Mechanisms for mechanical damage in the intervertebral disc annulus fibrosus

Intervertebral disc degeneration results in disorganization of the laminate structure of the annulus that may arise from mechanical microfailure. Failure mechanisms in the annulus were investigated using composite lamination theory and other analyses to calculate stresses in annulus layers, interlaminar shear stress, and the region of stress concentration around a fiber break. Scanning electron microscopy (SEM) was used to evaluate failure patterns in the annulus and evaluate novel structural features of the disc tissue. Stress concentrations in the annulus due to an isolated fiber break were localized to approximately 5 microm away from the break, and only considered a likely cause of annulus fibrosus failure (i.e., radial tears in the annulus) under extreme loading conditions or when collagen damage occurs over a relatively large region. Interlaminar shear stresses were calculated to be relatively large, to increase with layer thickness (as reported with degeneration), and were considered to be associated with propagation of circumferential tears in the annulus. SEM analysis of intervertebral disc annulus fibrosus tissue demonstrated a clear laminate structure, delamination, matrix cracking, and fiber failure. Novel structural features noted with SEM also included the presence of small tubules that appear to run along the length of collagen fibers in the annulus and a distinct collagenous structure representative of a pericellular matrix in the nucleus region.     

Pathophysiology of the human intervertebral disc

Intervertebral disc degeneration is a common invalidating disorder that can affect the musculoskeletal apparatus in both younger and older ages. The chief component of the intervertebral disc is the highly organized extracellular matrix; maintenance of its organization is essential for correct spinal mechanics. The matrix components, mainly proteoglycans and collagens, undergo a slow and continuous cell-mediated turnover process that enables disc cells to adapt their environment to external stimuli. Cellular senescence and a history of chronic abnormal loading can upset this balance, leading to progressive tissue failure that results in disc degeneration. Although biological treatment approaches to disc repair are still far to come, advances in our understanding of disc biochemistry and in defining the role of genetic inheritance have provided a starting point for developing new concepts in the diagnosis, therapy and prevention of disc degeneration.     

Neutral proteinases of the human intervertebral disc

Disc tissue consisting of pooled annuli fibrosus and nuclei pulposus from the cadaver of an adolescent aged 19 years was extracted with 4.0 M Gu-HCl. Proteins of low buoyant density (p less than or equal to 1.38 g/ml) containing the disc enzymes and inhibitors were separated from proteoglycans of high buoyant density (p greater than or equal to 1.50 g/ml) by density gradient ultracentrifugation. Sephadex G-75F gel chromatography followed by trypsin affinity chromatography was then used to resolve disc proteolytic and trypsin inhibitory activities. The results obtained were strongly suggestive of the presence of a high molecular weight zymogen which upon activation generated a population of smaller molecular weight proteinases. The disc proteinases obtained by this process showed similar properties in terms of: their pH optima (7.4-7.6); their inhibition patterns by class-specific proteinase inhibitors; their variation of activity as a function of NaCl and lysine concentrations; and the hydrodynamic size of their proteoglycan degradation products. The activated disc neutral proteinase demonstrated many characteristics in common with plasmin; however, unlike the latter, the disc proteinases also showed some calcium dependence.     

Thermal analysis of the human intervertebral disc

Intervertebral disc (IVD) degeneration is one of the most common musculuskeletal disorders affecting western society. Degeneration alters the morphology and the mechanical properties of the discs. According to previous reports, DSC proved to be a suitable method for the demonstration of thermal consequences of local as well as global conformational changes in the structure of the human intervertebral discs. In the present study, a wide spectrum of degenerated IVD was examined by DSC. The results suggest that definitive differences exist between the stages of disc degeneration in calorimetric measures.     

Aquaporin expression in the human intervertebral disc

The nucleus pulposus (NP) of the human intervertebral disc (IVD) is a hyperosmotic tissue that is subjected to daily dynamic compressive loads. In order to survive within this environment the resident chondrocyte-like cells must be able to control their cell volume, whilst also controlling the anabolism and catabolism of their extra-cellular matrix. Recent studies have demonstrated expression of a range of bi-directional, transmembrane water and solute transporters, named aquaporins (AQPs), within chondrocytes of articular cartilage. The aim of this study was to use immunohistochemsitry to investigate the expression of aquaporins 1, 2 and 3 within the human IVD. Results demonstrated expression of both AQP-1 and -3 by cells within the NP and inner annulus fibrosus (AF), while outer AF cells lacked expression of AQP-1 and showed very low numbers of AQP-3 immunopositive cells. Cells from all regions were negative for AQP-2. Therefore this study demonstrates similarities in the phenotype of NP cells and articular chondrocytes, which may be due to similarities in tissue osmolarity and mechanobiology. The decrease in expression of AQPs from the NP to the outer AF may signify changes in cellular phenotype in response to differences in mechanbiology, osmolarity and hydration between the gelatinous NP and the fibrous AF.     

Connective tissue growth factor expression in human intervertebral disc: implications for angiogenesis in intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is strongly associated with chronic low back pain, one of the most common causes of morbidity in the West. While normal healthy IVD is avascular, angiogenesis is a constant feature of IVD degeneration and has been shown to be associated with in-growth of nerves. Connective tissue growth factor (CTGF) plays a pivotal role in angiogenesis. To investigate the expression of CTGF in both normal and degenerated IVD, 21 IVDs were obtained from patients at surgery or postmortem examination and grouped according to the severity of histological degeneration. The immunohistochemical expression of CTGF was correlated with the degree of degeneration. CD31 immunohistochemistry was used to correlate IVD degeneration with vasculature. Our results showed that CTGF is expressed in non-degenerated and degenerated human IVDs and increased expression of CTGF is associated with degenerated discs, particularly within areas of neovascularization. We suggest that CTGF may play a role in angiogenesis in the human degenerated IVD.     

Temporal changes of mechanical signals and extracellular composition in human intervertebral disc during degenerative progression

In this study, a three-dimensional finite element model was used to investigate the changes in tissue composition and mechanical signals within human lumbar intervertebral disc during the degenerative progression. This model was developed based on the cell-activity coupled mechano-electrochemical mixture theory. The disc degeneration was simulated by lowering nutrition levels at disc boundaries, and the temporal and spatial distributions of the fixed charge density, water content, fluid pressure, Von Mises stress, and disc deformation were analyzed. Results showed that fixed charge density, fluid pressure, and water content decreased significantly in the nucleus pulposus (NP) and the inner to middle annulus fibrosus (AF) regions of the degenerative disc. It was found that, with degenerative progression, the Von Mises stress (relative to that at healthy state) increased within the disc, with a larger increase in the outer AF region. Both the disc volume and height decreased with the degenerative progression. The predicted results of fluid pressure change in the NP were consistent with experimental findings in the literature. The knowledge of the variations of temporal and spatial distributions of composition and mechanical signals within the human IVDs provide a better understanding of the progression of disc degeneration.     

Immunolocalization of MMP-19 in the human intervertebral disc: implications for disc aging and degeneration

Matrix metalloproteinases (MMPs) degrade components of the extracellular matrix of the disc, but the presence of MMP-19 has not been explored. In other tissues, MMP-19 is known to act in proteolysis of the insulin-like growth factor (IGF) binding protein-3, thereby exposing this protein to make it available to influence cell behavior. MMP-19 also has been shown to inhibit capillary-like formation and thus play a role in the avascular nature of the disc. Using immunohistochemistry, normal discs from six subjects aged newborn through 10 years and 20 disc specimens from control donors or surgical patients aged 15-76 (mean age 40.2 years) were examined for immunolocalization of MMP-19; six Thompson grade I discs, five Thompson grade II, eight Thompson grade III, five Thompson grade IV, and one Thompson grade V discs were analyzed. The results indicate that in discs from young subjects, MMP-19 was uniformly localized in the outer annulus. In discs from adult donors and surgical patients, outer and inner annulus cells only occasionally showed MMP-19 localization. The greatest expression of MMP-19 was observed in young discs, and little expression was seen in older or degenerating discs. Because MMP-19 has been shown to regulate IGF-mediated proliferation in other tissues, its decline in the aging/degenerating disc may contribute to the age-related decrease in disc cell numbers.     

Regulation of catabolic gene expression in normal and degenerate human intervertebral disc cells: implications for the pathogenesis of intervertebral disc degeneration

Introduction  

The aim of this study was to compare the effects of tumour necrosis factor-alpha (TNF-α) and interleukin-1-beta (IL-1β) on protease and catabolic cytokine and receptor gene expression in normal and degenerate human nucleus pulposus cells in alginate culture.     

Transcriptional profiling of bovine intervertebral disc cells: implications for identification of normal and degenerate human intervertebral disc cell phenotypes

Introduction  

Nucleus pulposus (NP) cells have a phenotype similar to articular cartilage (AC) cells. However, the matrix of the NP is clearly different to that of AC suggesting that specific cell phenotypes exist. The aim of this study was to identify novel genes that could be used to distinguish bovine NP cells from AC and annulus fibrosus (AF) cells, and to further determine their expression in normal and degenerate human intervertebral disc (IVD) cells.     

Expression of semaphorin 3A and its receptors in the human intervertebral disc: potential role in regulating neural ingrowth in the degenerate intervertebral disc

Introduction  

Intervertebral disc (IVD) degeneration is considered a major underlying factor in the pathogenesis of chronic low back pain. Although the healthy IVD is both avascular and aneural, during degeneration there is ingrowth of nociceptive nerve fibres and blood vessels into proximal regions of the IVD, which may contribute to the pain. The mechanisms underlying neural ingrowth are, however, not fully understood. Semaphorin 3A (sema3A) is an axonal guidance molecule with the ability to repel nerves seeking their synaptic target. This study aimed to identify whether members of the Class 3 semaphorins were expressed by chondrocyte-like cells of the IVD addressing the hypothesis that they may play a role in repelling axons surrounding the healthy disc, thus maintaining its aneural condition.     

Immunolocalization of MMP-19 in the human intervertebral disc: implications for disc aging and degeneration.

Matrix metalloproteinases (MMPs) degrade components of the extracellular matrix of the disc, but the presence of MMP-19 has not been explored. In other tissues, MMP-19 is known to act in proteolysis of the insulin-like growth factor (IGF) binding protein-3, thereby exposing this protein to make it available to influence cell behavior. MMP-19 also has been shown to inhibit capillary-like formation and thus play a role in the avascular nature of the disc. Using immunohistochemistry, normal discs from six subjects aged newborn through 10 years and 20 disc specimens from control donors or surgical patients aged 15-76 (mean age 40.2 years) were examined for immunolocalization of MMP-19; six Thompson grade I discs, five Thompson grade II, eight Thompson grade III, five Thompson grade IV, and one Thompson grade V discs were analyzed. The results indicate that in discs from young subjects, MMP-19 was uniformly localized in the outer annulus. In discs from adult donors and surgical patients, outer and inner annulus cells only occasionally showed MMP-19 localization. The greatest expression of MMP-19 was observed in young discs, and little expression was seen in older or degenerating discs. Because MMP-19 has been shown to regulate IGF-mediated proliferation in other tissues, its decline in the aging/degenerating disc may contribute to the age-related decrease in disc cell numbers.