Metabolism of the intervertebral disc: effects of low levels of oxygen, glucose, and pH on rates of energy metabolism of bovine nucleus pulposus cells

Responses of human adipose-derived mesenchymal stem cells to chemical microenvironment of the intervertebral disc

Background

Human adipose-derived mesenchymal stem cells (ADMSCs) may be ideal source of cells for intervertebral disc (IVD) regeneration, but the harsh chemical microenvironment of IVD may significantly influence the biological and metabolic vitality of ADMSCs and impair their repair potential. This study aimed to investigate the viability, proliferation and the expression of main matrix proteins of ADMSCs in the chemical microenvironment of IVD under normal and degeneration conditions.

Methods

ADMSCs were harvested from young (aged 8-12 years, n = 6) and mature (aged 33-42 years, n = 6) male donors and cultured under standard condition and IVD-like conditions (low glucose, acidity, high osmolarity, and combined conditions) for 2 weeks. Cell viability was measured by annexin V-FITC and PI staining and cell proliferation was measured by MTT assay. The expression of aggrecan and collagen-I was detected by real-time quantitative polymerase chain reaction and Western blot analysis.

Results

IVD-like glucose condition slightly inhibited cell viability, but increased the expression of aggrecan. In contrast, IVD-like osmolarity, acidity and the combined conditions inhibited cell viability and proliferation and the expression of aggrecan and collagen-I. ADMSCs from young and mature donors exhibited similar responses to the chemical microenvironments of IVD.

Conclusion

IVD-like low glucose is a positive factor but IVD-like high osmolarity and low pH are deleterious factors that affect the survival and biological behaviors of ADMSCs. These findings may promote the translational research of ADMSCs in IVD regeneration for the treatment of low back pain.

Simulated-physiological loading conditions preserve biological and mechanical properties of caprine lumbar intervertebral discs in ex vivo culture

Low-back pain (LBP) is a common medical complaint and associated with high societal costs. Degeneration of the intervertebral disc (IVD) is assumed to be an important causal factor of LBP. IVDs are continuously mechanically loaded and both positive and negative effects have been attributed to different loading conditions.

In order to study mechanical loading effects, degeneration-associated processes and/or potential regenerative therapies in IVDs, it is imperative to maintain the IVDs' structural integrity. While in vivo models provide comprehensive insight in IVD biology, an accompanying organ culture model can focus on a single factor, such as loading and may serve as a prescreening model to reduce life animal testing. In the current study we examined the feasibility of organ culture of caprine lumbar discs, with the hypothesis that a simulated-physiological load will optimally preserve IVD properties.

Lumbar caprine IVDs (n = 175) were cultured in a bioreactor up to 21 days either without load, low dynamic load (LDL), or with simulated-physiological load (SPL). IVD stiffness was calculated from measurements of IVD loading and displacement. IVD nucleus, inner- and outer annulus were assessed for cell viability, cell density and gene expression. The extracellular matrix (ECM) was analyzed for water, glycosaminoglycan and total collagen content.

IVD biomechanical properties did not change significantly with loading conditions. With SPL, cell viability, cell density and gene expression were preserved up to 21 days. Both unloaded and LDL resulted in decreased cell viability, cell density and significant changes in gene expression, yet no differences in ECM content were observed in any group.

In conclusion, simulated-physiological loading preserved the native properties of caprine IVDs during a 21-day culture period. The characterization of caprine IVD response to culture in the LDCS under SPL conditions paves the way for controlled analysis of degeneration- and regeneration-associated processes in the future.

Quantitative analysis of exogenous IGF-1 administration of intervertebral disc through intradiscal injection

Exogenous administration of IGF-1 has been proposed as a therapy for disc degeneration. The objectives of this study were to develop a numerical model for quantitatively analysing exogenous administration of IGF-1 into the intervertebral disc (IVD) via intradiscal injection and to investigate the effects of IGF-1 administration on distribution of glucose and oxygen in the IVD. In this study, the reversible binding reaction between IGF-1 and IGF binding proteins was incorporated into the mechano-electrochemical mixture model. The model was used to numerically analyse transport of IGF-1, glucose, oxygen and lactate in the IVD after IGF-1 administration. The enhancement of IGF-1 on lactate production was also taken into account in the theoretical model. The numerical analyses using finite element method demonstrated that the binding reactions significantly affect the time-dependent distribution of IGF-1 in the IVD. It was found that the region affected by IGF-1 was smaller and the duration of the therapeutic IGF-1 level was longer in the degenerated disc with a higher concentration of IGF binding proteins. It was also found that the IGF-1 injection can reduce glucose concentration and increase lactate accumulation (i.e., lower pH) in the IVD and these influences were regulated by the IGF-1 binding reactions. This study indicated the complexity of intradiscal administration of growth factors, which needs to be fully analysed in order to achieve a successful outcome. The new theoretical model developed in this study can serve as a powerful tool in analysing and designing the optimal treatments of growth factors for disc degeneration.

3D finite element analysis of nutrient distributions and cell viability in the intervertebral disc: effects of deformation and degeneration

The intervertebral disc (IVD) receives important nutrients, such as glucose, from surrounding blood vessels. Poor nutritional supply is believed to play a key role in disc degeneration. Several investigators have presented finite element models of the IVD to investigate disc nutrition; however, none has predicted nutrient levels and cell viability in the disc with a realistic 3D geometry and tissue properties coupled to mechanical deformation. Understanding how degeneration and loading affect nutrition and cell viability is necessary for elucidating the mechanisms of disc degeneration and low back pain. The objective of this study was to analyze the effects of disc degeneration and static deformation on glucose distributions and cell viability in the IVD using finite element analysis. A realistic 3D finite element model of the IVD was developed based on mechano-electrochemical mixture theory. In the model, the cellular metabolic activities and viability were related to nutrient concentrations, and transport properties of nutrients were dependent on tissue deformation. The effects of disc degeneration and mechanical compression on glucose concentrations and cell density distributions in the IVD were investigated. To examine effects of disc degeneration, tissue properties were altered to reflect those of degenerated tissue, including reduced water content, fixed charge density, height, and endplate permeability. Two mechanical loading conditions were also investigated: a reference (undeformed) case and a 10% static deformation case. In general, nutrient levels decreased moving away from the nutritional supply at the disc periphery. Minimum glucose levels were at the interface between the nucleus and annulus regions of the disc. Deformation caused a 6.2% decrease in the minimum glucose concentration in the normal IVD, while degeneration resulted in an 80% decrease. Although cell density was not affected in the undeformed normal disc, there was a decrease in cell viability in the degenerated case, in which averaged cell density fell 11% compared with the normal case. This effect was further exacerbated by deformation of the degenerated IVD. Both deformation and disc degeneration altered the glucose distribution in the IVD. For the degenerated case, glucose levels fell below levels necessary for maintaining cell viability, and cell density decreased. This study provides important insight into nutrition-related mechanisms of disc degeneration. Moreover, our model may serve as a powerful tool in the development of new treatments for low back pain.

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.

Mechanical loading affects the energy metabolism of intervertebral disc cells

Research has shown that mechanical loading affects matrix biosynthesis of intervertebral disc (IVD) cells; however, the pathway(s) to this effect is currently unknown. Cellular matrix biosynthesis is an energy demanding process. The objective of this study was to investigate the effects of static and dynamic compressive loading on energy metabolism of IVD cells. Porcine annulus fibrosus (AF) and nucleus pulposus (NP) cells seeded in 2% agarose were used in this experiment. Experimental groups included 15% static compression and 0.1 and 1 Hz dynamic compression at 15% strain magnitude for 4 h. ATP, lactate, glucose, and nitric oxide (NO) contents in culture media, and ATP content in cell-agarose construct were measured using biochemical assays. While the total ATP content of AF cells was promoted by static and dynamic loading, only 1 Hz dynamic loading increased total ATP content of NP cells. Increases in lactate production and glucose consumption of AF cells suggest that ATP production via glycolysis is promoted by dynamic compression. ATP release and NO production of AF and NP cells were significantly increased by dynamic loading. Thus, this study clearly illustrates that static and dynamic compressive loading affect IVD cell energy production while cellular responses to mechanical loading were both cell type and compression type dependent.

Effect of hyperbaric oxygenation on intervertebral disc degeneration: an in vitro study with human lumbar nucleus pulposus

STUDY DESIGN

An in vitro study with degenerated human lumbar intervertebral disc specimens cultured under hyperbaric oxygenation (HBO).

OBJECTIVE

To observe the changes in interleukin (IL)-1β, prostaglandin (PG)-E2, nitric oxide (NO), cell growth, and apoptosis of the human nucleus pulposus cell (NPC) after HBO.

SUMMARY OF BACKGROUND DATA

Intervertebral disc degeneration has been demonstrated as related to IL-1β, PG-E2, NO, and O2 concentration but the actual mechanism is not clear. HBO also has also been reported in the literature to influence changes in IL-1β, prostaglandin E2, NO, and O2 concentration. However, the direct effect of HBO on the disc cells has not been previously reported.

METHODS

We collected 12 human lumbar degenerated disc specimens and evaluated the effects of HBO on the cultured NPCs. The amounts of IL-1β, PG-E2, and NO in the conditioned medium were quantified by enzyme-linked immunosorbent assay and high performance liquid chromatography. Cell growth was measured by increase in cell number. Cell viability and proteoglycan content were evaluated by histologic study using safranin O staining. In situ analysis of apoptosis was performed using Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining.

RESULTS

Our data indicated that HBO treatment inhibited IL-1β, PG-E2, and NO production but increased cell number and matrix synthesis of cultured NPCs. TUNEL staining showed that HBO treatment suppressed the apoptosis of cultured NPCs.

CONCLUSION

HBO provides a potential treatment modality for disc degeneration.

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.

Structure and biology of the intervertebral disk in health and disease

The intervertebral disks along the spine provide motion and protection against mechanical loading. The 3 structural components, nucleus pulposus, annulus fibrosus, and cartilage endplate, function as a synergistic unit, though each has its own role. The cells within each of these components have distinct origins in development and morphology, producing specific extracellular matrix proteins that are organized into unique architectures fit for intervertebral disk function. This article focuses on various aspects of intervertebral disk biology and disruptions that could lead to diseases such as intervertebral disk degeneration.

Intervertebral disk nutrition: a review of factors influencing concentrations of nutrients and metabolites

The biomechanical behavior of the intervertebral disk ultimately depends on the viability and activity of a small population of resident cells that make and maintain the disk's extracellular matrix. Nutrients that support these cells are supplied by the blood vessels at the disks' margins and diffuse through the matrix of the avascular disk to the cells. This article reviews pathways of nutrient supply to these cells; examines factors that may interrupt these pathways, and discusses consequences for disk cell survival, disk degeneration, and disk repair.

On-line monitoring of oxygen as a non-destructive method to quantify cells in engineered 3D tissue constructs

Regulatory guidelines have established the importance of introducing quantitative quality controls during the production and/or at the time of release of cellular grafts for clinical applications. In this study we aimed to determine whether on-line measurements of oxygen can be used as a non-destructive method to estimate the number of chondrocytes within an engineered cartilage graft. Human chondrocytes were seeded and cultured in a perfusion bioreactor, and oxygen levels in the culture medium were continuously monitored at the inlet and outlet of the bioreactor chamber throughout the culture period. We found that the drop in oxygen across the perfused construct was linearly correlated with the number of cells per construct (R(2)  = 0.82, p < 0.0001). The method was valid for a wide range of cell numbers, including cell densities currently used in the manufacture of cartilage grafts for clinical applications. Given that few or no non-destructive assays that quantitatively characterize an engineered construct currently exist, this non-invasive method could represent a relevant instrument in regulatory compliant manufacturing of engineered grafts meeting specific quality criteria.

Porcine intervertebral disc repair using allogeneic juvenile articular chondrocytes or mesenchymal stem cells

Tissue engineering strategies for intervertebral disc repair have focused on the use of autologous disc-derived chondrocytes. Difficulties with graft procurement, harvest site morbidity, and functionality, however, may limit the utility of this cell source. We used an in vivo porcine model to investigate allogeneic non-disc-derived chondrocytes and allogeneic mesenchymal stem cells (MSCs) for disc repair. After denucleation, lumbar discs were injected with either fibrin carrier alone, allogeneic juvenile chondrocytes (JCs), or allogeneic MSCs. Discs were harvested at 3, 6, and 12 months, and cell viability and functionality were assessed qualitatively and quantitatively. JC-treated discs demonstrated abundant cartilage formation at 3 months, and to a lesser extent at 6 and 12 months. For the carrier and MSC-treated groups, however, there was little evidence of proteoglycan matrix or residual notochordal/chondrocyte cells, but rather a type I/II collagen-enriched scar tissue. By contrast, JCs produced a type II collagen-rich matrix that was largely absent of type I collagen. Viable JCs were observed at all time points, whereas no evidence of viable MSCs was found. These data support the premise that committed chondrocytes are more appropriate for use in disc repair, as they are uniquely suited for survival in the ischemic disc microenvironment.

The effect of sustained compression on oxygen metabolic transport in the intervertebral disc decreases with degenerative changes

Intervertebral disc metabolic transport is essential to the functional spine and provides the cells with the nutrients necessary to tissue maintenance. Disc degenerative changes alter the tissue mechanics, but interactions between mechanical loading and disc transport are still an open issue. A poromechanical finite element model of the human disc was coupled with oxygen and lactate transport models. Deformations and fluid flow were linked to transport predictions by including strain-dependent diffusion and advection. The two solute transport models were also coupled to account for cell metabolism. With this approach, the relevance of metabolic and mechano-transport couplings were assessed in the healthy disc under loading-recovery daily compression. Disc height, cell density and material degenerative changes were parametrically simulated to study their influence on the calculated solute concentrations. The effects of load frequency and amplitude were also studied in the healthy disc by considering short periods of cyclic compression. Results indicate that external loads influence the oxygen and lactate regional distributions within the disc when large volume changes modify diffusion distances and diffusivities, especially when healthy disc properties are simulated. Advection was negligible under both sustained and cyclic compression. Simulating degeneration, mechanical changes inhibited the mechanical effect on transport while disc height, fluid content, nucleus pressure and overall cell density reductions affected significantly transport predictions. For the healthy disc, nutrient concentration patterns depended mostly on the time of sustained compression and recovery. The relevant effect of cell density on the metabolic transport indicates the disturbance of cell number as a possible onset for disc degeneration via alteration of the metabolic balance. Results also suggest that healthy disc properties have a positive effect of loading on metabolic transport. Such relation, relevant to the maintenance of the tissue functional composition, would therefore link disc function with disc nutrition.

Regional cell density distribution and oxygen consumption rates in porcine TMJ discs: an explant study

OBJECTIVE

To determine the regional cell density distribution and basal oxygen consumption rates (based on tissue volume and cell number) of temporomandibular joint (TMJ) discs and further examine the impact of oxygen tension on these rates.

DESIGN

TMJ discs from pigs aged 6-8 months were divided into five regions: anterior, intermediate, posterior, lateral and medial. The cell density was determined using confocal laser scanning microscopy. The change in oxygen tension was recorded while TMJ disc explants were cultured in sealed metabolism chambers. The volume based oxygen consumption rate of explants was determined by theoretical curve-fitting of the recorded oxygen tension data with the Michaelis-Menten equation. The rate on a per-cell basis was calculated based on the cell density measurements and volume based rate measured in another group of discs.

RESULTS

The overall cell density [mean, 95% confidence interval (CI)] was 51.3 (21.3-81.3) × 10(6) cells/mL wet tissue. Along the anteroposterior axis, the anterior band had 25.5% higher cell density than the intermediate zone (P<0.02) and 29.1% higher than the posterior band (P<0.008). Along the mediolateral axes, the medial region had 26.2% higher cell density than the intermediate zone (P<0.04) and 25.4% higher than the lateral region (P<0.045). The overall volume and cell based maximum oxygen consumption rates were 1.44 (0.44-2.44) μmol/mL wet tissue/h and 28.7 (12.2-45.2)nmol/10(6)cells/h, respectively. The central regions (intermediate, lateral, and medial) had significantly higher volume based (P<0.02) and cell based (P<0.005) oxygen consumption rates than the anterior and posterior bands. At high oxygen tension, the oxygen consumption rate remained constant, but dropped as oxygen tension fell below 5%.

CONCLUSIONS

The TMJ disc had higher cell density and oxygen consumption rates than articular cartilage reported in the literature. These results suggest that a steeper oxygen gradient may exist in the TMJ disc and may be vulnerable to pathological events that impede nutrient supply.

Effect of endplate calcification and mechanical deformation on the distribution of glucose in intervertebral disc: a 3D finite element study

The intervertebral disc (IVD) is avascular, receiving nutrition from surrounding vasculature. Theoretical modelling can supplement experimental results to understand nutrition to IVD more clearly. A new, 3D finite element model of the IVD was developed to investigate effects of endplate calcification and mechanical deformation on glucose distributions in IVD. The model included anatomical disc geometry, non-linear coupling of cellular metabolism with pH and oxygen concentration and strain-dependent properties of the extracellular matrix. Calcification was simulated by reducing endplate permeability (∼79%). Mechanical loading was applied based on in vivo disc deformation during the transition from supine to standing positions. Three static strain conditions were considered: supine, standing and weight-bearing standing. Minimum glucose concentrations decreased 45% with endplate calcification, whereas disc deformation led to a 4.8-63% decrease, depending on the endplate condition (i.e. normal vs. calcified). Furthermore, calcification more strongly affected glucose concentrations in the nucleus compared to the annulus fibrous region. This study provides important insight into nutrient distributions in IVD under mechanical deformation.

Selection of reference genes for normalization of quantitative real-time PCR in organ culture of the rat and rabbit intervertebral disc

Background

The accuracy of quantitative real-time RT-PCR (qRT-PCR) is often influenced by experimental artifacts, resulting in erroneous expression profiles of target genes. The practice of employing normalization using a reference gene significantly improves reliability and its applicability to molecular biology. However, selection of an ideal reference gene(s) is of critical importance to discern meaningful results. The aim of this study was to evaluate the stability of seven potential reference genes (Actb, GAPDH, 18S rRNA, CycA, Hprt1, Ywhaz, and Pgk1) and identify most stable gene(s) for application in tissue culture research using the rat and rabbit intervertebral disc (IVD).

Findings

In vitro, four genes (Hprt1, CycA, GAPDH, and 18S rRNA) in rat IVD tissue and five genes (CycA, Hprt1, Actb, Pgk1, and Ywhaz) in rabbit IVD tissue were determined as most stable for up to 14 days in culture. Pair-wise variation analysis indicated that combination of Hprt1 and CycA in rat and the combination of Hprt1, CycA, and Actb in rabbit may most stable reference gene candidates for IVD tissue culture.

Conclusions

Our results indicate that Hprt1 and CycA are the most stable reference gene candidates for rat and rabbit IVD culture studies. In rabbit IVD, Actb could be an additional gene employed in conjunction with Hprt1 and CycA. Selection of optimal reference gene candidate(s) should be a pertinent exercise before employment of PCR outcome measures for biomedical research.

Computation models simulating notochordal cell extinction during early ageing of an intervertebral disc

Lower back pain due to intervertebral disc (IVD) degeneration is a prevalent problem which drastically affects the quality of life of millions of sufferers. Healthy IVDs begin with high populations of notochordal cells in the nucleus pulposus, while by the second stage of degeneration, these cells will be replaced by chondrocyte-like cells. Because the IVD is avascular, these cells rely on passive diffusion of nutrients to survive. It is thought that this transition in cell phenotype causes the shift of the IVD's physical properties, which impede the flow of nutrients. Our computational model of the IVD illustrates its ability to simulate the evolving chemical and mechanical environments occurring during the early ageing process. We demonstrate that, due to the insufficient nutrient supply and accompanying changes in physical properties of the IVD, there was a resultant exponential decay in the number of notochordal cells over time.

Difference in Energy Metabolism of Annulus Fibrosus and Nucleus Pulposus Cells of the Intervertebral Disc

Low back pain is associated with intervertebral disc degeneration. One of the main signs of degeneration is the inability to maintain extracellular matrix integrity. Extracellular matrix synthesis is closely related to production of adenosine triphosphate (i.e. energy) of the cells. The intervertebral disc is composed of two major anatomical regions: annulus fibrosus and nucleus pulposus, which are structurally and compositionally different, indicating that their cellular metabolisms may also be distinct. The objective of this study was to investigate energy metabolism of annulus fibrosus and nucleus pulposus cells with and without dynamic compression, and examine differences between the two cell types. Porcine annulus and nucleus tissues were harvested and enzymatically digested. Cells were isolated and embedded into agarose constructs. Dynamically loaded samples were subjected to a sinusoidal displacement at 2 Hz and 15% strain for 4 h. Energy metabolism of cells was analyzed by measuring adenosine triphosphate content and release, glucose consumption, and lactate/nitric oxide production. A comparison of those measurements between annulus and nucleus cells was conducted. Annulus and nucleus cells exhibited different metabolic pathways. Nucleus cells had higher adenosine triphosphate content with and without dynamic loading, while annulus cells had higher lactate production and glucose consumption. Compression increased adenosine triphosphate release from both cell types and increased energy production of annulus cells. Dynamic loading affected energy metabolism of intervertebral disc cells, with the effect being greater in annulus cells.

Human disc nucleus properties and vertebral endplate permeability

STUDY DESIGN

Experimental quantification of relationships between vertebral endplate morphology, permeability, disc cell density, glycosaminoglycan (GAG) content, and degeneration in samples harvested from human cadaveric spines.

OBJECTIVE

To test the hypothesis that variation in endplate permeability and porosity contributes to changes in intervertebral disc cell density and overall degeneration.

SUMMARY OF BACKGROUND DATA

Cells within the intervertebral disc are dependent on diffusive exchange with capillaries in the adjacent vertebral bone. Previous findings suggest that blocked routes of transport negatively affect disc quality, yet there are no quantitative relationships between human vertebral endplate permeability, porosity, cell density, and disc degeneration. Such relationships would be valuable for clarifying degeneration risk factors and patient features that may impede efforts at disc tissue engineering.

METHODS

Fifty-one motion segments were harvested from 13 frozen cadaveric human lumbar spines (32-85 years) and classified for degeneration using the magnetic resonance imaging-based Pfirrmann scale. A cylindrical core was harvested from the center of each motion segment that included vertebral bony and cartilage endplates along with adjacent nucleus tissue. The endplate mobility, a type of permeability, was measured directly using a custom-made permeameter before and after the cartilage endplate was removed. Cell density within the nucleus tissue was estimated using the picogreen method, while the nuclear GAG content was quantified using the dimethylmethylene blue technique. Specimens were imaged at 8 μm resolution using microCT; bony porosity was calculated. Analysis of variance, linear regression, and multiple comparison tests were used to analyze the data. RESULTS.: Nucleus cell density increased as the disc height decreased (R² = 0.13; P = 0.01) but was not related to subchondral bone porosity (P > 0.5), total mobility (P > 0.4), or age (P > 0.2). When controlling for disc height, however, a significant, negative effect of age on cell density was observed (P = 0.03). In addition to this, GAG content decreased with age nonlinearly (R² = 0.83, P < 0.0001) and a cell function measurement, GAGs/cell, decreased with degeneration (R² = 0.24; P < 0.0001). Total mobility (R² = 0.14; P < 0.01) and porosity (R² = 0.1, P < 0.01) had a positive correlation with age.

CONCLUSION

Although cell density increased with degeneration, cell function indicated that GAGs/cell decreased. Because permeability and porosity increase with age and degeneration, this implies that cell dysfunction, rather than physical barriers to transport, accelerates disc disease.

Adequacy of herniated disc tissue as a cell source for nucleus pulposus regeneration

OBJECT

The object of this study was to characterize the regenerative potential of cells isolated from herniated disc tissue obtained during microdiscectomy. The acquired data could help to evaluate the feasibility of these cells for autologous disc cell transplantation.

METHODS

From each of 5 patients (mean age 45 years), tissue from the nucleus pulposus compartment as well as from herniated disc was obtained separately during microdiscectomy of symptomatic herniated lumbar discs. Cells were isolated, and in vitro cell expansion for cells from herniated disc tissue was accomplished using human serum and fibroblast growth factor-2. For 3D culture, expanded cells were loaded in a fibrin-hyaluronan solution on polyglycolic acid scaffolds for 2 weeks. The formation of disc tissue was documented by histological staining of the extracellular matrix as well as by gene expression analysis of typical disc marker genes.

RESULTS

Cells isolated from herniated disc tissue showed significant signs of dedifferentiation and degeneration in comparison with cells from tissue of the nucleus compartment. With in vitro cell expansion, further dedifferentiation with distinct suppression of major matrix molecules, such as aggrecan and Type II collagen, was observed. Unlike in previous reports of cells from the nucleus compartment, the cells from herniated disc tissue showed only a weak redifferentiation process in 3D culture. However, propidium iodide/fluorescein diacetate staining documented that 3D assembly of these cells in polyglycolic acid scaffolds allows prolonged culture and high viability.

CONCLUSIONS

Study results suggested a very limited regenerative potential for cells harvested from herniated disc tissue. Further research on 2 major aspects in patient selection is suggested before conducting reasonable clinical trials in this matter: 1) diagnostic strategies to predict the regenerative potential of harvested cells at a radiological or cell biology level, and 2) clinical assessment strategies to elucidate the metabolic state of the targeted disc.

Structured coculture of stem cells and disc cells prevent disc degeneration in a rat model

BACKGROUND CONTEXT

Harnessing the potential of stem cells is an important strategy for regenerative medicine. This study explores the use of bilaminar coculture pellets (BCPs) of mesenchymal stem cells (MSCs) and nucleus pulposus cells (NPCs) as a cell-based therapy for intervertebral disc regeneration. Prior in vitro experiments have shown that BCP can help differentiate MSCs and substantially improve new matrix deposition.

PURPOSE

To evaluate the clinical relevance of BCPs by testing the system in vivo.

STUDY DESIGN/SETTING

We have designed a novel spherical BCP where MSCs are enclosed in a shell of NPCs. The pellets were tested in vivo in a rat tail model of disc degeneration.

METHODS

Rat caudal intervertebral discs were denucleated and treated with BCP in a fibrin sealant (FS) carrier (controls were MSCs suspended in FS; NPCs suspended in FS; MSCs and NPCs suspended in FS; FS only; and surgery only). At 14 and 35 days after implantation, the animals were euthanized and discs were evaluated for proteoglycan content, enzyme-linked immunosorbent assay for inflammatory cytokines, cell retention using polymerase chain reaction, disc height, histology, and disc grade based on a blinded scoring system.

RESULTS

The proteoglycan and cytokine levels were not significantly different among groups. The BCP group had higher cell retention than controls. Disc height and disc grade increased over time only in the BCP group. Bilaminar coculture pellets were the only treatment to show proteoglycan staining in the nucleus space at 35 days.

CONCLUSIONS

This study shows that BCPs may prevent postnucleotomy disc degeneration in vivo. Larger animals and longer time points will be necessary to further judge potential clinical impact. As opposed to strategies that require growth factor supplements, predifferentiation, or genetic manipulations, BCPs are a self-sustaining and targeted method for tissue regeneration in situ.

Influence of fluid-flow direction on effective permeability of the vertebral end plate: an analytical model

Convective transports in the vertebral end plate (VEP) play a significant role in the homeostasis of the spine. A few studies hypothesised that the hydraulic resistance or effective permeability of the VEP could be dependant upon fluid-flow direction. Results were influenced by species, region of interest within the end plate and pathology. Some results were contradictory. We propose an analytical model based on steady-state Newtonian flows in capillary media to develop a phenomenological analysis of convective transport through the VEP. This dependence was established using a biquadratic analytical function involving porosities of subchondral bone, capillary bed and cartilage end plate. Discussion of results provided a theoretical justification for variable and/or contradictory experimental results concerning the amount of energy lost by fluid during its course through the end plate. Tissue porosities and, especially, those relative to the capillary bed could strongly influence the dependence of fluid energy loss on flow direction and could potentially modify tissue homeostasis related to the day and night cycle.

Influence of low glucose supply on the regulation of gene expression by nucleus pulposus cells and their responsiveness to mechanical loading

Object

Environmental alterations resulting in a decrease in the nutrient supply have been associated with intervertebral disc (IVD) degeneration, particularly of the nucleus pulposus (NP). The goal of the present study was to examine the hypothesis that glucose deprivation alters the metabolism of NP cells and their responsiveness to mechanical loading. A possible interaction of glucose supply and hydrostatic pressure (HP) with gene expression by NP cells has not been investigated.

Methods

The influence of glucose supply (physiological concentration: 5 mM, reduction: 0 or 0.5 mM) and cyclic HP loading (2.5 MPa, 0.1 Hz, 30 minutes) on bovine and human NP cell matrix turnover was analyzed by quantitative real-time reverse transcriptase–polymerase chain reaction. Glucose-dependent effects on cell viability were determined by trypan blue exclusion. A glycosaminoglycan (GAG) assay was performed to determine nutritional effects on the protein level.

Results

Glucose reduction resulted in significant downregulations (p < 0.05) of aggrecan, collagen-I, and collagen-II gene expression by bovine NP cells. Exemplary human donors also displayed a similar trend for aggrecan and collagen-II, whereas matrix metalloproteinases (MMPs) tended to be upregulated under glucose deprivation. After HP loading, human NP cells showed individual upregulations of collagen-I and collagen-II expression, while MMP expression tended to be downregulated under glucose reduction relative to a normal glucose supply. Cell viability decreased with glucose deprivation. The GAG content was similar in all groups at Day 1, whereas at Day 3 there was a significant increase under physiological conditions.

Conclusions

Glucose deprivation strongly affected NP cell metabolism. The effects of an altered glucose supply on gene expression were more pronounced than the mechanically induced effects. Data in this study demonstrate that the glucose environment is more critical for disc cell metabolism than mechanical loads. In individual human donors, however, adequate mechanical stimuli might have a beneficial effect on matrix turnover during IVD degeneration.

Cryopreserved intervertebral disc with injected bone marrow-derived stromal cells: a feasibility study using organ culture

BACKGROUND CONTEXT

A recent clinical study demonstrated that cryopreserved allogeneic intervertebral disc transplantation relieved pain and preserved motion, thus opening up a new treatment option for degenerative disc disease. However, these transplanted discs continued to degenerate, possibly due to a lack of viable cells. Bone marrow-derived stromal cell (BMSC) implantation has been shown to delay disc degeneration.

PURPOSE

This study examined the viability over time of endogenous and injected BMSCs in cryopreserved disc under simulated-physiological loading conditions. STUDY DESIGN/ SETTING: An in vitro study of BMSCs injected into cryopreserved bovine caudal discs.

METHODS

Bovine caudal discs were harvested and cryopreserved at -196 degrees C. After thawing, PKH-26-labeled BMSCs embedded in peptide hydrogel carrier were injected into the nucleus pulposus. Two BMSC injection quantities, that is, 1x10(5) and 2.5x10(5) were examined. Discs with injected cells were maintained in a bioreactor for 7 days under simulated-physiological loading. Cell viability (staining), gene expression (reverse transcription-polymerase chain reaction) profile, and proteoglycan content (histologically) were evaluated.

RESULTS

Forty percent of endogenous cell viability was maintained after freeze thawing. Over the 7-day culture, this did not change further. However, there was upregulation of Col1a2 and Mmp-13 and downregulation of Col2a1gene expression. Sixty percent of BMSCs survived the initial injection procedure, and only 20% remained alive after 7 days of culture. Bone marrow-derived stromal cell implantation did not alter the viability of the endogenous cells, but discs injected with 1x105 BMSCs showed significantly higher ACAN expression than sham discs.

CONCLUSIONS

Although only 40% of cells survived cryopreservation, these endogeneous cells continued to survive over 7 days if maintained under simulated-physiological loading conditions. Although only a small portion of injected BMSCs survived, they did have some effect on the matrix protein gene expression profile. Their influence on native cells requires long-term evaluation.

Analysis of cell viability in intervertebral disc: Effect of endplate permeability on cell population

Responsible for making and maintaining the extracellular matrix, the cells of intervertebral discs are supplied with essential nutrients by diffusion from the blood supply through mainly the cartilaginous endplates (CEPs) and disc tissue. Decrease in transport rate and increase in cellular activity may adversely disturb the intricate supply-demand balance leading ultimately to cell death and disc degeneration. The present numerical study aimed to introduce for the first time cell viability criteria into nonlinear coupled nutrition transport equations thereby evaluating the dynamic nutritional processes governing viable cell population and concentrations of oxygen, glucose and lactic acid in the disc as CEP exchange area dropped from a fully permeable condition to an almost impermeable one. A uniaxial model of an in vitro cell culture analogue of the disc is first employed to examine and validate cell viability criteria. An axisymmetric model of the disc with four distinct regions was subsequently used to investigate the survival of cells at different CEP exchange areas. In agreement with measurements, predictions of the diffusion chamber model demonstrated substantial cell death as essential nutrient concentrations fell to levels too low to support cells. Cells died away from the nutrient supply and at higher cell densities. In the disc model, the nucleus region being farthest away from supply sources was most affected; cell death initiated first as CEP exchange area dropped below approximately 40% and continued exponentially thereafter to depletion as CEP calcified further. In cases with loss of endplate permeability and/or disruptions therein, as well as changes in geometry and fall in diffusivity associated with fluid outflow, the nutrient concentrations could fall to levels inadequate to maintain cellular activity or viability, resulting in cell death and disc degeneration.

A phenotypic comparison of proteoglycan production of intervertebral disc cells isolated from rats, rabbits, and bovine tails; which animal model is most suitable to study tissue engineering and biological repair of human disc disorders?

The nucleus pulposus (NP) of the intervertebral disc in cattle and humans shows the most dramatic changes with aging of any cartilaginous tissue. In humans, notochordal cells disappear from the NP and are replaced with chondrocytic cells by adolescence. However, notochordal cells of the NP persist into adult life in some species, such as rats and rabbits. Therefore, comparison of the metabolic activity of notochordal and nonnotochordal cells is considered to be important for determining the type of cell to use for transplantation to regenerate intervertebral discs. In this study, we investigated the notochordal NP cells of rats and rabbits, as well as nonnotochordal (chondrocyte-like) bovine NP cells, in a three-dimensional culture system to examine whether proteoglycan metabolism varied among these three cell types. As a result, bovine NP cells produced around 0.18 mg/mL of glycosaminoglycan after culture for 5 days, while rat and rabbit NP cells produced about four and two times more glycosaminoglycan than bovine cells, respectively. In conclusion, this study demonstrated marked differences of energy metabolism and production of matrix components between notochordal and nonnotochordal NP cells. Animals with notochordal cells in the NP, such as rats and rabbits, may not provide good models for investigation of biological repair and tissue engineering for human disc disorders.

Development of an in vitro model to test the efficacy of novel therapies for IVD degeneration

Low back pain (LBP) is a major cause of disability worldwide that has been linked to intervertebral disc (IVD) degeneration. An improved understanding of the pathogenesis of disc degeneration is now developing, which is leading to the development of a number of possible future therapies targeted at the underlying pathology and regeneration strategies. Although results thus far are promising, the investigation of such therapies in an environment that mimics the mechanical environment of the human disc in vivo is problematic. The development of an in vitro model system that can maintain metabolically active IVD tissue within a loading environment pertaining to that of the human spine is crucial for testing the efficacy of future cell-based and tissue-engineering therapies for IVD degeneration. Here, using our novel loading rig, capable of mimicking the loading environment experienced within the human spine, we have cultured nucleus pulposus tissue explants, applied a daily hydrostatic loading regime for up to 2 weeks and investigated proteoglycan retention, metabolic activity and cellular phenotype. IVD tissue cultured under a loading environment pertaining to the in vivo loading environment maintained metabolic cell activity, proteoglycan content and cellular phenotype. Indeed, all parameters were improved in IVD tissue cultured with load compared to unloaded controls. Such a model is invaluable for investigations assessing the feasibility and efficacy of future therapeutic approaches to inhibiting degeneration or stimulating regeneration of the IVD, where the in vivo loading environment may be crucial to their success or failure.

Strain-dependent oxygen diffusivity in bovine annulus fibrosus

The intervertebral disk (IVD) is the largest avascular structure in the human body. Transport of small molecules in IVD is mainly through diffusion from the endplates and the peripheral blood vessels surrounding IVD. Studies have investigated the structure, chemical components, and water content in IVD, but to our knowledge no study has investigated the effect of mechanical loading on oxygen transport in IVD. The objective of this study was to determine the strain-dependent behavior of oxygen diffusivity in IVD tissue. A one-dimensional steady-state diffusion experiment was designed and performed to determine the oxygen diffusivity in bovine annulus fibrosus (AF). The oxygen diffusivity was calculated using equation derived from Fick's law. A total of 20 AF specimens (d=6 mm, h approximately 0.5 mm) from bovine coccygeal IVD were used to determine oxygen diffusivity at three levels of compressive strain. The average oxygen diffusivity (mean+/-SD) of bovine AF in the axial direction was 1.43+/-0.242 x 10(-5) cm(2)/s (n=20) at 4.68+/-1.67% compressive strain level, 1.05+/-0.282 x 10(-5) cm(2)/s (n=20) at 14.2+/-1.50% strain level, and 7.71+/-1.63 x 10(-6) cm(2)/s (n=20) at 23.7+/-1.34% strain level. There was a significant decrease in oxygen diffusivity with increasing level of compressive strain (ANOVA, p<0.05). Oxygen diffusivity of bovine AF in the axial direction has been determined. The mechanical loading has a significant effect on oxygen transport in IVD tissues. This study is important in understanding nutritional transport in IVD tissues and related disk degeneration.

Expression of TRAIL and the death receptors DR4 and DR5 correlates with progression of degeneration in human intervertebral disks

Intervertebral disks degenerate far earlier than other musculoskeletal tissues and apoptosis has been suggested to have a vital function in promoting the degeneration process that is strongly associated with back pain. However, the molecular mediators of apoptosis in the intervertebral disk are poorly understood. Fas/FasL, TRAIL/DR4, TRAIL/DR5 and TNF-alpha/TNFR1 are ligand/receptor pairs of the tumor necrosis factor/nerve growth factor family, which are able to induce apoptosis by trimerization of the receptor by its corresponding ligand. We investigated which of these molecules are expressed in intervertebral disks and whether their expression correlates to disk degeneration. Intervertebral disks from 28 donors (age 12-70 years) suffering from scoliosis, vertebrae fracture or disk degeneration were scored histologically for degeneration and analyzed for gene expression of FasL/Fas, TRAIL/DR4, TNF-alpha/TNFR1 and caspase 8. Protein expression of FasL and TRAIL was assessed by immunohistology and apoptotic cell death was quantified by poly(ADP-ribose) polymerase (PARP) p85 staining. Isolated disk cells were analyzed by flow cytometry for Fas, FasL, TRAIL, DR4 and DR5 expression. Gene expression of TRAIL (P=0.002) and caspase 8 (P=0.027) significantly correlated with degeneration. TRAIL expression further correlated with cellularity (P=0.04), muccoid matrix changes (P=0.009) and tears and cleft formation (P=0.019). FasL and TRAIL expression was confirmed by immunohistology and PARP cleavage was significantly associated with degeneration (P=0.027). Flow cytometry on isolated disk cells revealed correlations between DR4 and degeneration (P=0.014), DR4/DR5 double-positive cells and degeneration (P=0.019), as well as DR5 and changes in tissue granularity (P=0.03). This is the first study that shows that intervertebral disk cells express TRAIL, DR4 and DR5, which correlate to the degenerative state of the disk. Therefore, disk cells inherit the molecular machinery to induce and undergo cellular apoptosis, and the frequency of cytokine expression suggests that the TRAIL/DR4/DR5 axis is an important molecular mediator of apoptosis induction in disk tissue.

Solute transport in intervertebral disc: experiments and finite element modeling

Loss of nutrient supply to the human intervertebral disc (IVD) cells is thought to be a major cause of disc degeneration in humans. To address this issue, transport of molecules of different size have been analyzed by a combination of experimental and modeling studies. Solute transport has been compared for steady-state and transient diffusion of several different solutes with molecular masses in the range 3-70 kDa, injected into parts of the disc where degeneration is thought most likely to occur first and into the blood supply to the disc. Diffusion coefficients of fluorescently tagged dextran molecules of different molecular weights have been measured in vitro using the concentration gradient technique in thin specimens of disc outer annulus and nucleus pulposus. Diffusion coefficients were found to decrease with molecular weight following a nonlinear relationship. Diffusion coefficients changed more rapidly for solutes with molecular masses less than 10 kDa. Although unrealistic or painful, solutes injected directly into the disc achieve the largest disc coverage with concentrations that would be high enough to be of practical use. Although more practical, solutes injected into the blood supply do not penetrate to the central regions of the disc and their concentrations dissipate more rapidly. Injection into the disc would be the best method to get drugs or growth factors to regions of degeneration in IVDs quickly; else concentrations of solute must be kept at a high value for several hours in the blood supply to the discs.

Effect of limited nutrition on in situ intervertebral disc cells under simulated-physiological loading

STUDY DESIGN

Whole ovine caudal intervertebral discs (IVD) were cultured in sufficient and limited nutrition under simulated-physiologic loading for 7 and 21 days.

OBJECTIVE

To study the effect of limited nutrition on disc cells embedded in their native tissue in short- and midterm whole organ disc culture.

SUMMARY OF BACKGROUND DATA

Nutrient-limited induction of disc cell death in vitro has been demonstrated and is believed to be a factor in disc degeneration. Nutrient-limited cell death and its consequences, as it relates to degeneration, have not been investigated in the intact IVD.

METHODS

Ovine IVDs with endplates were cultured for 7 and 21 days under simulated-physiologic loading, either in media with limited (2 g/L) or sufficient (4.5 g/L) glucose concentration. Cell viability, relative gene expression, newly synthesized chondroitin sulfate content, and matrix metalloproteinase (MMP) activity were measured after culture and compared to fresh tissue.

RESULTS

In sufficient glucose media, cell viability was maintained through 7 days to 21 days of culture. In limited glucose, it dropped significantly to 62% in the anulus fibrosus and to 56% in the nucleus pulposus after 7 days and remained so until 21 days (63% in the anulus fibrosus and 52% in the nucleus pulposus). No significant differences were found between culture conditions for relative gene expression, newly synthesized chondroitin sulfate and inactive and active forms of MMP13 and MMP7.

CONCLUSION

With this culture system, whole IVD explants could be maintained up to 21 days. Cell viability decreased to 50% to 60% under limited nutrition within days and remained so up to 3 weeks. The surviving cells did not compensate matrix production in this time frame.

Spinal hemiepiphysiodesis decreases the size of vertebral growth plate hypertrophic zone and cells

BACKGROUND

Hemiepiphysiodesis is a potential method to treat idiopathic juvenile scoliosis early. The purpose of the present study was to investigate a mechanism of curve creation in the pig thoracic model of spinal hemiepiphysiodesis by determining whether the structure of the vertebral growth plate varied with distance from the stapled, concave side of the spine. The hypotheses were that the heights of the hypertrophic zone, hypertrophic cells, and disc would be decreased on the treated side of the treated level as compared with both an unstapled control level and the side opposite the staple.

METHODS

Custom spine staples were implanted into six midthoracic vertebrae in each of five skeletally immature pigs. After eight weeks, the spines were harvested and histological sections were prepared. Hypertrophic zone height, hypertrophic cell height and width, and disc height were measured at discrete coronal plane locations at stapled and unstapled thoracic levels. Differences between stapled and unstapled levels and locations were compared with use of mixed linear modeling for repeated measures, followed by regression models to determine growth plate intercept and slope across the plane by thoracic level.

RESULTS

Zone height, cell height, and cell width were lowest on the stapled side of the stapled level, with significant differences in the overall statistical model (p < 0.02). Disc heights were significantly reduced (p < 0.0001) at the stapled levels across the coronal plane.

CONCLUSIONS

Unilateral control of intervertebral joint motion decreased growth plate height, cell size, and disc height.

MSC response to pH levels found in degenerating intervertebral discs

Painful degenerative disc disease is a major health problem and for successful tissue regeneration, MSCs must endure and thrive in a harsh disc microenvironment that includes matrix acidity as a critical factor. MSCs were isolated from bone marrow of Sprague-Dawley rats from two different age groups (<1 month, n=6 and 4-5 months, n=6) and cultured under four different pH conditions representative of the healthy, mildly or severely degenerated intervertebral disc (pH 7.4, 7.1, 6.8, and 6.5) for 5 days. Acidity caused an inhibition of aggrecan, collagen-1, and TIMP-3 expression, as well as a decrease in proliferation and viability and was associated with a change in cell morphology. Ageing had generally minor effects but young MSCs maintained greater mRNA expression levels. As acidic pH levels are typical of increasingly degenerated discs, our findings demonstrate the importance of early interventions and predifferentiation when planning to use MSCs for reparative treatments.

Investigation of solute concentrations in a 3D model of intervertebral disc

As the disc is the largest avascular structure in the body, disc cells depend for their normal function on an adequate supply of nutrients (oxygen and glucose) and the removal of metabolic by-products (lactic acid) via blood vessels at the cartilaginous endplates and annulus periphery. Concentration gradients develop depending on the balance between the rates of transport and rates of cellular activity. Since consumption and production rates are coupled via extracellular pH, the gradients are interdependent. This is a novel model study which takes into account the realistic 3D geometry of a L5-S1 lumbar disc in solving the nonlinear coupled diffusion equations. Effects of perturbations (calcification, sclerosis) in endplates, increases in cell metabolic rates following growth factor injection and changes in lumbar posture (kyphotic or lordotic) on extreme values of nutrient and metabolite concentrations and their spatial locations are investigated. Solute concentrations, particularly those of glucose, substantially diminish as a consequence of disturbances in supply at the endplates, increases in cell metabolic rate and more lordotic postures. Results, when compared to those from simplified axisymmetric models, demonstrate the importance of consideration of realistic 3D disc geometry.

Tissue engineering and the intervertebral disc: the challenges

Disc degeneration is a common disorder. Although the back pain that can develop in association with this is rarely life-threatening, the annual cost in terms of morbidity, lost productivity, medical expenses and workers' compensation benefits is significant. Surgical intervention as practised currently is directed towards removing the damaged or altered tissue. Development of new treatment modalities is critical as there is a growing consensus that the strategies used currently for symptomatic degenerative disc disease may not be effective. Accordingly, there is a need to develop an entirely new way to treat this disorder; regenerative medicine and tissue engineering approaches appear particularly promising in this regard. This paper reviews some of the challenges that currently are limiting the clinical application of this approach to the treatment of disc degeneration.

Effects of mechanical compression on metabolism and distribution of oxygen and lactate in intervertebral disc

The objective of this study was to examine the effects of mechanical compression on metabolism and distributions of oxygen and lactate in the intervertebral disc (IVD) using a new formulation of the triphasic theory. In this study, the cellular metabolic rates of oxygen and lactate were incorporated into the newly developed formulation of the mechano-electrochemical mixture model [Huang, C.-Y., Gu, W.Y., 2007. Effect of tension-compression nonlinearity on solute transport in charged hydrated fibrosus tissues under dynamic unconfined compression. Journal of Biomechanical Engineering 129, 423-429]. The model was used to numerically analyze metabolism and transport of oxygen and lactate in the IVD under static or dynamic compression. The theoretical analyses demonstrated that compressive loading could affect transport and metabolism of nutrients. Dynamic compression increased oxygen concentration, reduced lactate accumulation, and promoted oxygen consumption and lactate production (i.e., energy conversion) within the IVD. Such effects of dynamic loading were dependent on strain level and loading frequency, and more pronounced in the IVD with less permeable endplate. In contrast, static compression exhibited inverse effects on transport and metabolism of oxygen and lactate. The theoretical predictions in this study are in good agreement with those in the literature. This study established a new theoretical model for analyzing cellular metabolism of nutrients in hydrated, fibrous soft tissues under mechanical compression.

Behavior of mesenchymal stem cells in the chemical microenvironment of the intervertebral disc

STUDY DESIGN

Responses of mesenchymal stem cells (MSCs) from 2 age groups was analyzed under chemical conditions representative of the intervertebral disc (IVD) (low glucose levels, acidic pH, high osmolarity, and combined conditions). OBJECTIVE.: To determine the microenvironmental conditions of the IVD that are critical for MSC-based tissue repair and to determine whether MSCs from different age groups respond differently.

SUMMARY OF BACKGROUND DATA

MSCs offer promise for IVD repair, but their potential is limited by the harsh chemical microenvironment in which they must survive.

METHODS

MSCs were isolated from bone marrow from mature (4-5 month old) and young (1 month old) rats and cultured in monolayer under IVD-like glucose, osmolarity, and pH conditions as well as under a combination of these conditions and under standard media conditions for 2 weeks. The response of MSCs was examined by measuring gene expression (real-time RT-PCR), proliferation (MTT assay), and viability (fluorescence staining).

RESULTS

Culturing under IVD-like glucose conditions (1.0 mg/mL glucose) stimulated aggrecan and collagen-1 expression and caused a small increase in proliferation. In contrast, IVD-like osmolarity (485 mOsm) and pH (pH = 6.8) conditions strongly decreased proliferation and expression of matrix proteins, with more pronounced effects for osmolarity. Combining these 3 conditions also resulted in decreased proliferation, and gene expression of matrix proteins, demonstrating that osmolarity and pH dominated the effects of glucose. Both age groups showed a similar response pattern to the disc microenvironment.

CONCLUSION

IVD repair using MSCs requires increased knowledge of MSC response to the chemical microenvironment. IVD-like low glucose enhanced matrix biosynthesis and maintained cell proliferation whereas IVD-like high osmolarity and low pH conditions were critical factors that reduced biosynthesis and proliferation of young and mature MSCs. Since osmolarity decreases and acidity increases during degeneration, we speculate that pH may be the major limitation for MSC-based IVD repair.

Adipose stem cells for intervertebral disc regeneration: current status and concepts for the future

New regenerative treatment strategies are being developed for intervertebral disc degeneration of which the implantation of various cell types is promising. All cell types used so far require in vitro expansion prior to clinical use, as these cells are only limited available. Adipose-tissue is an abundant, expendable and easily accessible source of mesenchymal stem cells. The use of these cells therefore eliminates the need for in vitro expansion and subsequently one-step regenerative treatment strategies can be developed. Our group envisioned, described and evaluated such a one-step procedure for spinal fusion in the goat model. In this review, we summarize the current status of cell-based treatments for intervertebral disc degeneration and identify the additional research needed before adipose-derived mesenchymal stem cells can be evaluated in a one-step procedure for regenerative treatment of the intervertebral disc. We address the selection of stem cells from the stromal vascular fraction, the specific triggers needed for cell differentiation and potential suitable scaffolds. Although many factors need to be studied in more detail, potential application of a one-step procedure for intervertebral disc regeneration seems realistic.

The occurrence and regional distribution of DR4 on herniated disc cells: a potential apoptosis pathway in lumbar intervertebral disc

STUDY DESIGN

Intervertebral discs surgically obtained from 60 herniated patients and 5 normal individuals were examined to correlate the regional distribution of DR4-receptor and apoptosis.

OBJECTIVE

To explore the role of a tumor necrosis factor superfamily member DR4 and the TRAIL/DR4 mediated apoptosis in the human lumbar intervertebral disc.

SUMMARY OF BACKGROUND DATA

The pathogenesis of lumbar degenerative intervertebral discs remains not completely understood. In herniated lumbar disc tissues, increased apoptosis and higher expression of Fas/Fas ligand and caspase-3 have been reported, suggesting a pivotal role of apoptotic mechanisms in intervertebral disc degeneration. However, it is not clear that apoptosis mediators such as TRAIL and Death Receptor 4 (DR4), which often represent different apoptosis signal pathways, contribute to the apoptosis process during the development of the degenerated intervertebral discs.

METHODS

Apoptosis was determined by poly(ADP-ribose) polymerase (PARP) p85 immunohistochemistry. Expression of DR4 was revealed by immunohistochemistry analysis. Statistical difference among groups was analyzed using one-way ANOVA with LSD post hoc multiple comparisons and the bivariate correlations.

RESULTS

Apoptotic cells were detected in the nucleus pulposus and anulus fibrosus of all samples. However, the number of apoptotic cells was significantly higher in the nucleus compared with the anulus. Further, there were significantly more apoptotic cells in the herniated discs compared with the normal discs. Within herniated discs, a remarkably higher percentage of positive staining cells were detected in the uncontained discs than the contained ones. Strong expression of DR4 was detected in all samples of degenerative herniated discs, whereasmuch weaker expression was sporadically identified in normal discs. In addition, the prevalence of apoptosis positively correlated with the severity of disc degeneration.

CONCLUSION

The concomitant increase of DR4 expression in the regions of heavy apoptotic cell aggregation suggests that TRAIL/DR4-mediated pathway may play an important role in the apoptosis in herniated discs.