The challenge and advancement of annulus fibrosus tissue engineering

Background

Intervertebral disc degeneration, a main cause of back pain, is an endemic problem and a big economic burden for the health care system. Current treatments are symptom relieving but do not address underlying problems—biological and structural deterioration of the disc. Tissue engineering is an emerging approach for the treatment of intervertebral disc degeneration since it restores the functionality of native tissues. Although numerous studies have focused on the nucleus pulposus tissue engineering and achieved successes in laboratory settings, disc tissue engineering without annulus fibrosus for the end stage of disc degeneration is deemed to fail. The purpose of this article is to review the advancement of annulus fibrosus tissue engineering.

Material and Methods

Relevant articles regarding annulus fibrosus tissue engineering were identified in PubMed and Medline databases.

Results

The ideal strategy for disc regeneration is to restore the function and integrity of the disc by using biomaterials, native matrices, growth factors, and cells that producing matrices. In the past decades there are tremendous advancement in annulus fibrosus tissue engineering including cell biology, biomaterials, and whole disc replacement. The recent promising results on whole disc tissue engineering—a composite of annulus fibrosus and nucleus pulposus—make the tissue engineering approach more appealing.

Conclusion

Despite the promising results in disc tissue engineering, there is still much work to be done regarding the clinical application.     

The effects of dynamic loading on the intervertebral disc

Loading is important to maintain the balance of matrix turnover in the intervertebral disc (IVD). Daily cyclic diurnal assists in the transport of large soluble factors across the IVD and its surrounding circulation and applies direct and indirect stimulus to disc cells. Acute mechanical injury and accumulated overloading, however, could induce disc degeneration. Recently, there is more information available on how cyclic loading, especially axial compression and hydrostatic pressure, affects IVD cell biology. This review summarises recent studies on the response of the IVD and stem cells to applied cyclic compression and hydrostatic pressure. These studies investigate the possible role of loading in the initiation and progression of disc degeneration as well as quantifying a physiological loading condition for the study of disc degeneration biological therapy. Subsequently, a possible physiological/beneficial loading range is proposed. This physiological/beneficial loading could provide insight into how to design loading regimes in specific system for the testing of various biological therapies such as cell therapy, chemical therapy or tissue engineering constructs to achieve a better final outcome. In addition, the parameter space of ‘physiological’ loading may also be an important factor for the differentiation of stem cells towards most ideally ‘discogenic’ cells for tissue engineering purpose.     

Relevance of in vitro and in vivo models for intervertebral disc degeneration

Models available for the study of intervertebral disc degeneration are designed to answer many important questions. In vitro biologic models employ a variety of cell, tissue, or organ culture techniques with culture conditions that partially mimic the cellular environment of the degenerated human intervertebral disc. In vitro biomechanical models include intervertebral disc or motion-segment loading experiments as well as finite element modeling techniques. The literature describes numerous in vivo animal models for use in the study of intervertebral disc degeneration, each of which has its own advantages and disadvantages. Human-subject studies have included the use of magnetic resonance imaging and other techniques to assess diffusion into the intervertebral disc, to measure intradiscal pressure, to conduct kinematic or stiffness studies of lumbar motion segments, and to evaluate muscular forces on the spine. Although all of these studies are helpful in answering specific questions, their relevance in assessing disc degeneration in patients with symptoms of discogenic pain must be carefully considered.     

New optical diagnostic tools in renal cell cancer

In modern medicine the profound use of abdominal imaging has led to a dramatic change of presentation of renal tumors. Smaller tumors and therefore more benign masses are being discovered, and as systemic use of renal mass biopsies is not recommended by the general guidelines, an appropriate tool to assess the biology of renal tumors is highly desirable. Apart from new developments of currently applied diagnostic modalities, several research groups focus on the potential of optical diagnostic techniques to contribute to the diagnostic process of renal tumors. They use the interaction of light with biological tissue to gather information on the optical properties of a tissue sample and therefore providing information on the histological composition of this tissue in a non-invasive manner and in real-time. In this review we provide an overview of novel diagnostic techniques starting with the future of conventional diagnostics like Contrast-enhanced ultrasonography (CEUS) and positron emission tomography-computed tomography (PET-CT) followed by optical technologies that are potentially employed in the nearby future to improve the diagnostic process of renal tumors with a focus on optical diagnostic techniques.     

Biological repair of the degenerated intervertebral disc by the injection of growth factors

The homeostasis of intervertebral disc (IVD) tissues is accomplished through a complex and precise coordination of a variety of substances, including cytokines, growth factors, enzymes and enzyme inhibitors. Recent biological therapeutic strategies for disc degeneration have included attempts to up-regulate the production of key matrix proteins or to down-regulate the catabolic events induced by pro-inflammatory cytokines. Several approaches to deliver these therapeutic biologic agents have been proposed and tested in a preclinical setting. One of the most advanced biological therapeutic approaches to regenerate or repair a degenerated disc is the injection of a recombinant growth factor. Abundant evidence for the efficacy of growth factor injection therapy for the treatment of IVD degeneration can be found in preclinical animal studies. Recent data obtained from animal studies on changes in cytokine expression following growth factor injection illustrate the great potential for patients with chronic discogenic low back pain. The first clinical trial for growth factor injection has been initiated and the results of that study may prove the usefulness of growth factor injection for treating the symptoms of patients with degenerative disc diseases. The focus of this review article is the effects of an in vivo injection of growth factors on the biological repair of the degenerated intervertebral disc in animal models. The effects of growth factor injection on the symptoms of patients with low back pain, the therapeutic target of growth factor injection and the limitations of the efficacy of growth factor therapy are also reviewed. Further quantitative studies on the effect of growth factor injection on pain generation and the long term effects on the endplate and cell survival after an injection using large animals are needed. An international academic-industrial consortium addressing these aims, such as was achieved for osteoarthritis (The Osteoarthritis Initiative), may further the development of biological therapies for degenerative disc diseases.

     

骨髓间充质干细胞治疗椎间盘退行性病变的研究进展

对椎间盘退变机制、BMSCs生物学特性、支架材料及细胞因子认识的深入,为BMSCs修复退变的椎间盘组织奠定了基础。研究表明,BMSCs治疗椎间盘退行性病变具有可行性,临床应用前景广阔。本文就BMSCs治疗椎间盘退行性病变的研究进展进行综述。     

Histologic changes after vertebroplasty

BACKGROUND: Vertebroplasty with use of polymethylmethacrylate cement is gaining popularity in the treatment of some specific painful lesions of the spine. It remains unclear, however, what possible side effects this type of cement might have upon the vertebral body. We performed a histologic and radiographic analysis of the end plate and disc to determine whether there was a difference between vertebroplasty with polymethylmethacrylate cement and vertebroplasty with calcium phosphate cement in the surrounding tissue of the goat spine. Furthermore, we assessed whether a defect in the end plate, simulating end-plate fracture and allowing for direct contact of cement with disc tissue, had any effect on end-plate or disc degeneration. METHODS: Twenty-four mature goats were divided between two follow-up periods (six weeks and six months). All animals underwent a bilateral transpedicular vertebroplasty at two lumbar levels, where one of the following treatments was applied: vertebroplasty with calcium phosphate cement with or without an end-plate defect, and vertebroplasty with polymethylmethacrylate cement with or without an end-plate defect. The effect of the various treatments on the integrity of the intervertebral disc, end plate, and surrounding tissue was examined with semiquantitative histologic analysis and radiography. RESULTS: No sign of disc or end-plate degeneration was seen in any of the analyzed sections. The mean disc height did not decrease from the postoperative period to the time that the animals were killed in any group, thereby supporting the histologic findings. A mild inflammatory reaction was found in four vertebral bodies in the polymethylmethacrylate groups only. CONCLUSIONS: Calcium phosphate cement and polymethylmethacrylate cement both seem to be adequate bone-void fillers in terms of biological behavior in the vertebral body.     

椎间盘退变生物学治疗的研究进展

椎间盘退变所致的颈肩腰腿痛严重影响许多患者的生活及工作,目前的治疗方法主要侧重于缓解疼痛症状或神经受压症状,而无法阻止椎间盘退变的进程,导致疾病具有高复发率。近年来学者们开始广泛研究椎间盘退变的生物学治疗方法,即通过生物分子治疗、基因治疗、细胞治疗和组织工程等方法来修复和重塑椎间盘,以期从根本上解决椎间盘退变的问题,而上述方法大多处于动物实验或体外实验阶段,临床应用尚存在诸多挑战。     

Cell therapy for disc degeneration--potentials and pitfalls

Disc degeneration is considered a major source of pain in patients with chronic low back pain. Novel strategies to cure or decrease the symptoms and increase the patient's quality of life and function are under development. Until recently conservative treatment and fusion surgery were the main therapeutic options. Disc prostheses are undergoing clinical evaluation. The potential for cell transplantation to the intervertebral disc with mature autologous disc cells, chondrocytes, or stem cells is in early stages of investigation. Cell transplantation potentially can increase proteoglycan production and induce disc regeneration or slow down the degeneration process. In animal models, transplantation of autologous disc cells and chondrocytes (derived from costal cartilage) has been demonstrated to be feasible and may slow disc degeneration.     

椎间盘退变与细胞死亡的相关研究进展

腰椎间盘退行性变被认为是临床下腰痛的重要原因,其分子机制尚未明确.近年来,椎间盘退变的分子基础研究已经成为热门.椎间盘独特的生理结构和生物力学特性导致了它易于退变的特点.椎间盘退变开始与椎间盘细胞学行为的改变有关,包括细胞死亡的增加和细胞外基质的降解.然而,在退变椎间盘中的细胞死亡机制仍不明确,主要包括细胞凋亡和自噬.对椎间盘退变分子机制的深入研究能够为将来进一步改善和治疗椎闻盘退变打下基础.虽然椎间盘退变的生物学研究方面已经取得了一定的进展,但是椎间盘本身的生物环境对生物学治疗的发展仍具有挑战性.     

Anulus fibrosus tension inhibits degenerative structural changes in lamellar collagen

Mechanical stress is one of the risk factors believed to influence intervertebral disc degeneration. Animal models have shown that certain regimes of compressive loading can induce a cascade of biological effects that ultimately results in cellular and structural changes in the disc. It has been proposed that both cell-mediated breakdown of collagen and the compromised stability of collagen with loss of anular tension could result in degradation of lamellae in the anulus fibrosus (AF). To determine whether this may be important in the AF, we subjected entire rings of de-cellularized AF tissue to MMP-1 digestion with or without tension. Biomechanical testing found trends of decreasing strength and stiffness when tissues were digested without tension compared with those with tension. To determine the physiologic significance of tissue level tension in the AF, we used an established in vivo murine model to apply a disc compression insult known to cause degeneration. Afterward, that motion segment was placed in fixed-angle bending to impose tissue level tension on part of the AF and compression on the contralateral side. We found that the AF on the convex side of bending retained a healthy lamellar appearance, while the AF on the concave side resembled tissues that had undergone degeneration by loading alone. Varying the time of onset and duration of bending revealed that even a brief duration applied immediately after cessation of compression was beneficial to AF structure on the convex side of bending. Our results suggest that both cell-mediated events and cell-independent mechanisms may contribute to the protective effect of tissue level tension in the AF.     

水凝胶再生修复退变椎间盘的研究进展

目的总结生物材料水凝胶治疗椎间盘退变的研究进展,探讨水凝胶再生修复退变椎间盘的临床应用潜能。方法广泛查阅国内外有关水凝胶修复退变椎间盘尤其是髓核组织的研究文献,并进行总结和分析。结果水凝胶与髓核组织具有相似特性,在退变椎间盘再生修复中扮演重要角色,主要可体现在人工髓核假体、以水凝胶为载体的细胞治疗、非细胞治疗和组织工程修复。结论水凝胶作为生物材料广泛应用于椎间盘的再生修复治疗,为治疗椎间盘退变提供了新方向。     

Nucleus pulposus cells expressing hBMP7 can prevent the degeneration of allogenic IVD in a canine transplantation model

We have previously explored the possibilities of allogenic intervertebral disc (IVD) curing disc degeneration disease in clinical practice. The results showed that the motion and stability of the spinal unit was preserved after transplantation of allogenic IVD in human beings at 5‐year follow‐up. However, mild degeneration was observed in the allogenic transplanted IVD cases. In this study, we construct the biological tissue engineering IVD by injecting the nucleus pulposus cells (NPCs) expressing human bone morphogenetic protein 7 (hBMP7) into cryopreserved IVD, and transplant the biological tissue engineering IVD into a beagle dog to investigate whether NPCs expressing hBMP7 could prevent the degeneration of the transplanted allogenic IVDs. At 24 weeks after transplantation, MRI scan showed that IVD allografts injected NPCs expressing hBMP7 have a slighter signs of degeneration than IVD allografts with NPCs or without NPCs. The range of motion of left–right rotation in the group without NPCs was bigger than that of two cells injection group. PKH‐26‐labeled cells were identified at IVD allograft. The study demonstrated that NPCs expressing hBMP7 could survive at least 24 weeks and prevent the degeneration of the transplanted IVD. This solution might have a potential role in preventing the IVD allograft degeneration in long time follow‐up. © 2013 Orthopaedic Research Society Published by Wiley Periodicals, Inc. J Orthop Res 31:1366–1373, 2013     

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.

     

Biologic modification of animal models of intervertebral disc degeneration

Intervertebral disc degeneration is a chronic process that can become manifest in clinical disorders such as idiopathic low back pain, sciatica, disc herniation, spinal stenosis, and myelopathy. The limited available treatment options (including discectomy and spinal fusion) for these and other disabling conditions that arise from intervertebral disc degeneration are highly invasive, achieve limited success, and only address acute symptoms while doing nothing to halt the process of degeneration. Although the precise pathophysiology of intervertebral disc degeneration has yet to be clearly delineated, the progressive decline in aggrecan, the primary proteoglycan of the nucleus pulposus, appears to be a final common pathway. Animal models as well as in vitro studies of the process of disc degeneration have yielded many potentially useful targets for the reversal of disc degeneration. One current research trend is the use of established animal models of disc degeneration to study the role of therapeutic modalities in reversing the process of degeneration, often with use of the delivery of genes or gene products that influence the anabolic and catabolic pathways of the disc. This article reviews the ability of gene-product delivery systems and gene therapy to alter biologic processes in animal models of disc degeneration and examines future trends in this field.     

The molecular basis of intervertebral disc degeneration

BackgroundIntervertebral disc (IVD) degeneration remains a clinically important condition for which treatment is costly and relatively ineffective. The molecular basis of degenerative disc disease has been an intense focus of research recently, which has greatly increased our understanding of the biology underlying this process.PurposeTo review the current understanding of the molecular basis of disc degeneration.Study designReview article.MethodsA literature review was performed to identify recent investigations and current knowledge regarding the molecular basis of IVD degeneration.ResultsThe unique structural requirements and biochemical properties of the disc contribute to its propensity toward degeneration. Mounting evidence suggests that genetic factors account for up to 75% of individual susceptibility to IVD degeneration, far more than the environmental factors such as occupational exposure or smoking that were previously suspected to figure prominently in this process. Decreased extracellular matrix production, increased production of degradative enzymes, and increased expression of inflammatory cytokines contribute to the loss of structural integrity and accelerate IVD degeneration. Neurovascular ingrowth occurs, in part, because of the changing degenerative phenotype.ConclusionsA detailed understanding of the biology of IVD degeneration is essential to the design of therapeutic solutions to treat degenerative discs. Although significant advances have been made in explaining the biologic mediators of disc degeneration, the inhospitable biochemical environment of the IVD remains a challenging environment for biological therapies.     

Are animal models useful for studying human disc disorders/degeneration?

Intervertebral disc (IVD) degeneration is an often investigated pathophysiological condition because of its implication in causing low back pain. As human material for such studies is difficult to obtain because of ethical and government regulatory restriction, animal tissue, organs and in vivo models have often been used for this purpose. However, there are many differences in cell population, tissue composition, disc and spine anatomy, development, physiology and mechanical properties, between animal species and human. Both naturally occurring and induced degenerative changes may differ significantly from those seen in humans. This paper reviews the many animal models developed for the study of IVD degeneration aetiopathogenesis and treatments thereof. In particular, the limitations and relevance of these models to the human condition are examined, and some general consensus guidelines are presented. Although animal models are invaluable to increase our understanding of disc biology, because of the differences between species, care must be taken when used to study human disc degeneration and much more effort is needed to facilitate research on human disc material. All of the authors contributed equally to this publication and are listed simply in alphabetical order.     

Tissue engineering in urology

Techniques that are aimed at regeneration of human tissues and organs (tissue engineering) have recently entered into clinical practice. Tissue engineering is currently among the fastest growing areas in medicine, and involves the application of the principles of biology and engineering to the development of functional substitutes for damaged tissues. One of the main limitations of reconstructive surgery in the genitourinary tract is the lack of autologous tissue. This could be changed by the ability to cultivate the patient's own tissues in vitro, or by stimulating the cells in vivo into regeneration of new tissues. The present review discusses how tissue engineering can be used to regenerate some of the tissues of the genitourinary tract. Even though these methods have only recently been introduced clinically into genitourinary medicine, numerous scientific studies have been reported that indicate that these techniques may be of great importance in the near future.     

Cell transplantation in lumbar spine disc degeneration disease

Low back pain is an extremely common symptom, affecting nearly three-quarters of the population sometime in their life. Given that disc herniation is thought to be an extension of progressive disc degeneration that attends the normal aging process, seeking an effective therapy that staves off disc degeneration has been considered a logical attempt to reduce back pain. The most apparent cellular and biochemical changes attributable to degeneration include a decrease in cell density in the disc that is accompanied by a reduction in synthesis of cartilage-specific extracellular matrix components. With this in mind, one therapeutic strategy would be to replace, regenerate, or augment the intervertebral disc cell population, with a goal of correcting matrix insufficiencies and restoring normal segment biomechanics. Biological restoration through the use of autologous disc chondrocyte transplantation offers a potential to achieve functional integration of disc metabolism and mechanics. We designed an animal study using the dog as our model to investigate this hypothesis by transplantation of autologous disc-derived chondrocytes into degenerated intervertebral discs. As a result we demonstrated that disc cells remained viable after transplantation; transplanted disc cells produced an extracellular matrix that contained components similar to normal intervertebral disc tissue; a statistically significant correlation between transplanting cells and retention of disc height could displayed. Following these results the Euro Disc Randomized Trial was initiated to embrace a representative patient group with persistent symptoms that had not responded to conservative treatment where an indication for surgical treatment was given. In the interim analyses we evaluated that patients who received autologous disc cell transplantation had greater pain reduction at 2 years compared with patients who did not receive cells following their discectomy surgery and discs in patients that received cells demonstrated a significant difference as a group in the fluid content of their treated disc when compared to control. Autologous disc-derived cell transplantation is technically feasible and biologically relevant to repairing disc damage and retarding disc degeneration. Adipose tissue provides an alternative source of regenerative cells with little donor site morbidity. These regenerative cells are able to differentiate into a nucleus pulposus-like phenotype when exposed to environmental factors similar to disc, and offer the inherent advantage of availability without the need for transporting, culturing, and expanding the cells. In an effort to develop a clinical option for cell placement and assess the response of the cells to the post-surgical milieu, adipose-derived cells were collected, concentrated, and transplanted under fluoroscopic guidance directly into a surgically damaged disc using our dog model. This study provides evidence that cells harvested from adipose tissue might offer a reliable source of regenerative potential capable of bio-restitution.

     

Disc cell therapies: critical issues

Background

Disc cell therapies, in which cells are injected into the degenerate disc in order to regenerate the matrix and restore function, appear to be an attractive, minimally invasive method of treatment. Interest in this area has stimulated research into disc cell biology in particular. However, other important issues, some of which are discussed here, need to be considered if cell-based therapies are to be brought to the clinic.

Purpose

Firstly, a question which is barely addressed in the literature, is how to identify patients with ‘degenerative disc disease’ who would benefit from cell therapy. Pain not disc degeneration is the symptom which drives patients to the clinic. Even though there are associations between back pain and disc degeneration, many people with even severely degenerate discs, with herniated discs or with spinal stenosis, are pain-free. It is not possible using currently available techniques to identify whether disc repair or regeneration would remove symptoms or prevent symptoms from occurring in future. Moreover, the repair process in human discs is very slow (years) because of the low cell density which can be supported nutritionally even in healthy human discs. If repair is necessary for relief of symptoms, questions regarding quality of life and rehabilitation during this long process need consideration.

Also, some serious technical issues remain. Finding appropriate cell sources and scaffolds have received most attention, but these are not the only issues determining the feasibility of the procedure. There are questions regarding the safety of implanting cells by injection through the annulus whether the nutrient supply to the disc is sufficient to support implanted cells and whether, if cells are able to survive, conditions in a degenerate human disc will allow them to repair the damaged tissue.

Conclusions

If cell therapy for treatment of disc-related disorders is to enter the clinic as a routine treatment, investigations must examine the questions related to patient selection and the feasibility of achieving the desired repair in an acceptable time frame. Few diagnostic tests that examine whether cell therapies are likely to succeed are available at present, but definite exclusion criteria would be evidence of major disc fissures, or disturbance of nutrient pathways as measured by post-contrast MRI.