Biochemistry of the intervertebral disc

Diverse and multifunctional roles for perlecan (HSPG2) in repair of the intervertebral disc

Perlecan is a widely distributed, modular, and multifunctional heparan sulfate proteoglycan, which facilitates cellular communication with the extracellular environment to promote tissue development, tissue homeostasis, and optimization of biomechanical tissue functions. Perlecan-mediated osmotic mechanotransduction serves to regulate the metabolic activity of cells in tissues subjected to tension, compression, or shear. Perlecan interacts with a vast array of extracellular matrix (ECM) proteins through which it stabilizes tissues and regulates the proliferation or differentiation of resident cell populations. Here we examine the roles of the HS-proteoglycan perlecan in the normal and destabilized intervertebral disc. The intervertebral disc cell has evolved to survive in a hostile weight bearing, acidic, low oxygen tension, and low nutrition environment, and perlecan provides cytoprotection, shields disc cells from excessive compressive forces, and sequesters a range of growth factors in the disc cell environment where they aid in cellular survival, proliferation, and differentiation. The cells in mechanically destabilized connective tissues attempt to re-establish optimal tissue composition and tissue functional properties by changing the properties of their ECM, in the process of chondroid metaplasia. We explore the possibility that perlecan assists in these cell-mediated tissue remodeling responses by regulating disc cell anabolism. Perlecan's mechano-osmotic transductive property may be of potential therapeutic application.

Understanding the etiopathogenesis of lumbar intervertebral disc herniation: From clinical evidence to basic scientific research

Lumbar intervertebral disc herniation, as a leading cause of low back pain, productivity loss, and disability, is a common musculoskeletal disorder that results in significant socioeconomic burdens. Despite extensive clinical and basic scientific research efforts, herniation etiopathogenesis, particularly its initiation and progression, is not well understood. Understanding herniation etiopathogenesis is essential for developing effective preventive measures and therapeutic interventions. Thus, this review seeks to provide a thorough overview of the advances in herniation-oriented research, with a discussion on ongoing challenges and potential future directions for clinical, translational, and basic scientific investigations to facilitate innovative interdisciplinary research aimed at understanding herniation etiopathogenesis. Specifically, risk factors for herniation are identified and summarized, including familial predisposition, obesity, diabetes mellitus, smoking tobacco, selected cardiovascular diseases, disc degeneration, and occupational risks. Basic scientific experimental and computational research that aims to understand the link between excessive mechanical load, catabolic tissue remodeling due to inflammation or insufficient nutrient supply, and herniation, are also reviewed. Potential future directions to address the current challenges in herniation-oriented research are explored by combining known progressive development in existing research techniques with ongoing technological advances. More research on the relationship between occupational risk factors and herniation, as well as the relationship between degeneration and herniation, is needed to develop preventive measures for working-age individuals. Notably, researchers should explore using or modifying existing degeneration animal models to study herniation etiopathogenesis, as such models may allow for a better understanding of how to prevent mild-to-moderately degenerated discs from herniating.

Textile Design of an Intervertebral Disc Replacement Device from Silk Yarn

Low back pain is often due to degeneration of the intervertebral discs (IVD). It is one of the most common age- and work-related problems in today's society. Current treatments are not able to efficiently restore the full function of the IVD. Therefore, the aim of the present work was to reconstruct the two parts of the intervertebral disc-the annulus fibrosus (AF) and the nucleus pulposus (NP)-in such a way that the natural structural features were mimicked by a textile design. Silk was selected as the biomaterial for realization of a textile IVD because of its cytocompatibility, biodegradability, high strength, stiffness, and toughness, both in tension and compression. Therefore, an embroidered structure made of silk yarn was developed that reproduces the alternating fiber structure of +30° and -30° fiber orientation found in the AF and mimics its lamellar structure. The developed embroidered ribbons showed a tensile strength that corresponded to that of the natural AF. Fiber additive manufacturing with 1 mm silk staple fibers was used to replicate the fiber network of the NP and generate an open porous textile 3D structure that may serve as a reinforcement structure for the gel-like NP.

Aggrecan governs intervertebral discs development by providing critical mechanical cues of the extracellular matrix

Aggrecan (ACAN) is localized in the intervertebral disc (IVD) in unique compartment-specific patterns where it contributes to the tissue structure and mechanical function together with collagens. The extracellular matrix (ECM) of the IVD undergoes degenerative changes during aging, misuse or trauma, which inevitably alter the biochemical and biomechanical properties of the tissue. A deeper understanding of these processes can be achieved in genetically engineered mouse models, taking into account the multifaceted aspects of IVD development. In this study, we generated aggrecan insertion mutant mice (Acan ^iE5/iE5 ) by interrupting exon 5 coding for the G1 domain of ACAN, and analyzed the morphological and mechanical properties of the different IVD compartments during embryonic development. Western blotting using an antibody against the total core protein failed to detect ACAN in cartilage extracts, whereas immunohistochemistry by a G1-specific antibody showed weak signals in vertebral tissues of Acan ^iE5/iE5 mice. Homozygous mutant mice are perinatally lethal and characterized by short snout, cleft palate and disproportionate dwarfism. Whole-mount skeletal staining and µ-CT analysis of Acan ^iE5/iE5 mice at embryonic day 18.5 revealed compressed vertebral bodies with accelerated mineralization compared to wild type controls. In Acan ^iE5/iE5 mice, histochemical staining revealed collapsed extracellular matrix with negligible sulfated glycosaminoglycan content accompanied by a high cellular density. Collagen type II deposition was not impaired in the IVD of Acan ^iE5/iE5 mice, as shown by immunohistochemistry. Mutant mice developed a severe IVD phenotype with deformed nucleus pulposus and thinned cartilaginous endplates accompanied by a disrupted growth plate structure in the vertebral body. Atomic force microscopy (AFM) imaging demonstrated a denser collagen network with thinner fibrils in the mutant IVD zones compared to wild type. Nanoscale AFM indentation revealed bimodal stiffness distribution attributable to the softer proteoglycan moiety and harder collagenous fibrils of the wild type IVD ECM. In Acan ^iE5/iE5 mice, loss of aggrecan resulted in a marked shift of the Young's modulus to higher values in all IVD zones. In conclusion, we demonstrated that aggrecan is pivotal for the determination and maintenance of the proper stiffness of IVD and vertebral tissues, which in turn could play an essential role in providing developmental biomechanical cues.

Glis1 and oxaloacetate in nucleus pulposus stromal cell somatic reprogramming and survival

Regenerative medicine aims to repair degenerate tissue through cell refurbishment with minimally invasive procedures. Adipose tissue (FAT)-derived stem or stromal cells are a convenient autologous choice for many regenerative cell therapy approaches. The intervertebral disc (IVD) is a suitable target. Comprised of an inner nucleus pulposus (NP) and an outer annulus fibrosus (AF), the degeneration of the IVD through trauma or aging presents a substantial socio-economic burden worldwide. The avascular nature of the mature NP forces cells to reside in a unique environment with increased lactate levels, conditions that pose a challenge to cell-based therapies. We assessed adipose and IVD tissue-derived stromal cells through in vitro transcriptome analysis in 2D and 3D culture and suggested that the transcription factor Glis1 and metabolite oxaloacetic acid (OAA) could provide NP cells with survival tools for the harsh niche conditions in the IVD.

Collagen-Alginate Composite Hydrogel: Application in Tissue Engineering and Biomedical Sciences

Alginate (ALG), a polysaccharide derived from brown seaweed, has been extensively investigated as a biomaterial not only in tissue engineering but also for numerous biomedical sciences owing to its wide availability, good compatibility, weak cytotoxicity, low cost, and ease of gelation. Nevertheless, alginate lacks cell-binding sites, limiting long-term cell survival and viability in 3D culture. Collagen (Col), a major component protein found in the extracellular matrix (ECM), exhibits excellent biocompatibility and weak immunogenicity. Furthermore, collagen contains cell-binding motifs, which facilitate cell attachment, interaction, and spreading, consequently maintaining cell viability and promoting cell proliferation. Recently, there has been a growing body of investigations into collagen-based hydrogel trying to overcome the poor mechanical properties of collagen. In particular, collagen-alginate composite (CAC) hydrogel has attracted much attention due to its excellent biocompatibility, gelling under mild conditions, low cytotoxicity, controllable mechanic properties, wider availability as well as ease of incorporation of other biomaterials and bioactive agents. This review aims to provide an overview of the properties of alginate and collagen. Moreover, the application of CAC hydrogel in tissue engineering and biomedical sciences is also discussed.

A microstructure-based modeling approach to assess aging-sensitive mechanics of human intervertebral disc

BACKGROUND AND OBJECTIVE

The human body soft tissues are hierarchic structures interacting in a complex manner with the surrounding biochemical environment. The loss of soft tissues functionality with age leads to more vulnerability regarding to the external mechanical loadings and increases the risk of injuries. As a main example of the human body soft tissues, the intervertebral disc mechanical response evolution with age is explored. Although the age-dependence of the intervertebral disc microstructure is a well-known feature, no noticeable age effect on the disc stiffness is evidenced in the in-vitro experimental studies of the literature. So, if the disc intrinsic mechanics remains constant, how to explain the correlation of disc degeneration and disc functionality loss with age.

METHODS

A microstructure-based modeling approach was developed to assess in-silico the aging-sensitive mechanics of human intervertebral disc. The model considers the relationship between stress/volumetric macro-response and microstructure along with effective age effects acting at the lamellar and multi-lamellar scales. The stress-stretch and transversal responses of the different disc regions were computed for various age groups (13-18, 36, 58, 69 and 82 years old) and their evolution with age was studied.

RESULTS

While matching with in-vitro experimental data, the predicted stiffness was found to increase while passing from adolescent young discs to mature older discs and then to remain almost constant for the rest of life. Important age-related changes in the disc transversal behavior were also predicted affecting the flexibility of the disc, changing its volumetric behavior, and modifying its dimensions.

CONCLUSION

The developed approach was found able to bring new conclusions about age-dependent mechanical properties including regional dependency. The disc mechanics in terms of rigidity, radial and axial transversal responses were found to alter going from adolescent to middle age where the disc reaches a certain maturity. After reaching maturity, the mechanical properties undergo very slight changes until becoming almost constant with age.

Evaluation of Bone Marrow-derived Stem Cells and Adipose-derived Stem Cells Co-cultured on Human Nucleus Pulposus Cells: A Pilot Study

Objective

We aimed to determine whether bone marrow-derived mesenchymal stem cells (BDMSCs) effectively attenuate the degeneration of human nucleus pulposus cells (NPCs).

Methods

Four NPC lines were obtained from 3 subjects who underwent spinal surgery for cervical disc herniation (n=1) or lumbar disc herniation (n=2). For co-culture wells without contact, BDMSCs and adipose-derived mesenchymal stem cells (ADMSCs) were seeded on tissue culture plates and maintained for 3 days. Senescence-associated β-gal (SA-β-gal) staining was represented as a percentage of the total number of stained cells (%). The cells with intracellular lipid droplets (LDs) were represented as the percentage of the number of cells with LDs. Glycosaminoglycan (GAG) secretion was measured at 450 nm, using a commercial kit, to analyze optical density.

Results

The ratio of cells stained with SA-β-gal to the total number of cells reduced significantly when co-cultured with BDMSCs and ADMSCs (p<0.001 vs. p<0.001). The proportion of NPCs containing LDs was lower when co-cultured with BDMSCs than with ADMSCs (p<0.001). The optical density related to GAG secretion was lower in BDMSCs and ADMSCs co-cultured with NPCs than in the controls (p<0.001 vs. p<0.001).

Conclusion

SA-β-gal staining showed significant attenuation of degenerative changes in NPCs co-cultured with BDMSCs. Moreover, the unexpected increase in LDs was significantly higher in NPCs co-cultured with ADMSCs than in those co-cultured with BDMSCs. However, GAG secretion was significantly decreased in NPCs co-cultured with MSCs.

Osmo-inelastic response of the intervertebral disc annulus fibrosus tissue

The aim of this article is to provide some insights on the osmo-inelastic response under stretching of annulus fibrosus of the intervertebral disc. Circumferentially oriented specimens of square cross section, extracted from different regions of bovine cervical discs (ventral-lateral and dorsal-lateral), are tested under different strain-rates and saline concentrations within normal range of strains. An accurate optical strain measuring technique, based upon digital image correlation, is used in order to determine the full-field displacements in the lamellae and fibers planes of the layered soft tissue. Annulus stress-stretch relationships are measured along with full-field transversal strains in the two planes. The mechanical response is found hysteretic, rate-dependent and osmolarity-dependent with a Poisson's ratio higher than 0.5 in the fibers plane and negative (auxeticity) in the lamellae plane. While the stiffness presents a regional-dependency due to variations in collagen fibers content/orientation, the strain-rate sensitivity of the response is found independent on the region. A significant osmotic effect is found on both the auxetic response in the lamellae plane and the stiffness rate-sensitivity. These local experimental observations will result in more accurate chemo-mechanical modeling of the disc annulus and a clearer multi-scale understanding of the disc intervertebral function.

Interlamellar-induced time-dependent response of intervertebral disc annulus: A microstructure-based chemo-viscoelastic model

The annulus fibrosus of the intervertebral disc exhibits an unusual transversal behavior for which a constitutive representation that considers as well regional effect, chemical sensitivity and time-dependency has not yet been developed, and it is hence the aim of the present contribution. A physically-based model is proposed by introducing a free energy function that takes into account the actual disc annulus structure in relation with the surrounding biochemical environment. The response is assumed to be dominated by the viscoelastic contribution of the extracellular matrix, the elastic contribution of the oriented collagen fibers and the osmo-induced volumetric contribution of the internal fluid content variation. The regional dependence of the disc annulus response due to variation in fibers content/orientation allows a micromechanical treatment of the soft tissue. A finite element model of the annulus specimen is designed while taking into consideration the 'interlamellar' ground substance zone between lamellae of the layered soft tissue. The kinetics is designed using full-field strain measurements performed on specimens extracted from two disc annulus regions and tested under different osmotic conditions. The time-dependency of the tissue response is reported on stress-free volumetric changes, on hysteretic stress and transversal strains during quasi-static stretching at different strain-rates and on their temporal changes during an interrupted stretching. Considering the effective contributions of the internal fluid transfer and the extracellular matrix viscosity, the microstructure-based chemo-mechanical model is found able to successfully reproduce the significant features of the macro-response and the unusual transversal behavior including the strong regional dependency from inner to outer parts of the disc: Poisson's ratio lesser than 0 (auxetic) in lamellae plane, higher than 0.5 in fibers plane, and their temporal changes towards usual values (between 0 and 0.5) at chemo-mechanical equilibrium. The underlying time-dependent mechanisms occurring in the tissue are analyzed via the local numerical fields and important insights about the effective role of the interlamellar zone are revealed for the different disc localizations. STATEMENT OF SIGNIFICANCE: The structural complexity of the annulus fibrosus has only been appreciated through recent experimental contributions and a constitutive representation that considers as well regional effect, chemical sensitivity and time-dependency of the unusual transversal behavior has not yet been developed. Here, a microstructure-based chemo-viscoelastic model is developed to highlight the interlamellar-induced time-dependent response by means of a two-scale strategy. The model provides important insights about the origin of the time-dependent phenomena in disc annulus along with regional dependency, essential for understanding disc functionality.

Crystallinity dependency of the time-dependent mechanical response of polyethylene: application in total disc replacement

Degeneration of the intervertebral disc (IVD) is a leading source of chronic low back pain or neck pain, and represents the main cause of long-term disability worldwide. In the aim to relieve pain, total disc replacement (TDR) is a valuable surgical treatment option, but the expected benefit strongly depends on the prosthesis itself. The present contribution is focused on the synthetic mimic of the native IVD in the aim to optimally restore its functional anatomy and biomechanics, and especially its time-dependency. Semi-crystalline polyethylene (PE) materials covering a wide spectrum of the crystallinity are used to propose new designs of TDR. The influence of the crystallinity on various features of the time-dependent mechanical response of the PE materials is reported over a large strain range by means of dynamic mechanical thermo-analysis and video-controlled tensile mechanical tests. The connection of the stiffness and the yield strength with the microstructure is reported in the aim to propose a model predicting the crystallinity dependency of the response variation with the frequency. New designs of TDR are proposed and implemented into an accurate computational model of a cervical spine segment in order to simulate the biomechanical response under physiological conditions. Predicted in-silico motions are found in excellent agreement with experimental data extracted from published in-vitro studies under compression and different neck movements, namely, rotation, flexion/extension and lateral bending. The simulation results are also criticized by analyzing the local stresses and the predicted biomechanical responses provided by the different prosthetic solutions in terms of time-dependency manifested by the hysteretic behavior under a cyclic movement and the frequency effect.

Potential biomarkers of the mature intervertebral disc identified at the single cell level

Intervertebral disc (IVD) degeneration and trauma is a major socio-economic burden and the focus of cell-based regenerative medicine approaches. Despite numerous ongoing clinical trials attempting to replace ailing IVD cells with mesenchymal stem cells, a solid understanding of the identity and nature of cells in a healthy mature IVD is still in need of refinement. Although anatomically simple, the IVD is comprised of heterogeneous cell populations. Therefore, methods involving cell pooling for RNA profiling could be misleading. Here, by using RNA in situ hybridization and z proportion test, we have identified potential novel biomarkers through single cell assessment. We quantified the proportion of RNA transcribing cells for 50 genetic loci in the outer annulus fibrosus (AF) and nucleus pulposus (NP) in coccygeal bovine discs isolated from tails of four skeletally mature animals. Our data reconfirm existing data and suggest 10 novel markers such as Lam1 and Thy1 in the outer AF and Gli1, Gli3, Noto, Scx, Ptprc, Sox2, Zscan10 and LOC101904175 in the NP, including pluripotency markers, that indicate stemness potential of IVD cells. These markers could be added to existing biomarker panels for cell type characterization. Furthermore, our data once more demonstrate heterogeneity in cells of the AF and NP, indicating the need for single cell assessment by methods such as RNA in situ hybridization. Our work refines the molecular identity of outer AF and NP cells, which can benefit future regenerative medicine and tissue engineering strategies in humans.

Glycosaminoglycan synthesis in the nucleus pulposus: Dysregulation and the pathogenesis of disc degeneration

Few human tissues have functions as closely linked to the composition of their extracellular matrices as the intervertebral disc. In fact, the hallmark of intervertebral disc degeneration, commonly accompanying low back and neck pain, is the progressive loss of extracellular matrix molecules - specifically the GAG-substituted proteoglycans. While this loss is often associated with increased extracellular catabolism via metalloproteinases and pro-inflammatory cytokines, there is strong evidence that disc degeneration is related to dysregulation of the enzymes involved in GAG biosynthesis. In this review, we discuss those environmental factors, unique to the disc, that control expression and function of XT-1, GlcAT-I, and ChSy/ChPF in the healthy and degenerative state. Additionally, we address the pathophysiology of aberrant GAG biosynthesis and highlight therapeutic strategies designed to augment the loss of extracellular matrix molecules that afflict the degenerative state.

[Comparative study on biological characteristics between different generations of rabbit nucleus pulposus cells]

Objective

To research the biological characteristics of different generations of rabbit nucleus pulposus cells (NPCs) that were cultured with natural culture and subculture method.

Methods

The thoracolumbar segments of New Zealand white rabbits (6-8 weeks old and weighing 1.5-2.5 kg) were obtained and nucleus pulposus were isolated from disc regions. And NPCs were harvested by enzymatic digestion from nucleus pulposus. Primary NPCs were counted as P0 generation. Then, NPCs were passaged by trypsin and counted as P1, P2, P3 with a totle of 4 generations. P0 to P3 generations NPCs were separately examined by observation of cell morphology and proliferation time, detection of apoptosis rates of cells by flow cytometry, and detection of hypoxia-inducible factor 1α (HIF-1α), matrix metalloproteinases 2 (MMP-2), Aggrecan, and collagen type Ⅱ proteins by immunofluorescence and Western blot.

Results

The morphology of NPCs transformed from triangular or polygonal in P0 generation to spindle in P3 generation; the characteristic vacuolated cells gradually disappeared; and the cell volume and cell proliferation time increased. The cell apoptosis rates were 5.47%±0.91%, 13.77%±2.42%, 33.46%±1.82%, and 38.76%±1.50% from P0 to P3 generations, with the increase of culture time, and there were significant differences between 4 generations ( P<0.05). Immunofluorescence staining showed that with the increase of cells generation, the fluorescence intensity of HIF-1α, collagen type Ⅱ, and Aggrecan decreased, and the fluorescence intensity of MMP-2 increased. Western blot results showed that the relative expression of HIF-1α protein was high in P0 generation, the P1 generation has a rising trend, and then gradually decreased; the differences between generations were significant ( P<0.05). The relative expression of collagen type Ⅱ protein decreased from P0 to P3 generations and there were significant differences between generations ( P<0.05). The relative expression of Aggrecan protein decreased from P0 to P2 generations and there were significant differences between generations ( P<0.05); but no significant difference was found between P2 and P3 generations ( P>0.05). The relative expression of MMP-2 protein increased significantly in P3 generation; except that the difference between P0 and P2 generations was not significant ( P>0.05), the significant differences were found between the other generations ( P<0.05).

Conclusion

Rabbit NPCs degeneration model was successfully established by the natural culture and subculture method. Transforming of NPCs morphology, increasing of cell apoptosis rates, decreasing of anabolism, and increasing of catabolism were presented in NPCs degeneration model.

Quantitative MRI in early intervertebral disc degeneration: T1rho correlates better than T2 and ADC with biomechanics, histology and matrix content

Introduction

Low-back pain (LBP) has been correlated to the presence of intervertebral disc (IVD) degeneration on T2-weighted (T2w) MRI. It remains challenging, however, to accurately stage degenerative disc disease (DDD) based on T2w MRI and measurements of IVD height, particularly for early DDD. Several quantitative MRI techniques have been introduced to detect changes in matrix composition signifying early DDD. In this study, we correlated quantitative T2, T1rho and Apparent Diffusion Coefficient (ADC) values to disc mechanical behavior and gold standard early DDD markers in a graded degenerated lumbar IVD caprine model, to assess their potential for early DDD detection.

Methods

Lumbar caprine IVDs were injected with either 0.25 U/ml or 0.5 U/ml Chondroïtinase ABC (Cabc) to trigger early DDD-like degeneration. Injection with phosphate-buffered saline (PBS) served as control. IVDs were cultured in a bioreactor for 20 days under axial physiological loading. High-resolution 9.4 T MR images were obtained prior to intervention and after culture. Quantitative MR results were correlated to recovery behavior, histological degeneration grading, and the content of glycosaminoglycans (GAGs) and water.

Results

Cabc-injected IVDs showed aberrancies in biomechanics and loss of GAGs without changes in water-content. All MR sequences detected changes in matrix composition, with T1rho showing largest changes pre-to-post in the nucleus, and significantly more than T2 and ADC. Histologically, degeneration due to Cabc injection was mild. T1rho nucleus values correlated strongest with altered biomechanics, histological degeneration score, and loss of GAGs.

Conclusions

T2- and T1rho quantitative MR-mapping detected early DDD changes. T1rho nucleus values correlated better than T2 and ADC with biomechanical, histological, and GAG changes. Clinical implementation of quantitative MRI, T1rho particularly, could aid in distinguishing DDD more reliably at an earlier stage in the degenerative process.

A Histopathological Scheme for the Quantitative Scoring of Intervertebral Disc Degeneration and the Therapeutic Utility of Adult Mesenchymal Stem Cells for Intervertebral Disc Regeneration

The purpose of this study was to develop a quantitative histopathological scoring scheme to evaluate disc degeneration and regeneration using an ovine annular lesion model of experimental disc degeneration. Toluidine blue and Haematoxylin and Eosin (H&E) staining were used to evaluate cellular morphology: (i) disc structure/lesion morphology; (ii) proteoglycan depletion; (iii) cellular morphology; (iv) blood vessel in-growth; (v) cell influx into lesion; and (vi) cystic degeneration/chondroid metaplasia. Three study groups were examined: 5 × 5 mm lesion; 6 × 20 mm lesion; and 6 × 20 mm lesion plus mesenchymal stem cell (MSC) treatment. Lumbar intervertebral discs (IVDs) were scored under categories (i-vi) to provide a cumulative score, which underwent statistical analysis using STATA software. Focal proteoglycan depletion was associated with 5 × 5 mm annular rim lesions, bifurcations, annular delamellation, concentric and radial annular tears and an early influx of blood vessels and cells around remodeling lesions but the inner lesion did not heal. Similar features in 6 × 20 mm lesions occurred over a 3-6-month post operative period. MSCs induced a strong recovery in discal pathology with a reduction in cumulative histopathology degeneracy score from 15.2 to 2.7 (p = 0.001) over a three-month recovery period but no recovery in carrier injected discs.

Effect of Hydration on Healthy Intervertebral Disk Mechanical Stiffness

The intervertebral disk has an excellent swelling capacity to absorb water, which is thought to be largely due to the high proteoglycan composition. Injury, aging, degeneration, and diurnal loading are all noted by a significant decrease in water content and tissue hydration. The objective of this study was to evaluate the effect of hydration, through osmotic loading, on tissue swelling and compressive stiffness of healthy intervertebral disks. The wet weight of nucleus pulposus (NP) and annulus fibrosus (AF) explants following swelling was 50% or greater, demonstrating significant ability to absorb water under all osmotic loading conditions (0.015 M-3.0 M phosphate buffered saline (PBS)). Estimated NP residual strains, calculated from the swelling ratio, were approximately 1.5 × greater than AF residual strains. Compressive stiffness increased with hyperosmotic loading, which is thought to be due to material compaction from osmotic-loading and the nonlinear mechanical behavior. Importantly, this study demonstrated that residual strains and material properties are greatly dependent on osmotic loading. The findings of this study support the notion that swelling properties from osmotic loading will be important for accurately describing the effect of degeneration and injury on disk mechanics. Furthermore, the tissue swelling will be an important consideration for developing biological repair strategies aimed at restoring mechanical behavior toward a healthy disk.

Macro- to microscale strain transfer in fibrous tissues is heterogeneous and tissue-specific

Mechanical deformation applied at the joint or tissue level is transmitted through the macroscale extracellular matrix to the microscale local matrix, where it is transduced to cells within these tissues and modulates tissue growth, maintenance, and repair. The objective of this study was to investigate how applied tissue strain is transferred through the local matrix to the cell and nucleus in meniscus, tendon, and the annulus fibrosus, as well as in stem cell-seeded scaffolds engineered to reproduce the organized microstructure of these native tissues. To carry out this study, we developed a custom confocal microscope-mounted tensile testing device and simultaneously monitored strain across multiple length scales. Results showed that mean strain was heterogeneous and significantly attenuated, but coordinated, at the local matrix level in native tissues (35-70% strain attenuation). Conversely, freshly seeded scaffolds exhibited very direct and uniform strain transfer from the tissue to the local matrix level (15-25% strain attenuation). In addition, strain transfer from local matrix to cells and nuclei was dependent on fiber orientation and tissue type. Histological analysis suggested that different domains exist within these fibrous tissues, with most of the tissue being fibrous, characterized by an aligned collagen structure and elongated cells, and other regions being proteoglycan (PG)-rich, characterized by a dense accumulation of PGs and rounder cells. In meniscus, the observed heterogeneity in strain transfer correlated strongly with cellular morphology, where rounder cells located in PG-rich microdomains were shielded from deformation, while elongated cells in fibrous microdomains deformed readily. Collectively, these findings suggest that different tissues utilize distinct strain-attenuating mechanisms according to their unique structure and cellular phenotype, and these differences likely alter the local biologic response of such tissues and constructs in response to mechanical perturbation.

Elastic, permeability and swelling properties of human intervertebral disc tissues: A benchmark for tissue engineering

The aim of functional tissue engineering is to repair and replace tissues that have a biomechanical function, i.e., connective orthopaedic tissues. To do this, it is necessary to have accurate benchmarks for the elastic, permeability, and swelling (i.e., biphasic-swelling) properties of native tissues. However, in the case of the intervertebral disc, the biphasic-swelling properties of individual tissues reported in the literature exhibit great variation and even span several orders of magnitude. This variation is probably caused by differences in the testing protocols and the constitutive models used to analyze the data. Therefore, the objective of this study was to measure the human lumbar disc annulus fibrosus (AF), nucleus pulposus (NP), and cartilaginous endplates (CEP) biphasic-swelling properties using a consistent experimental protocol and analyses. The testing protocol was composed of a swelling period followed by multiple confined compression ramps. To analyze the confined compression data, the tissues were modeled using a biphasic-swelling model, which augments the standard biphasic model through the addition of a deformation-dependent osmotic pressure term. This model allows considering the swelling deformations and the contribution of osmotic pressure in the analysis of the experimental data. The swelling stretch was not different between the disc regions (AF: 1.28±0.16; NP: 1.73±0.74; CEP: 1.29±0.26), with a total average of 1.42. The aggregate modulus (Ha) of the extra-fibrillar matrix was higher in the CEP (390kPa) compared to the NP (100kPa) or AF (30kPa). The permeability was very different across tissue regions, with the AF permeability (64 E(-16)m(4)/Ns) higher than the NP and CEP (~5.5 E(-16)m(4)/Ns). Additionally, a normalized time-constant (3000s) for the stress relaxation was similar for all the disc tissues. The properties measured in this study are important as benchmarks for tissue engineering and for modeling the disc's mechanical behavior and transport.

Mechanical properties of the extra-fibrillar matrix of human annulus fibrosus are location and age dependent

The mechanical behavior of the annulus fibrosus (AF) of the intervertebral disc can be modeled as a mixture of fibers, extra-fibrillar matrix (EFM), ions, and fluid. However, the properties of the EFM have not been measured directly. We measured mechanical properties of the human EFM at several locations, determined the effect of age and degeneration, and evaluated whether changes in EFM properties correspond to AF compositional changes. EFM mechanical properties were measured using a method that combines osmotic loading and confined compression. AF samples were dissected from several locations, and mechanical properties were correlated with age, degeneration, and composition. EFM modulus was found to range between 10 and 50 kPa, increasing nonlinearly with compression magnitude and being highest in the AF outer-anterior region. EFM properties were not correlated with composition or degeneration. However, the EFM modulus, its relative contribution to tissue modulus, and model parameters were correlated with age. These measurements will result in more accurate predictions of deformations in the intervertebral disc. Additionally, parameters such as permeability and diffusivity used for biotransport analysis of glucose and other solutes depend on EFM deformation. Consequently, the accuracy of biotransport simulations will be greatly improved.

Compressive properties of fibrous repair tissue compared to nucleus and annulus

The wound healing process includes filling the void between implant and tissue edges by collagenous connective repair tissue. This fibrous repair tissue may load share or stabilize implants such as spinal disc replacements. The objective of this study was the biomechanical characterization of human fibrous tissue compared to annulus fibrosus and nucleus pulposus. Human lumbar discs (10 nucleus and annulus) and 10 lumbar deep wound fibrous tissue specimens were sectioned into 12mm diameter×6mm high cylindrical samples. Confined compression testing, after 2h swelling at 0.11MPa, was performed at 5%, 10% and 15% strain over 3.5h. Unconfined dynamic testing (2-0.001Hz) was performed at 5-15% strain. Semi-quantitative histology estimated the proportion of proteoglycan to collagen. Fibrous tissue exhibited a decrease in height during the swelling period whereas annulus and nucleus tissues did not. The aggregate modulus was significantly less for fibrous tissue (p<0.002). Percent stress relaxation was greatest for the fibrous tissue and similar for annulus and nucleus. Dynamic testing found the storage modulus (E') was greater than the loss modulus (E″) for all tissues. Annulus were found to have greater E' and E″ than nucleus, whereas E' and E″ were similar between annulus and fibrous tissue. Fibrous tissue had the greatest increase in both moduli at greater frequencies, but had the lowest hydration and proteoglycan content. Fibrous tissue would not be a substitute for native tissue within the disc space but if adjacent to a disc prosthesis may impart some degree of intersegmental stability during acute loading activities.

The effects of microenvironment in mesenchymal stem cell-based regeneration of intervertebral disc

BACKGROUND CONTEXT

Recent studies have demonstrated new therapeutic strategy using transplantation of mesenchymal stem cells (MSCs), especially bone marrow-derived MSCs (BM-MSCs), to preserve intervertebral disc (IVD) structure and functions. It is important to understand whether and how the MSCs survive and thrive in the hostile microenvironment of the degenerated IVD. Therefore, this review majorly examines how resident disc cells, hypoxia, low nutrition, acidic pH, mechanical loading, endogenous proteinases, and cytokines regulate the behavior of the exogenous MSCs.

PURPOSE

To review and summarize the effect of the microenvironment in biological characteristics of BM-MSCs for IVD regeneration; the presence of endogenous stem cells and the state of the art in the use of BM-MSCs to regenerate the IVD in vivo were also discussed.

STUDY DESIGN

Literature review.

METHODS

MEDLINE electronic database was used to search for articles concerning stem/progenitor cell isolation from the IVD, regulation of the components of microenvironment for MSCs, and MSC-based therapy for IVD degeneration. The search was limited to English language.

RESULTS

Stem cells are probably resident in the disc, but exogenous stem cells, especially BM-MSCs, are currently the most popular graft cells for IVD regeneration. The endogenous disc cells and the biochemical and biophysical components in the degenerating disc present a complicated microenvironment to regulate the transplanted BM-MSCs. Although MSCs regenerate the mildly degenerative disc effectively in the experimental and clinical trials, many underlying questions are in need of further investigation.

CONCLUSIONS

There has been a dramatic improvement in the understanding of potential MSC-based therapy for IVD regeneration. The use of MSCs for IVD degeneration is still at the stage of preclinical and Phase 1 studies. The effects of the disc microenvironment in MSCs survival and function should be closely studied for transferring MSC transplantation from bench to bedside successfully.

Multi-scale structural and tensile mechanical response of annulus fibrosus to osmotic loading

This study investigates differential multi-scale structure and function relationships of the outer and inner annulus fibrosus (AF) to osmotic swelling in different buffer solutions by quantifying tensile mechanics, glycoasamino-glycan(GAG) content, water content and tissue swelling, and collagen fibril ultrastructure. In the outer AF, the tensile modulus decreased by over 70% with 0.15 M PBS treatment but was unchanged with 2 M PBS treatment. Moreover, the modulus loss following 0.15 M PBS treatment was reversed when followed by 2 M PBS treatment, potentially from increased interfibrillar and interlamellar shearing associated with fibril swelling. In contrast, the inner AF tensile modulus was unchanged by 0.15 M PBS treatment and increased following 2 M treatment. Transmission electron microscopy revealed that the mean collagen fibril diameters of the untreated outer and inner AF were 87.8 ± 27.9 and 71.0 ± 26.9 nm, respectively. In the outer AF, collagen fibril swelling was observed with both 0.15 M and 2 M PBS treatments, but inherently low GAG content remained unchanged. In the inner AF, 2 M PBS treatment caused fibril swelling and GAG loss, suggesting that GAG plays a role in maintaining the structure of collagen fibrils leading to modulation of the native tissue mechanical properties. These results demonstrate important regional variations in structure and composition, and their influence on the heterogeneous mechanics of the AF. Moreover, because the composition and structure is altered as a consequence of progressive disk degeneration, quantification of these interactions is critical for study of the AF pathogenesis of degeneration and tissue engineering

Comparative immunolocalisation of fibrillin-1 and perlecan in the human foetal, and HS-deficient hspg2 exon 3 null mutant mouse intervertebral disc

The aim of this study was to examine the comparative localisations of fibrillin-1 and perlecan in the foetal human, wild-type C57BL/6 and HS-deficient hspg2Δ³⁻/Δ³⁻ exon 3 null mouse intervertebral disc (IVD) using fluorescent laser scanning confocal microscopy. Fibrillin-1 fibrils were prominent components of the outer posterior and anterior annulus fibrosus (AF) of the foetal human IVD. Finer fibrillin-1 fibrils were evident in the inner AF where they displayed an arcade-type arrangement in the developing lamellae. Relatively short but distinct fibrillin-1 fibrils were evident in the central region of the IVD and presumptive cartilaginous endplate and defined the margins of the nuclear sheath in the developing nucleus pulposus (NP). Fibrillin-1 was also demonstrated in the AF of C57BL/6 wild-type mice but to a far lesser extent in the HS-deficient hspg2Δ³⁻/Δ³⁻ exon 3 null mouse. This suggested that the HS chains of perlecan may have contributed to fibrillin-1 assembly or its deposition in the IVD. The cell-matrix interconnections provided by the fibrillin fibrils visualised in this study may facilitate communication between disc cells and their local biomechanical microenvironment in mechanosensory processes which regulate tissue homeostasis. The ability of fibrillin-1 to sequester TGF-β a well-known anabolic growth factor in the IVD also suggests potential roles in disc development and/or remodelling.

Quantitative analysis of lumbar intervertebral disc abnormalities at 3.0 Tesla: value of T(2) texture features and geometric parameters

T(2) relaxation time mapping provides information about the biochemical status of intervertebral discs. The present study aimed to determine whether texture features extracted from T(2) maps or geometric parameters are sensitive to the presence of abnormalities at the posterior aspect of lumbar intervertebral discs, i.e. bulging and herniation. Thirty-one patients (21 women and 10 men; age range 18-51 years) with low back pain were enrolled. MRI of the lumbar spine at 3.0 Tesla included morphological T(1) - and T(2) -weighted fast spin-echo sequences, and multi-echo spin-echo sequences that were used to construct T(2) maps. On morphological MRI, discs were visually graded into 'normal', 'bulging' or 'herniation'. On T(2) maps, texture analysis (based on the co-occurrence matrix and wavelet transform) and geometry analysis of the discs were performed. The three T(2) texture features and geometric parameters best-suited for distinguishing between normal discs and discs with bulging or herniation were determined using Fisher coefficients. Statistical analysis comprised ANCOVA and post hoc t-tests. Eighty-two discs were classified as 'normal', 49 as 'bulging' and 20 showed 'herniation.' The T(2) texture features Entropy and Difference Variance, and all three pre-selected geometric parameters differed significantly between normal and bulging, normal and herniated, and bulging and herniated discs (p < 0.05). These findings suggest that T(2) texture features and geometric parameters are sensitive to the presence of abnormalities at the posterior aspect of lumbar intervertebral discs, and may thus be useful as quantitative biomarkers that predict disease.

Recent progress in histochemistry and cell biology

Studies published in Histochemistry and Cell Biology in the year 2011 represent once more a manifest of established and newly sophisticated techniques being exploited to put tissue- and cell type-specific molecules into a functional context. The review is therefore the Histochemistry and Cell Biology's yearly intention to provide interested readers appropriate summaries of investigations touching the areas of tissue biology, developmental biology, the biology of the immune system, stem cell research, the biology of subcellular compartments, in order to put the message of such studies into natural scientific-/human- and also pathological-relevant correlations.

Mechanical destabilization induced by controlled annular incision of the intervertebral disc dysregulates metalloproteinase expression and induces disc degeneration

STUDY DESIGN

An investigation of mechanical destabilization of the lumbar ovine intervertebral disc (IVD) inducing IVD degeneration (IVDD) as determined by multiparameter outcome measures (magnetic resonance imaging [MRI], IVD composition, biomechanical testing, gene profiling, immunohistochemistry, and immunoblotting).

OBJECTIVE

To assess the effect of IVD mechanical destabilization on matrix protein and metalloproteinase gene expression to investigate the pathophysiological mechanisms of lumbar IVDD.

SUMMARY OF BACKGROUND DATA

Several earlier studies have used annular transection to induce IVDD in sheep, but none have optimized or validated the most appropriate lesion size.

METHODS

The annulus fibrosus (AF) incision inducing maximal change in IVD biomechanics was applied to L1-L2, L3-L4, and L5-L6 discs in vivo to compare with a sham procedure at 3 months post operation. IVDs were evaluated by MRI, biomechanics, histopathology, proteoglycan and collagen content, gene expression, and aggrecan proteolysis by Western blotting.

RESULTS

Significant changes were observed in lesion (6 × 20 mm(2)) compared with sham IVDs at 3 months post operation: reduced disc height on MRI; increased neutral zone in biomechanical testing; depleted proteoglycan and collagen content in the nucleus pulposus (NP) and lesion half of the AF but not in the contralateral AF; increased messenger RNA for collagen I and II, aggrecan, versican, perlecan, matrix metalloproteinase (MMP)-1 & 13, and ADAMTS-5, in the lesion-site AF and NP but not in the contralateral AF. ADAMTS-4 messenger RNA was increased in the lesion-site AF but decreased in the NP. Despite an upregulation in MMPs, there was no change in MMP- or ADAMTS-generated aggrecan neoepitopes in any region of the IVD in lesion or sham discs.

CONCLUSION

Lumbar IVDD was reproducibly induced with a 6 × 20 mm(2) annular lesion, with focal dysregulation of MMP gene expression, cell cloning in the inner AF, loss of NP aggrecan, and disc height. Loss of aggrecan from the NP was not attributable to increased proteolysis in the interglobular domain by MMPs or ADAMTS.

Human Annulus Fibrosus Dynamic Tensile Modulus Increases with Degeneration

The annulus fibrosus (AF) of the intervertebral disc experiences cyclic tensile loading in vivo at various states of mechanical equilibrium. Disc degeneration is associated with alterations in the biochemical composition of the AF including decreased water content, decreased proteoglycan concentration, and increased collagen deposition that affect mechanical function of the AF in compression and shear. Such changes may also affect the dynamic viscoelastic properties of the AF and thus alter the disc's ability to dissipate energy under physiologic loading. The objectives of this study were to quantify the dynamic viscoelastic properties of human AF in circumferential tension and to determine the effect of degeneration on these properties. Nondegenerated and degenerated human AF tensile samples were tested in uniaxial tension over a spectrum of loading frequencies spanning 0.01Hz to 2Hz at several states of equilibrium strain to determine the dynamic viscoelastic properties (dynamic modulus, phase angle) using a linear viscoelastic model. The AF dynamic modulus increased at higher equilibrium strain levels. The AF behaved more elastically at higher frequencies with a decreased phase angle. Degeneration resulted in a higher dynamic modulus at all strain levels but had no effect on phase angle. The findings from this study elucidate the effect of degeneration on the dynamic viscoelastic properties of human AF and lend insight into the mechanical role of the AF in cyclic loading conditions.

Comparative immunolocalization of the elastin fiber-associated proteins fibrillin-1, LTBP-2, and MAGP-1 with components of the collagenous and proteoglycan matrix of the fetal human intervertebral disc

STUDY DESIGN

A comparative immunolocalization study of elastin-associated proteins and established intervertebral disc (IVD) extracellular matrix (ECM) components.

OBJECTIVE

To localize for the first time, elastic fiber–associated proteins with structural fibrillar components in the annulus fibrosus (AF) of the fetal IVD.

SUMMARY OF BACKGROUND DATA

Elastin has been identified histochemically in adult bovine, human, and immature rat IVDs, and in fetal human IVDs using electron microscopy; however, no immunolocalization studies have been undertaken for associated components in human fetal IVDs.

METHODS

En-bloc fixation of thoracolumbar spinal segments in formalin and Histochoice followed by standard histochemical processing, paraffin embedding, microtome sectioning, and identification of IVD ECM components using a range of specific mono- and polyclonal antibodies and bright-field and laser scanning confocal microscopy.

RESULTS

The elastic fiber-associated proteins fibrillin-1, LTBP-2, and MAGP-1 were prominently immunolocalized in the outer lamellar layers of the AF of the human fetal IVD. Dual localization of selected components by confocal microscopy demonstrated that versican and LTBP-2 were colocalized with fibrillin-1 microfibrils in the AF lamellae with a similar distribution to the elastin fibers. LTBP-2 was also associated with pericellular perlecan in the outer AF. These interconnections between elastin-associated proteins resulted in an elastic network, which connected the AF cells with the adjacent cartilaginous vertebral bodies.

CONCLUSION

Specific immunolocalization of fibrillin-1, MAGP-1, and versican with elastin in the outer AF of the fetal human IVD has been demonstrated. We deduce from the established distributions of the elastin-associated proteins and their known interactivities with matrix components that these stabilize and aid in the integration of the elastic fibers in the annular lamellae and may be responsible for the generation of tensional forces in the outer AF, which direct the assembly of this tissue.

Effect of synthetic link N peptide on the expression of type I and type II collagens in human intervertebral disc cells

Intervertebral disc (IVD) degeneration is associated with proteolytic degradation of proteoglycan aggregates present within the extracellular matrix of the disc. Link N peptide (DHLSDNYTLDHDRAIH) is the N-terminal peptide of link protein, which stabilizes the proteoglycan aggregates. It is generated in vivo by proteolytic degradation during tissue turnover. It has been previously shown that this peptide can stimulate the synthesis of collagens by articular cartilage and bovine IVD cells in vitro. Being a synthetic peptide, Link N has considerable financial benefits for clinical use over recombinant growth factors because it is extremely cheap to produce. The purpose of the present study was to determine the effect of Link N on the expression of types I and II collagen and investigate the cellular mechanisms of Link N signal transduction in human IVD cells. The present results suggest that Link N stimulates the expression of types I and II collagen in human IVD cells. More specifically, Link N stimulated the expression of type I in nucleus pulposus (NP) cells, but not in annulus fibrosus cells. As Link N also decreased the phosphorylation of p38 in NP cells only, results suggest that p38 is a mediator of the effect of Link N on type I collagen expression. p38 is a member of the mitogen-activated protein kinase family highlighted by three major cascades: p38, c-Jun amino-terminal kinase, and extracellular signal-regulated kinase pathways. Link N showed no effect on the latter two pathways, suggesting a specific effect of Link N on the p38 cascade. On the other hand, Link N stimulated the expression of type II collagen in both NP and annulus fibrosus, suggesting that other mechanisms are implicated in the control of type II collagen expression in disc cells, without excluding p38 for the NP. In conclusion, the present study showed that Link N can modulate the expression of collagen in human IVD cells.

Electrospun nanofibrous polycaprolactone scaffolds for tissue engineering of annulus fibrosus

The annulus fibrosus comprises concentric lamellae that can be damaged due to intervertebral disc degeneration; to provide permanent repair of these acquired structural defects, one solution is to fabricate scaffolds that are designed to support the growth of annulus fibrosus cells. In this study, electrospun nanofibrous scaffolds of polycaprolactone are fabricated in random, aligned, and round-end configurations. Primary porcine annulus fibrosus cells are grown on the scaffolds and evaluated for attachment, proliferation, and production of extracellular matrix. The scaffold consisting of round-end nanofibers substantially outperforms the random and aligned scaffolds on cell adhesion; additionally, the scaffold with aligned nanofibers strongly affects the orientation of cells.

A detailed microscopic examination of alterations in normal anular structure induced by mechanical destabilization in an ovine model of disc degeneration

STUDY DESIGN

Microstructural investigation of anular structure.

OBJECTIVE

To reveal the effect of mechanical destabilization on the anular architecture both locally and distantly.

SUMMARY OF BACKGROUND DATA

Several longitudinal ovine-induced disc degeneration studies have documented degenerative changes in disc components using histologic, biomechanical, and biochemical approaches; however, changes in intervertebral disc (IVD) microstructure have largely remained neglected. In recent years, the use of structurally relevant section planes has improved our understanding of disc microstructure, including the presence of significant bridging structures radially linking the lamellae. It has been suggested that the translamellar cross-bridges offer a mechanism by which the anular wall can adaptively remodel itself in response to a changing biomechanical microenvironment.

METHODS

IVDs harvested from lesion and sham-operated groups of Merino wethers were subjected to en face oblique and vertical sectioning. The macrostructural effect of the destabilization was examined in the vertically sectioned group with conventional histologic techniques. The second group was serially sectioned into 30-μm slices allowing a global examination of the anular microstructure in its fully hydrated state using a differential interference contrast microscope.

RESULTS

The previously described induced disc degeneration in the mid-inner anulus fibrosus (AF) and a spontaneous repair process in the outer AF was confirmed. Increased translamellar bridging was observed contralaterally to the lesion in the mechanically destabilized IVD and development of atypical broad bridging elements in the outer lamellae. Structural alterations in the lamellar anchorages to the cartilaginous endplates in destabilized IVDs, including lamellar branching and discontinuities atypical of normal lamellar attachments were also observed.

CONCLUSION

The present investigation has offered a glimpse of an anular wall apparently capable of remodeling in response to perturbations in its normal mechanical environment. The translamellar cross-bridges undergo adaptations in structure, in response to altered stresses locally at the anular defect site but also distantly in the contralateral AF in the destabilized disc. It is currently not known whether such changes in anular microarchitecture, however, predispose the anulus to further mechanical damage or have a stabilizing role to play in this structure.

Fibronectin splicing variants in human intervertebral disc and association with disc degeneration

STUDY DESIGN

In this study, normal intervertebral disc (IVD) tissues and degenerative human IVD tissues were compared for presence of fibronectin (FN) mRNA splice variants and for FN fragments (FN-f).

OBJECTIVE

To further understand FN RNA splice forms and protein fragments in disc degeneration.

SUMMARY OF BACKGROUND DATA

FN splice variants play important roles in regulating cell-matrix and matrix-matrix interactions in skeletogenesis and skeletal function in limbs and other sites. However, presence and possible roles of FN splice variants and fragments in human IVD have not been determined.

METHODS

Normal infant and adult IVD tissues were obtained from organ donors, and degenerative human IVD tissues were obtained from patients undergoing spinal surgeries. FN splice patterns were assessed by reverse transcriptase polymerase chain reaction. Relative expression levels were semiquantified by densitometry. FN and its fragments were studied by Western blot analysis.

RESULTS

Both the EDB and EDB splice variants were present in normal and degenerative IVD tissues. The EDB to EDB ratio was highest in moderately degenerative tissue. The EDA domain was only expressed in infant but not adult tissue. Variable-region (V) splice forms were present in all tissues studied. A splice form with the entire V-region, the 15th type III domain, and 10th type I domain adjacent to the 3'end of V region omitted (referred to as [V+III-15+I-10], also known as [V+C] splice form) was present at higher levels in adult than in infant samples. FN-f were also detected in degenerating tissue, but not in normal IVD tissue samples.

CONCLUSION

The data indicate that higher levels of EDB isoform and FN-f are associated with IVD degeneration. This shift in alternative splicing may reflect an attempt of tissue repair and remodeling. Novel information gathered in this study will lead to a better understanding of pathologic processes associated with disc malfunction and degeneration.

Self-assembly of aligned tissue-engineered annulus fibrosus and intervertebral disc composite via collagen gel contraction

Many cartilaginous tissues such as intervertebral disc (IVD) display a heterogeneous collagen microstructure that results in mechanical anisotropy. These structures are responsible for mechanical function of the tissue and regulate cellular interactions and metabolic responses of cells embedded within these tissues. Using collagen gels seeded with ovine annulus fibrosus cells, constructs of varying structure and heterogeneity were created to mimic the circumferential alignment of the IVD. Alignment was induced within gels by contracting annular gels around an inner boundary using both a polyethylene center and alginate center to create a composite engineered IVD. Collagen alignment and heterogeneity were measured using second harmonic generation microscopy. Decreasing initial collagen density from 2.5 mg/mL to 1 mg/mL produced greater contraction of constructs, resulting in gels that were 55% and 6.2% of the original area after culture, respectively. As a result, more alignment occurred in annular-shaped 1 mg/mL gels compared with 2.5 mg/mL gels (p < 0.05). This alignment was also produced in a composite-engineered IVD with alginate nucleus pulposus. The resulting collagen alignment could promote further aligned collagen development necessary for the creation of a mechanically functional tissue-engineered IVD.

Theoretical and uniaxial experimental evaluation of human annulus fibrosus degeneration

The highly organized structure and composition of the annulus fibrosus provides the tissue with mechanical behaviors that include anisotropy and nonlinearity. Mathematical models are necessary to interpret and elucidate the meaning of directly measured mechanical properties and to understand the structure-function relationships of the tissue components, namely, the fibers and extrafibrillar matrix. This study models the annulus fibrosus as a combination of strain energy functions describing the fibers, matrix, and their interactions. The objective was to quantify the behavior of both nondegenerate and degenerate annulus fibrosus tissue using uniaxial tensile experimental data. Mechanical testing was performed with samples oriented along the circumferential, axial, and radial directions. For samples oriented along the radial direction, the toe-region modulus was 2x stiffer with degeneration. However, no other differences in measured mechanical properties were observed with degeneration. The constitutive model fit well to samples oriented along the radial and circumferential directions (R(2)> or =0.97). The fibers supported the highest proportion of stress for circumferential loading at 60%. There was a 70% decrease in the matrix contribution to stress from the toe-region to the linear-region of both the nondegenerate and degenerate tissue. The shear fiber-matrix interaction (FMI) contribution increased by 80% with degeneration in the linear-region. Samples oriented along the radial and axial direction behaved similarly under uniaxial tension (modulus=0.32 MPa versus 0.37 MPa), suggesting that uniaxial testing in the axial direction is not appropriate for quantifying the mechanics of a fiber reinforcement in the annulus. In conclusion, the structurally motivated nonlinear anisotropic hyperelastic constitutive model helps to further understand the effect of microstructural changes with degeneration, suggesting that remodeling in the subcomponents (i.e., the collagen fiber, matrix and FMI) may minimize the overall effects on mechanical function of the bulk material with degeneration.

Identification of phenotypic discriminating markers for intervertebral disc cells and articular chondrocytes

OBJECTIVE

The present study was conducted to improve our knowledge of intervertebral disc (IVD) cell biology by comparing the phenotype of nucleus pulposus (NP) and annulus fibrosus (AF) cells with that of articular chondrocytes (ACs).

METHODS

Rabbit cells from NP and AF were isolated and their phenotype was compared with that of AC by real-time PCR analysis of type I (COL1A1), II (COL2A1) and V (COL5A1) collagens, aggrecan transcript (AGC1), matrix Gla protein (MGP) and Htra serine peptidase 1 (Htra1).

RESULTS

Transcript analysis indicated that despite certain similarities, IVD cells exhibit distinct COL2A1/COL1A1 and COL2A1/AGC1 ratios as compared with AC. The expression pattern of COL5A1, MGP and Htra1 makes it possible to define a phenotypic signature for NP and AF cells.

CONCLUSIONS

Our study shows that NP and AF cells exhibit a clearly distinguishable phenotype from that of AC. Type V collagen, MGP and HtrA1 could greatly help to discriminate among NP, AF and AC cells.

Topographical variation in the distributions of versican, aggrecan and perlecan in the foetal human spine reflects their diverse functional roles in spinal development

We evaluated the immunohistochemical distribution of three major proteoglycans of cartilage, i.e., aggrecan, versican and perlecan vis-a-vis collagens I and II in the developing human spine of first-trimester foetuses. Aggrecan and perlecan were prominently immunolocalised in the cartilaginous vertebral body rudiments and to a lesser extent within the foetal intervertebral disc. In contrast, versican was only expressed in the developing intervertebral disc interspace. Using domain-specific monoclonal antibodies against the various modules of versican, we discovered the V0 isoform as the predominant form present. Versican immunolocalisations conducted with antibodies directed to epitopes in its N and C termini and GAG-alpha and GAG-beta core protein domains provided evidence that versican in the nucleus pulposus was either synthesised devoid of a G3 domain or this domain was proteolytically removed in situ. The V0 versican isoform was localised with prominent fibrillar components in the annular lamellae of the outer annulus fibrosus. Perlecan was a notable pericellular proteoglycan in the annulus fibrosus and nucleus pulposus but poorly immunolocalised in the marginal tissues of the developing intervertebral disc, apparently delineating the intervertebral disc-vertebral body interface region destined to become the cartilaginous endplate in the mature intervertebral disc. The distribution of collagens I and II in the foetal spine was mutually exclusive with type I present in the outer annulus fibrosus, marginal tissues around the vertebral body rudiment and throughout the developing intervertebral disc, and type II prominent in the vertebral rudiment, absent in the outer annulus fibrosus and diffusely distributed in the inner annulus fibrosus and nucleus pulposus. Collectively, our findings suggest the existence of an intricate and finely balanced interplay between various proteoglycans and collagens and the spinal cell populations which synthesise and assemble these components during spinal development.

Primary bovine intervertebral disc cells transduced with adenovirus overexpressing 12 BMPs and Sox9 maintain appropriate phenotype

OBJECTIVE

To confirm that primary intervertebral disc cells cultured in monolayer transduced with adenovirus maintained their phenotype, hence is an appropriate system to test gene therapy agents.

DESIGN

Adult bovine nucleus pulposus and anulus fibrosus cells cultured in monolayer were transduced with adenoviruses expressing human bone morphogenetic proteins (AdBMPs) or Sox9 (AdSox9), or green fluorescence protein (AdGFP, as control). Chondrocyte phenotypic markers (e.g., type II collagen and aggrecan) and the chondrocyte hypertrophy marker (type X collagen) were measured 6 days after viral transduction by reverse-transcription polymerase chain reaction.

RESULTS

Primary nucleus pulposus and anulus fibrosus cells transduced with AdBMPs, AdSox9, or adenovirus-expressing green fluorescence protein only (AdGFP, as control) continue to express healthy chondrocyte phenotypic markers and showed no evidence of the expression of the chondrocyte hypertrophy marker (type X collagen gene). Thus, we have shown that bovine nucleus pulposus and anulus fibrosus cells transduced with adenovirus overexpressing 12 different bone morphogenetic proteins or Sox9 maintain their phenotype in short-term culture.

CONCLUSIONS

In this study, primary bovine intervertebral disc cells transduced with adenovirus overexpressing 12 bone morphogenetic proteins or Sox9 preserved their phenotype in short-term culture. These cells did not express the type X collagen gene, an undesirable chondrocyte hypertrophic gene that could lead to ossification. Therefore, low-passage intervertebral disc cells cultured in monolayer is an appropriate culture system to test therapeutic genes. We further suggest that these cells may also be appropriate for engineering tissues or for cell therapy for degenerative disc diseases.

Ultrastructure of inclusion bodies in annulus cells in the degenerating human intervertebral disc

The rough endoplasmic reticulum (rER) of the cell has an architectural editing function that checks whether protein structure and three-dimensional assembly have occurred properly prior to export of newly synthesized material out of the cell. If these have been faulty, the material is retained within the rER as an inclusion body. Inclusion bodies have been identified previously in chondrocytes and osteoblasts in chondrodysplasias and osteogenesis imperfecta. Inclusion bodies in intervertebral disc cells, however, have only recently been recognized. Our objectives were to use transmission electron microscopy to analyze more fully inclusion bodies in the annulus pulposus and to study the extracellular matrix (ECM) surrounding cells containing inclusion bodies. ECM frequently encapsulated cells with inclusion bodies, and commonly contained prominent banded aggregates of Type VI collagen. Inclusion body material had several morphologies, including relatively smooth, homogeneous material, or a rougher, less homogeneous feature. Such findings expand our knowledge of the fine structure of the human disc cell and ECM during disc degeneration, and indicate the potential utility of ultrastructural identification of discs with intracellular inclusion bodies as a screening method for molecular studies directed toward identification of defective gene products in degenerating discs.

The elastic fibre network of the human lumbar anulus fibrosus: architecture, mechanical function and potential role in the progression of intervertebral disc degeneration

Elastic fibres are critical constituents of dynamic biological structures that functionally require elasticity and resilience. The network of elastic fibres in the anulus fibrosus of the intervertebral disc is extensive, however until recently, the majority of histological, biochemical and biomechanical studies have focussed on the roles of other extracellular matrix constituents such as collagens and proteoglycans. The resulting lack of detailed descriptions of elastic fibre network architecture and mechanical function has limited understanding of the potentially important contribution made by elastic fibres to healthy disc function and their possible roles in the progression of disc degeneration. In addition, it has made it difficult to postulate what the consequences of elastic fibre related disorders would be for intervertebral disc behaviour, and to develop treatments accordingly. In this paper, we review recent and historical studies which have examined both the structure and the function of the human lumbar anulus fibrosus elastic fibre network, provide a synergistic discussion in an attempt to clarify its potentially critical contribution both to normal intervertebral disc behaviour and the processes relating to its degeneration, and recommend critical areas for future research.

Quantitative MRI as a diagnostic tool of intervertebral disc matrix composition and integrity

Degenerative disc disease has been implicated as a major component of spine pathology. The current major clinical procedures for treating disc degeneration have been disappointing, because of altered spinal mechanics leading to subsequent degeneration at adjacent disc levels. Disc pathology treatment is shifting toward prevention and treatment of underlying etiologic processes at the level of the disc matrix composition and integrity and the biomechanics of the disc. The ability to perform such treatment relies on one's ability to accurately and objectively assess the state of the matrix and the effectiveness of treatment by a non-invasive technique. In this review, we will summarize our advances in efforts to develop an objective, accurate, non-invasive diagnostic tool (quantitative MRI) in the detection and quantification of matrix composition and integrity and of biomechanical changes in early intervertebral disc degeneration.

The application of 23Na double-quantum-filter (DQF) NMR spectroscopy for the study of spinal disc degeneration

Degenerative disc disease is an irreversible process that leads to a loss of mechanical integrity and back pain in millions of people. In this report, (23)Na double-quantum-filtered (DQF) NMR spectroscopy is used to study disc tissues in two stages of degeneration. Initial results indicate that the (23)Na DQF signal may be useful for determining the degree of degeneration. The spectral analysis reveals the presence of sodium environments with different residual quadrupolar couplings and T(2) relaxation times that we attribute to different regions, or compartments, corresponding to different biochemical regions in the tissue. In general it is found that there are compartments with no residual quadrupolar couplings, compartments with moderate couplings (200 to 1000 Hz), and compartments with couplings ranging from 1500 to 3000 Hz. The results indicate that (23)Na DQF NMR spectroscopy provides a probe of the degenerative state of the intervertebral disc tissues, and might hold potential as a novel diagnostic method for detection of disc degeneration.