus tissue engineering.

Generation of a biological nucleus pulposus (NP) replacement by tissue engineering appears to be a promising approach for the therapy of early stages of intervertebral disc degeneration. Thereby, autologous mesenchymal stem cells (MSCs) represent an attractive cell source compared to cells of the NP that are already altered in their phenotype due to degenerative processes. This study compares the influence of 3D pellet culture and alginate beads, as well as that of different media compositions, by the addition of human platelet-rich plasma (PRP) or transforming growth factor (TGF-ß1 ) in interaction with hydrostatic pressure on chondrogenic differentiation of human MSCs compared to NP cells. We found that gene expression of the chondrogenic markers aggrecan, collagen type 2 and collagen type 1 and Sox9 was considerably lower in cells cultivated with PRP compared to TGF-ß1 . Immunohistology confirmed this result at protein level in pellet culture. Additionally, the pellet culture system was found to be more suitable than alginate beads. A positive influence of hydrostatic pressure could only be shown for individual donors. In summary, in comparison to TGF-ß1 , human PRP did not induce adequate chondrogenic differentiation for both culture systems and cell types used. The mixture of growth factors in PRP promoted proliferation rather than chondrogenic differentiation. Based on these results, an application of PRP in human NP tissue-engineering approaches cannot be recommended.

Interactions of environmental conditions and mechanical loads have influence on matrix turnover by nucleus pulposus cells.

Disc degeneration is associated with several changes in the physicochemical environment of intervertebral disc cells. Nucleus pulposus (NP) cells in the center of degenerated discs are exposed to decreased glucose supply, osmolarity, pH, and oxygen levels. To understand the complexity of these interactions on a cellular level, we designed standardized experiments in which we compared responses to these environmental factors under normal levels with those seen under two different degrees of disc degeneration. We hypothesized that these changes in environmental stimuli influence gene expression of matrix proteins and matrix degrading enzymes and alter their responses to cyclic hydrostatic pressure (HP). Our results suggest that a simulation of degenerative conditions influences the degradation of disc matrix through impairing matrix formation and accelerating matrix resorption via up- or down-regulation of the respective target genes. The greatest effects were seen for decreases in glucose concentration and pH. Low oxygen had little influence. HP had little direct effect but appeared to counteract matrix degradation by reducing or inverting some of the adverse effects of other stimuli. For ongoing in vitro studies, interactions between mechanical stimuli and factors in the physicochemical environment should not be ignored as these could markedly influence results.