Fibrin promotes proliferation and matrix production of intervertebral disc cells cultured in three-dimensional poly(lactic-co-glycolic acid) scaffold

Discogenic Low Back Pain: Anatomy, Pathophysiology and Treatments of Intervertebral Disc Degeneration

Intervertebral disc (IVD) degeneration is a major contributing factor for discogenic low back pain (LBP), causing a significant global disability. The IVD consists of an inner core proteoglycan-rich nucleus pulposus (NP) and outer lamellae collagen-rich annulus fibrosus (AF) and is confined by a cartilage end plate (CEP), providing structural support and shock absorption against mechanical loads. Changes to degenerative cascades in the IVD cause dysfunction and instability in the lumbar spine. Various treatments include pharmacological, rehabilitation or surgical interventions that aim to relieve pain; however, these modalities do not halt the pathologic events of disc degeneration or promote tissue regeneration. Loss of stem and progenitor markers, imbalance of the extracellular matrix (ECM), increase of inflammation, sensory hyperinnervation and vascularization, and associated signaling pathways have been identified as the onset and progression of disc degeneration. To better understand the pain originating from IVD, our review focuses on the anatomy of IVD and the pathophysiology of disc degeneration that contribute to the development of discogenic pain. We highlight the key mechanisms and associated signaling pathways underlying disc degeneration causing discogenic back pain, current clinical treatments, clinical perspective and directions of future therapies. Our review comprehensively provides a better understanding of healthy IVD and degenerative events of the IVD associated with discogenic pain, which helps to model painful disc degeneration as a therapeutic platform and to identify signaling pathways as therapeutic targets for the future treatment of discogenic pain.

Intervertebral Disc Degeneration: Biomaterials and Tissue Engineering Strategies toward Precision Medicine

Intervertebral disc degeneration is a common cause of discogenic low back pain resulting in significant disability. Current conservative or surgical intervention treatments do not reverse the underlying disc degeneration or regenerate the disc. Biomaterial-based tissue engineering strategies exhibit the potential to regenerate the disc due to their capacity to modulate local tissue responses, maintain the disc phenotype, attain biochemical homeostasis, promote anatomical tissue repair, and provide functional mechanical support. Despite preliminary positive results in preclinical models, these approaches have limited success in clinical trials as they fail to address discogenic pain. This review gives insights into the understanding of intervertebral disc pathology, the emerging concept of precision medicine, and the rationale of personalized biomaterial-based tissue engineering tailored to the severity of the disease targeting early, mild, or severe degeneration, thereby enhancing the efficacy of the treatment for disc regeneration and ultimately to alleviate discogenic pain. Further research is required to assess the relationship between disc degeneration and lower back pain for developing future clinically relevant therapeutic interventions targeted towards the subgroup of degenerative disc disease patients.

Morphology Comparison Between Goat Bone Marrow Mesenchymal Stem Cells and Adhesive Fibrin for the Repair of Annulus Fibrosus Defect of Intervertebral Discs

Introduction: The purpose of this study was to compare the histological findings of goat bone marrow mesenchymal stem cell (BMSC) transplantation and adhesive fibrin repair for annulus fibrosus defects in intervertebral discs. Material and methods: The goats were spanided into three groups: the control group, the adhesive group and the transplantation group. In the control group, surgical instruments were used to create a fibrous ring defect in the intervertebral disc of the goats. In the adhesive group, a 1.5*1.5-cm defect was also created by surgical intervention, and the broken fiber ring was then bonded with adhesive fibrin. In the transplantation group, a gelatine sponge containing the goat BMSCs was implanted into the broken annulus fibrosus, and the wound was closed layer by layer. At 6 weeks and 12 weeks after the operation, the damaged tissues were removed, and haematoxylin and eosin (HE), trichrome gelatine (Masson), Alcian blue periodic acid-Schiff (AB-PAS) and Collagen II staining was performed. Then, the tissues from the different groups were histologically compared and analyzed. Results: Goat BMSCs have a better ability to repair defects in the fibrous ring than adhesive fibrin. Over time, the number of cells or the amount of tissue following cell transplantation was greater, indicating that the degree of repair is greater with BMSCs than with adhesive fibrin. Conclusion: Histologically, repair of the defect of the fibrous ring and prevention of nucleus pulposus protrusion were more effective in the cell transplantation group than in the other two groups.

Injectable Functional Biomaterials for Minimally Invasive Surgery

Injectable materials represent very attractive ready-to-use biomaterials for application in minimally invasive surgical procedures. It is shown that this approach to treat, for example, vertebral fracture, craniofacial defects, or tumor resection has significant clinical potential in the biomedical field. In the last four decades, calcium phosphate cements have been widely used as injectable materials for orthopedic surgery due to their excellent properties in terms of biocompatibility and osteoconductivity. However, few clinical studies have demonstrated certain weaknesses of these cements, which include high viscosity, long degradation time, and difficulties being manipulated. To overcome these limitations, the use of sol-gel technology has been investigated, which has shown good results for synthesis of injectable calcium phosphate-based materials. In the last few decades, injectable hydrogels have gained increasing attention owing to their structural similarities with the extracellular matrix, easy process conditions, and potential applications in minimally invasive surgery. However, the need to protect cells during injection leads to the development of double network injectable hydrogels that are capable of being cross-linked in situ. This review will provide the current state of the art and recent advances in the field of injectable biomaterials for minimally invasive surgery.

Comparing Hydrogels for Human Nucleus Pulposus Regeneration: Role of Osmolarity During Expansion

Hydrogels can facilitate nucleus pulposus (NP) regeneration, either for clinical application or research into mechanisms of regeneration. However, many different hydrogels and culture conditions for human degenerated NP have been employed, making literature data difficult to compare. Therefore, we compared six different hydrogels of natural polymers and investigated the role of serum in the medium and of osmolarity during expansion or redifferentiation in an attempt to provide comparators for future studies. Human NP cells of Thompson grade III discs were cultured in alginate, agarose, fibrin, type II collagen, gelatin methacryloyl (gelMA), and hyaluronic acid-poly(ethylene glycol) hydrogels. Medium containing fetal bovine serum and a serum-free (SF) medium were compared in agarose, gelMA, and type II collagen hydrogels. Isolation and expansion of NP cells in low compared to high osmolarity medium were performed before culture in agarose and type II collagen hydrogels in media of varying osmolarity. NP cells in agarose produced the highest amounts of proteoglycans, followed by cells in type II collagen hydrogels. The absence of serum reduced the total amount of proteoglycans produced by the cells, although incorporation efficiency was higher in type II collagen hydrogels in the absence than in the presence of serum. Isolation and expansion of NP cells in high osmolarity medium improved proteoglycan production during culture in hydrogels, but variation in osmolarity during redifferentiation did not have any effect. Agarose hydrogels seem to be the best option for in vitro culture of human NP cells, but for clinical application, type II collagen hydrogels may be better because, as opposed to agarose, it degrades in time. Although culture in SF medium reduces the amount of proteoglycans produced during redifferentiation culture, isolating and expanding the cells in high osmolarity medium can largely compensate for this loss.

Microfluidic production of bioactive fibrin micro-beads embedded in crosslinked collagen used as an injectable bulking agent for urinary incontinence treatment

Endoscopic injection of bulking agents has been widely used to treat urinary incontinence, often due to urethral sphincter complex insufficiency. The aim of the study was to develop a novel injectable bioactive collagen-fibrin bulking agent restoring long-term continence by functional muscle tissue regeneration. Fibrin micro-beads were engineered using a droplet microfluidic system. They had an average diameter of 140 μm and recombinant fibrin-binding insulin-like growth factor-1 (α₂PI_1-8-MMP-IGF-1) was covalently conjugated to the beads. A plasmin fibrin degradation assay showed that 72.5% of the initial amount of α₂PI_1-8-MMP-IGF-1 loaded into the micro-beads was retained within the fibrin micro-beads. In vitro, the growth factor modified fibrin micro-beads enhanced cell attachment and the migration of human urinary tract smooth muscle cells, however, no change of the cellular metabolic activity was seen. These bioactive micro-beads were mixed with genipin-crosslinked homogenized collagen, acting as a carrier. The collagen concentration, the degree of crosslinking, and the mechanical behavior of this bioactive collagen-fibrin injectable were comparable to reference samples. This novel injectable showed no burst release of the growth factor, had a positive effect on cell behavior and may therefore induce smooth muscle regeneration in vivo, necessary for the functional treatment of stress and other urinary incontinences.

STATEMENT OF SIGNIFICANCE

Urinary incontinence is involuntary urine leakage, resulting from a deficient function of the sphincter muscle complex. Yet there is no functional cure for this devastating condition using current treatment options. Applied physical and surgical therapies have limited success. In this study, a novel bioactive injectable bulking agent, triggering new muscle regeneration at the injection site, has been evaluated. This injectable consists of cross-linked collagen and fibrin micro-beads, functionalized with bound insulin-like growth factor-1 (α₂PI_1-8-MMP-IGF-1). These bioactive fibrin micro-beads induced human smooth muscle cell migration in vitro. Thus, this injectable bulking agent is apt to be a good candidate for regeneration of urethral sphincter muscle, ensuring a long-lasting treatment for urinary incontinence.

Controlled release of basic fibroblast growth factor for angiogenesis using acoustically-responsive scaffolds

The clinical translation of pro-angiogenic growth factors for treatment of vascular disease has remained a challenge due to safety and efficacy concerns. Various approaches have been used to design spatiotemporally-controlled delivery systems for growth factors in order to recapitulate aspects of endogenous signaling and thus assist in translation. We have developed acoustically-responsive scaffolds (ARSs), which are fibrin scaffolds doped with a payload-containing, sonosensitive emulsion. Payload release can be controlled non-invasively and in an on-demand manner using focused, megahertz-range ultrasound (US). In this study, we investigate the in vitro and in vivo release from ARSs containing basic fibroblast growth factor (bFGF) encapsulated in monodispersed emulsions. Emulsions were generated in a two-step process utilizing a microfluidic device with a flow focusing geometry. At 2.5 MHz, controlled release of bFGF was observed for US pressures above 2.2 ± 0.2 MPa peak rarefactional pressure. Superthreshold US yielded a 12.6-fold increase in bFGF release in vitro. The bioactivity of the released bFGF was also characterized. When implanted subcutaneously in mice, ARSs exposed to superthreshold US displayed up to 3.3-fold and 1.7-fold greater perfusion and blood vessel density, respectively, than ARSs without US exposure. Scaffold degradation was not impacted by US. These results highlight the utility of ARSs in both basic and applied studies of therapeutic angiogenesis.

Injectable hydrogels for cartilage and bone tissue engineering

Tissue engineering has become a promising strategy for repairing damaged cartilage and bone tissue. Among the scaffolds for tissue-engineering applications, injectable hydrogels have demonstrated great potential for use as three-dimensional cell culture scaffolds in cartilage and bone tissue engineering, owing to their high water content, similarity to the natural extracellular matrix (ECM), porous framework for cell transplantation and proliferation, minimal invasive properties, and ability to match irregular defects. In this review, we describe the selection of appropriate biomaterials and fabrication methods to prepare novel injectable hydrogels for cartilage and bone tissue engineering. In addition, the biology of cartilage and the bony ECM is also summarized. Finally, future perspectives for injectable hydrogels in cartilage and bone tissue engineering are discussed.

Tissue Engineering of the Intervertebral Disc's Annulus Fibrosus: A Scaffold-Based Review Study

Tissue engineering as a high technology solution for treating disc's problem has been the focus of some researches recently; however, the upcoming successful results in this area depends on understanding the complexities of biology and engineering interface. Whereas the major responsibility of the nucleus pulposus is to provide a sustainable hydrated environment within the disc, the function of the annulus fibrosus (AF) is more mechanical, facilitating joint mobility and preventing radial bulging by confining of the central part, which makes the AF reconstruction important. Although the body of knowledge regarding the AF tissue engineering has grown rapidly, the opportunities to improve current understanding of how artificial scaffolds are able to mimic the AF concentric structure-including inter-lamellar matrix and cross-bridges-addressed unresolved research questions. The aim of this literature review was to collect and discuss, from the international scientific literature, information about tissue engineering of the AF based on scaffold fabrication and material properties, useful for developing new strategies in disc tissue engineering. The key parameter of this research was understanding if role of cross-bridges and inter-lamellar matrix has been considered on tissue engineering of the AF.

Fabrication and characterization of three-dimensional poly(lactic acid-co-glycolic acid), atelocollagen, and fibrin bioscaffold composite for intervertebral disk tissue engineering application

The use of synthetically derived poly(lactic- co-glycolic acid) scaffold and naturally derived materials in regeneration of intervertebral disks has been reported in many previous studies. However, the potential effect of poly(lactic- co-glycolic acid) in combination with atelocollagen or fibrin or both atelocollagen and fibrin bioscaffold composite have not been mentioned so far. This study aims to fabricate and characterize three-dimensional poly(lactic- co-glycolic acid) scaffold incorporated with (1) atelocollagen, (2) fibrin, and (3) both atelocollagen and fibrin combination for intervertebral disk tissue engineering application. The poly(lactic- co-glycolic acid) without any natural, bioscaffold composites was used as control. The chemical conformation, morphology, cell–scaffold attachment, porosity, water uptake capacity, thermal properties, mechanical strength, and pH level were evaluated on all scaffolds using attenuated total reflectance Fourier transform infrared, scanning electron microscope, gravimetric analysis, swelling test, differential scanning calorimetry, and Instron E3000, respectively. Biocompatibility test was conducted to assess the intervertebral disk, annulus fibrosus cells viability using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. The attenuated total reflectance Fourier transform infrared results demonstrated notable peaks of amide bond suggesting interaction of atelocollagen, fibrin, and both atelocollagen and fibrin combination into the poly(lactic- co-glycolic acid) scaffold. Based on the scanning electron microscope observation, the pore size of the poly(lactic- co-glycolic acid) structure significantly reduced when it was incorporated with atelocollagen and fibrin. The poly(lactic- co-glycolic acid)–atelocollagen scaffolds demonstrated higher significant swelling ratios, mechanical strength, and thermal stability than the poly(lactic- co-glycolic acid) scaffold alone. All the three bioscaffold composite groups exhibited the ability to reduce the acidic poly(lactic- co-glycolic acid) by-product. In this study, the biocompatibility assessment using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide cells proliferation assay demonstrated a significantly higher annulus fibrosus cells viability in poly(lactic- co-glycolic acid)–atelocollagen–fibrin compared to poly(lactic- co-glycolic acid) alone. The cellular attachment is comparable in poly(lactic- co-glycolic acid)–atelocollagen–fibrin and poly(lactic- co-glycolic acid)–fibrin scaffolds. Overall, these results may suggest potential use of poly(lactic- co-glycolic acid) combined with atelocollagen and fibrin bioscaffold composite for intervertebral disk regeneration.

Minimal invasive annulotomy for induction of disc degeneration and implantation of poly (lactic-co-glycolic acid) (PLGA) plugs for annular repair in a rabbit model

Background

The rabbit disc model is useful for the study of intervertebral disc (IVD) degeneration and experimental therapeutic interventions. The annulotomy-induced disc models present several drawbacks, particularly an excessive disruption of disc integrity and a rapidly disc degeneration; therefore, this study sought to establish a minimal invasive annulotomy for induction of disc degeneration model, combined to annulus repair using implantation of a PLGA (poly lactic-co-glycolic acid) plug.

Methods

New Zealand white rabbits (n = 24) received annular injuries in three discs (L3/4, L4/5 and L5/6). The experimental discs were randomly assigned to four groups: (a) annular defect with a 1.8 mm diameter mini-trephine; (b) annular puncture by 16G needle; (c) annular defect with a PLGA plug implanted by press-fit fashion; (d) uninjured L2/3 disc served as control. Postsurgical x-ray, MRI examination, and real-time PCR analysis were performed at 1, 3 and 6 months. Gross morphology and histology were evaluated at postoperative 6 months.

Results

Radiographic examinations showed a slow, progressive disc space narrowing and a significant degree of disc degeneration on MRI grade in the injured discs at 6 months in all rabbits. Histological examinations and aggrecan, Col1A1, Col2A1 and matrix metalloprotease (MMP)-3 mRNA expression confirmed the disc degeneration, supporting the imaging results. The PLGA implantation reduced the marked loss of T2-weighted signal intensity seen at MRI in the injured discs and slowly decreased the disc height index (DHI) over the follow-up period. HE/Safranin O staining showed that annular defect was replaced by regenerated connective tissue with significant loss of proteoglycan content.

Conclusions

The minimally invasive approach for the creation of annular defects by an appropriately sized mini-Trephine is a suitable option for the study of disc degeneration in a rabbit model. Implantation of a suitable PLGA plug induced a successful repair of the annulus fibrosus within the degenerated disc, and retarded the degenerative process in the annular injury model.

The formation of human auricular cartilage from microtic tissue: An in vivo study

OBJECTIVES

This study aimed to isolate, culture-expand and characterize the chondrocytes isolated from microtic cartilage and evaluate its potential as a cell source for ear cartilage reconstruction. Specific attention was to construct the auricular cartilage tissue by using fibrin as scaffold.

STUDY DESIGN

Cell culture experiment with the use of microtic chondrocytes.

DESIGN

Cell culture experiment with the use of microtic chondrocytes.

METHODS

After ear reconstructive surgery at the Universiti Kebangsaan Malaysia Medical Center, chondrocytes were isolated from microtic cartilage. Chondrocytes isolated from the tissue were cultured expanded until passage 4 (P4). Upon confluency at P4, chondrocytes were harvested and tissue engineered constructs were made with human plasma polymerized to fibrin. Constructs formed later is implanted at the dorsal part of nude mice for 8 weeks, followed by post-implantation evaluation with histology staining (Hematoxylin and Eosin (H&E) and Safranin O), immunohistochemistry and RT-PCR for chondrogenic associated genes expression level.

RESULTS

Under gross assessment, the construct after 8 weeks of implantation showed similar physical characteristics that of cartilage. Histological staining showed abundant lacunae cells embedded in extracellular matrix similar to that of native cartilage. Safranin O staining showed positive staining which indicates the presence of proteoglycan-rich matrix. Immunohistochemistry analysis showed the strong positive staining for collagen type II, the specific collagen type in the cartilage. Gene expression quantification showed no significant differences in the expression of chondrogenic gene used which is collagen type I, collagen type II, aggrecan core protein (ACP), elastin and sox9 genes when compared to construct formed from normal auricular tissue.

CONCLUSION

Chondrocytes isolated from microtia cartilage has the potential to be used as an alternative cell source for external ear reconstruction in future clinical application.

Scaffolds and tissue regeneration: An overview of the functional properties of selected organic tissues

Tissue engineering plays a significant role both in the re-establishment of functions and regeneration of organic tissues. Success in manufacturing projects for biological scaffolds, for the purpose of tissue regeneration, is conditioned by the selection of parameters such as the biomaterial, the device architecture, and the specificities of the cells making up the organic tissue to create, in vivo, a microenvironment that preserves and further enhances the proliferation of a specific cell phenotype. To support this approach, we have screened scientific publications that show biomedical applications of scaffolds, biomechanical, morphological, biochemical, and hemodynamic characteristics of the target organic tissues, and the possible interactions between different cell matrices and biological scaffolds. This review article provides an overview on the biomedical application of scaffolds and on the characteristics of the (bio)materials commonly used for manufacturing these biological devices used in tissue engineering, taking into consideration the cellular specificity of the target tissue. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1483-1494, 2016.

The potential of 3-dimensional construct engineered from poly(lactic-co-glycolic acid)/fibrin hybrid scaffold seeded with bone marrow mesenchymal stem cells for in vitro cartilage tissue engineering

Articular cartilage is well known for its simple uniqueness of avascular and aneural structure that has limited capacity to heal itself when injured. The use of three dimensional construct in tissue engineering holds great potential in regenerating cartilage defects. This study evaluated the in vitro cartilaginous tissue formation using rabbit's bone marrow mesenchymal stem cells (BMSCs)-seeded onto poly(lactic-co-glycolic acid) PLGA/fibrin and PLGA scaffolds. The in vitro cartilaginous engineered constructs were evaluated by gross inspection, histology, cell proliferation, gene expression and sulphated glycosaminoglycan (sGAG) production at week 1, 2 and 3. After 3 weeks of culture, the PLGA/fibrin construct demonstrated gross features similar to the native tissue with smooth, firm and glistening appearance, superior histoarchitectural and better cartilaginous extracellular matrix compound in concert with the positive glycosaminoglycan accumulation on Alcian blue. Significantly higher cell proliferation in PLGA/fibrin construct was noted at day-7, day-14 and day-21 (p<0.05 respectively). Both constructs expressed the accumulation of collagen type II, collagen type IX, aggrecan and sox9, showed down-regulation of collagen type I as well as produced relative sGAG content with PLGA/fibrin construct exhibited better gene expression in all profiles and showed significantly higher relative sGAG content at each time point (p<0.05). This study suggested that with optimum in vitro manipulation, PLGA/fibrin when seeded with pluripotent non-committed BMSCs has the capability to differentiate into chondrogenic lineage and may serve as a prospective construct to be developed as functional tissue engineered cartilage.

Stem Cell-assisted Approaches for Cartilage Tissue Engineering

The regeneration of damaged articular cartilage remains challenging due to its poor intrinsic capacity for repair. Tissue engineering of articular cartilage is believed to overcome the current limitations of surgical treatment by offering functional regeneration in the defect region. Selection of proper cell sources and ECM-based scaffolds, and incorporation of growth factors or mechanical stimuli are of primary importance to successfully produce artificial cartilage for tissue repair. When designing materials for cartilage tissue engineering, biodegradability and biocompatibility are the key factors in selecting material candidates, for either synthetic or natural polymers. The unique environment of cartilage makes it suitable to use a hydrogel with high water content in the cross-linked or thermosensitive (injectable) form. Moreover, design of composite scaffolds from two polymers with complementary physicochemical and biological properties has been explored to provide residing chondrocytes with a combination of the merits that each component contributes.

Biomimetic fibrin–hyaluronan hydrogels for nucleus pulposus regeneration

Aim: To develop a biomimetic polymeric injectable hydrogel that can support nucleus pulposus (NP) regeneration. Materials & methods: Natural polymer-based hydrogels were synthesized using fibrinogen (FBG) and hyaluronic acid (HA), conjugated by a novel two-step procedure. Bovine NP cells were cultured in FBG–HA conjugate-based 3D beads in vitro and in a nucleotomized organ culture model. Results: FBG–HA conjugate-based hydrogels prepared with 235 KDa HA at a FBG/HA w/w ratio of 17:1 showed superior gel stability and mechanical properties and markedly increased glycosaminoglycan synthesis compared with a FBG/HA mixture-based hydrogels or fibrin gels. Gene-expression levels of NP markers were maintained in vitro. In organ culture, NP cells seeded in FBG–HA conjugate-based hydrogels showed better integration with native NP tissue compared with fibrin gels. Moreover, FBG–HA conjugate-based hydrogels restored compressive stiffness and disc height after nucleotomy under dynamic load. Conclusion: Specific FBG–HA conjugate-based hydrogels may be suitable as injectable materials for minimally invasive, biological NP regeneration.

Treatment of the degenerated intervertebral disc; closure, repair and regeneration of the annulus fibrosus

Degeneration of the intervertebral disc (IVD) and disc herniation are two causes of low back pain. The aetiology of these disorders is unknown, but tissue weakening, which primarily occurs due to inherited genetic factors, ageing, nutritional compromise and loading history, is the basic factor causing disc degeneration. Symptomatic disc herniation mainly causes radicular pain. Current treatments of intervertebral disc degeneration and low back pain are based on alleviating the symptoms and comprise administration of painkillers or surgical methods such as spinal fusion. None of these methods is completely successful. Current research focuses on regeneration of the IVD and particularly on regeneration of the nucleus pulposus. Less attention has been directed to the repair or regeneration of the annulus fibrosus, although this is the key to successful nucleus pulposus, and therewith IVD, repair. This review focuses on the importance of restoring the function of the annulus fibrosus, as well as on the repair, replacement or regeneration of the annulus fibrosus in combination with restoration of the function of the nucleus pulposus, to treat low back pain.

Challenges and strategies in the repair of ruptured annulus fibrosus

Lumbar discectomy is the surgical procedure most frequently performed for patients suffering from low back pain and sciatica. Disc herniation as a consequence of degenerative or traumatic processes is commonly encountered as the underlying cause for the painful condition. While discectomy provides favourable outcome in a majority of cases, there are conditions where unmet requirements exist in terms of treatment, such as large disc protrusions with minimal disc degeneration; in these cases, the high rate of recurrent disc herniation after discectomy is a prevalent problem. An effective biological annular repair could improve the surgical outcome in patients with contained disc herniations but otherwise minor degenerative changes. An attractive approach is a tissue-engineered implant that will enable/stimulate the repair of the ruptured annulus. The strategy is to develop three-dimensional scaffolds and activate them by seeding cells or by incorporating molecular signals that enable new matrix synthesis at the defect site, while the biomaterial provides immediate closure of the defect and maintains the mechanical properties of the disc. This review is structured into (1) introduction, (2) clinical problems, current treatment options and needs, (3) biomechanical demands, (4) cellular and extracellular components, (5) biomaterials for delivery, scaffolding and support, (6) pre-clinical models for evaluation of newly developed cell- and material-based therapies, and (7) conclusions. This article highlights that an interdisciplinary approach is necessary for successful development of new clinical methods for annulus fibrosus repair. This will benefit from a close collaboration between research groups with expertise in all areas addressed in this review.

Development of poly(lactide-co-glycolide) scaffold-impregnated small intestinal submucosa with pores that stimulate extracellular matrix production in disc regeneration

The pore size and microstructure of scaffolds influences cell attachment, migration, proliferation and ingrowth, but the optimal pore size of scaffolds for disc tissue formation is not clearly understood. We developed porous poly(lactide-co-glycolide) (PLGA) scaffolds with various pore sizes for nucleus pulposus (NP) cell cultures and examined the effects of pore size on cell ingrowth and extracellular matrix (ECM) synthesis. High cell density in the small pores of scaffolds promotes collagen synthesis and cell migration through interconnected pores. Scaffolds with large pores exhibited slower cell proliferation and collagen synthesis. Guided by these results, we investigated a novel, biodegradable, synthetic/natural hybrid scaffold composed of PLGA and small intestinal submucosa (SIS) (PLGA-SIS) with the proper pore size for NP regeneration. We tested the morphological and physical properties of PLGA-SIS scaffolds and initial cell attachment and ECM production of NP in scaffolds. The mechanical and degradable properties of the PLGA-SIS scaffold were superior to those of SIS sponge and were similar to the properties of PLGA scaffold. NP cells grown on PLGA-SIS scaffold exhibited higher initial cell adhesion and ECM production than those grown on pure PLGA scaffold in a biological assay. In conclusion, this study suggests that a proper pore size of scaffolds is critical in NP regeneration, and that PLGA-SIS scaffolds with suitable pores might be useful as substrates for tissue-engineered biodiscs.

Fabrication of a Layered Microstructured Polymeric Microspheres as a Cell Carrier for Nucleus Pulposus Regeneration

This study aimed to investigate the feasibility of nanostructured 3D poly(lactide-co-glycolide) (PLGA) constructs, which are loaded with dexamethasone (DEX) and growth factor embedded hepaiin/poly(L-lysine) nanoparticles by a layer-by-layer system, to serve as an effective scaffold for nucleus pulposus (NP) tissue engineering. Our results demonstrated that the microsphere constructs were capable of simultaneously releasing basic fibroblast growth factor and DEX with approximately zero-order kinetics. The dual bead microspheres showed no cytotoxicity, and promoted the proliferation of the rat mesenchymal stem cells (rMSCs) by lactate dehydrogenase assay and CCK-8 assay. After 4 weeks of culture in vitro, the rMSCs- scaffold hybrids contained significantly higher levels of sulfated GAG/DNA and type-II collagen than the control samples. Moreover, quantity real-time PCR analysis revealed that the expression of disc-matrix proteins, including type-II collagen, aggrecan and versican, in the rMSCs-scaffold hybrids was significantly higher than the control group, whereas the expression of osteogenic differentiation marker type-I collagen was decreased. Taken together, these data indicate that the heparin bound bFGF-coated and DEX-loaded PLGA microsphere constructs is an effective bioactive scaffold for the regeneration of NP tissue.

Demineralized bone particle impregnated poly(l-lactide-co-glycolide) scaffold for application in tissue-engineered intervertebral discs

Demineralized bone particle (DBP) contains powerful bioactive molecules that facilitate new bone or cartilage growth. We developed hybrid scaffolds of poly(l-lactide-co-glycolide) (PLGA) with various concentrations of DBP (DBP/PLGA), of which phenotypes on intervertebral disc (IVD) cells were investigated. The hybrid scaffold has a cylindrical donut shape with two distinct parts; the inner is for the nucleus pulposus (NP) and the outer is for annulus fibrosus (AF). Rabbit NP and AF cells were seeded into the inner and outer regions of the DBP/PLGA scaffolds separately. Disc cell viability in DBP/PLGA scaffolds was superior to pure PLGA scaffold and increased with increasing DBP concentration. In vitro- and in vivo-formed tissues were characterized by RT-PCR, Safranin-O, Masson's trichrome staining and immunohistochemi- cal staining for type-I and type-II collagen. DBP/PLGA hybrid scaffolds revealed more active expression of disc phenotypes, as characterized by protein and mRNA expression, than the PLGA control. This study provides valuable information for potential disc replacement using DBP and PLGA.

Intervertebral disk tissue engineering using biphasic silk composite scaffolds

Scaffolds composed of synthetic, natural, and hybrid materials have been investigated as options to restore intervertebral disk (IVD) tissue function. These systems fall short of the lamellar features of the native annulus fibrosus (AF) tissue or focus only on the nucleus pulposus (NP) tissue. However, successful regeneration of the entire IVD requires a combination approach to restore functions of both the AF and NP. To address this need, a biphasic biomaterial structure was generated by using silk protein for the AF and fibrin/hyaluronic acid (HA) gels for the NP. Two cell types, porcine AF cells and chondrocytes, were utilized. For the AF tissue, two types of scaffold morphologies, lamellar and porous, were studied with the porous system serving as a control. Toroidal scaffolds formed out of the lamellar, and porous silk materials were used to generate structures with an outer diameter of 8 mm, inner diameter of 3.5 mm, and a height of 3 mm (the interlamellar distance in the lamellar scaffold was 150-250 μm, and the average pore sizes in the porous scaffolds were 100-250 μm). The scaffolds were seeded with porcine AF cells to form AF tissue, whereas porcine chondrocytes were encapsulated in fibrin/HA hydrogels for the NP tissue and embedded in the center of the toroidal disk. Histology, biochemical assays, and gene expression indicated that the lamellar scaffolds supported AF-like tissue over 2 weeks. Porcine chondrocytes formed the NP phenotype within the hydrogel after 4 weeks of culture with the AF tissue that had been previously cultured for 2 weeks, for a total of 6 weeks of cultivation. This biphasic scaffold simulating in combination of both AF and NP tissues was effective in the formation of the total IVD in vitro.

Three-dimensional culture of rabbit nucleus pulposus cells in collagen microspheres

BACKGROUND

Degenerative disc disease poses an increasing threat to our quality of life as we age. Existing treatments have limitations. New treatment modalities focusing on biologic rather than surgical approach would be appealing.

PURPOSE

Culturing intervertebral disc cells in a three-dimensional (3D) model that can retain cellular characteristics and phenotype is a critical step toward understanding how the disc cells respond to and interact with extrinsic signals before better therapeutics can be derived.

STUDY DESIGN

In this work, we studied the culture of rabbit nucleus pulposus (NP) cells in a collagen microsphere system and compared their cell morphology and expression of a few potential phenotypic markers with that in monolayer culture.

METHODS

Specifically, rabbit NP cells isolated from both young and old animals were encapsulated and cultured in collagen microspheres with different monomeric concentrations and with different cell encapsulation density for different period of time. Evaluation on the growth kinetics, the viability, the cell morphology, the expression of Types I and II collagen, glycosaminoglycans (GAGs), and Keratin 19, and the ultrastructure of the fiber meshwork were conducted to compare the microsphere 3D culture system and the traditional monolayer cultures.

RESULTS

Nucleus pulposus cells in two-dimensional culture lost the phenotypic expression of Type II collagen and keratin 19 and expressed Type I collagen. In contrast, the 3D collagen microsphere culture system consistently outperformed the traditional monolayer culture in maintaining a round morphology and preserving the phenotypes of NP cells with persistent expression of Type II collagen and Keratin 19. These cells also remodeled the template collagen matrix in the microspheres by depositing new matrices, including collagen Type II and GAGs in a cell seeding density and collagen concentration dependent manner.

CONCLUSIONS

This study demonstrates the appeal of the 3D collagen microsphere system for NP cell culture over traditional monolayer culture because it preserves the phenotypic characteristics of NP cells. This system also enables the NP cells to remodel the template collagen matrix by depositing new matrices, suggesting an innovative way to reconstitute cell-specific and native tissue-like environment in vitro for future studies on stem cell matrix niche and interactions of NP cell with extrinsic factors.

Silk-fibrin/hyaluronic acid composite gels for nucleus pulposus tissue regeneration

Scaffold designs are critical for in vitro culture of tissue-engineered cartilage in three-dimensional environments to enhance cellular differentiation for tissue engineering and regenerative medicine. In the present study we demonstrated silk and fibrin/hyaluronic acid (HA) composite gels as scaffolds for nucleus pulposus (NP) cartilage formation, providing both biochemical support for NP outcomes as well as fostering the retention of size of the scaffold during culture due to the combined features of the two proteins. Passage two (P2) human chondrocytes cultured in 10% serum were encapsulated within silk-fibrin/HA gels. Five study groups with fibrin/HA gel culture (F/H) along with varying silk concentrations (2% silk gel only, fibrin/HA gel culture with 1% silk [F/H+1S], 1.5% silk [F/H+1.5S], and 2% silk [F/H+2S]) were cultured in serum-free chondrogenic defined media (CDM) for 4 weeks. Histological examination with alcian blue showed a defined chondrogenic area at 1 week in all groups that widened homogenously until 4 weeks. In particular, chondrogenic differentiation observed in the F/H+1.5S had no reduction in size throughout the culture period. The results of biochemical and molecular biological evaluations supported observations made during histological examination. Mechanical strength measurements showed that the silk mixed gels provided stronger mechanical properties for NP tissue than fibrin/HA composite gels in CDM. This effect could potentially be useful in the study of in vitro NP tissue engineering as well as for clinical implications for NP tissue regeneration.

Biological and biomechanical effects of fibrin injection into porcine intervertebral discs

STUDY DESIGN

Surgically denucleated porcine intervertebral discs (IVD) were injected with BIOSTAT BIOLOGX Fibrin Sealant (FS), and the in vivo effects were assessed over time by histological, biochemical, and mechanical criteria.

OBJECTIVE

The objectives were to test whether the intradiscal injection of FS stimulates disc healing.

SUMMARY OF BACKGROUND DATA

Disc avascularity prevents the deposition of a provisional fibrin scaffold that typically facilitates soft tissue repair. Poor disc wound healing leads to disc damage accumulation and chronic inflammation characterized by overproduction of proinflammatory cytokines and proteolytic enzymes.

METHODS

Four lumbar IVDs from each of 31 Yucatan minipigs were randomized to untreated controls; degenerative injury (nucleotomy); and nucleotomy plus FS injection. Animals were killed at 1, 2, 3, 6, and 12 weeks postsurgery. IVDs were harvested to quantify (1) architecture using morphological and histological grading; (2) proteoglycan composition using DMMB assay; (3) cytokine content using ELISA; and (4) mechanical properties using quantitative pressure/volume testing.

RESULTS

There was progressive invasion of annular tissue into the nucleus of nucleotomy discs and concomitant reduction in proteoglycan content. By contrast, FS supplementation inhibited nuclear fibrosis and facilitated proteoglycan content recovery over time. FS discs synthesized significantly less TNF-α than degenerate discs (66% vs. 226%, P < 0.05) and had upregulation of IL-4 (310% vs. 166%) and TGF-β (400% vs. 117%) at 2 to 3 weeks posttreatment. At the third week postsurgery, the denucleated discs were less stiff than controls (pressure modulus 779.9 psi vs. 2754.8 psi; P < 0.05) and failed at lower pressures (250.5 psi vs. 492.5 psi; P < 0.05). The stiffness and leakage pressure of the FS-treated discs recovered to control values after 6 and 12 weeks, respectively.

CONCLUSION

FS facilitated structural, compositional, and mechanical repair of the surgically damaged IVD. These FS-derived benefits are likely due to its conductive scaffold properties and metabolically active constituents such as thrombin, factor XIII, and aprotinin acetate.

Expansion of human articular chondrocytes and formation of tissue-engineered cartilage: a step towards exploring a potential use of matrix-induced cell therapy

Monolayer culture expansion remains as a fundamental step to acquire sufficient number of cells for 3D constructs formation. It has been well-documented that cell expansion is however accompanied by cellular dedifferentiation. In order to promote cell growth and circumvent cellular dedifferentiation, we evaluated the effects of Transforming Growth Factor Beta-2 (TGF-β2), Insulin-like Growth Factor-I (IGF-I) and basic Fibroblast Growth Factor (bFGF) combination on articular chondrocytes culture and 'chondrocytes-fibrin' construct formation. Chondrocytes were serially cultured in: (1) F12:DMEM+10% Foetal Bovine Serum (FBS) with growth factors (FD10GFs), (2) F12:DMEM+2%FBS with the growth factors (FD2GFs) and, (3) F12:DMEM+10%FBS without growth factors (FD) as control. Cultured chondrocytes were evaluated by means of growth kinetics parameters, cell cycle analysis, quantitative phenotypic expression of collagen type II, aggrecan core protein sox-9 and collagen type I and, immunochemistry technique. Harvested chondrocytes were incorporated with plasma-derived fibrin and were polymerized to form the 3D constructs and implanted subcutaneously at the dorsum of athymic nude mice for eight (8) weeks. Resulted constructs were assigned for gross inspections and microscopic evaluation using standard histochemicals staining, immunochemistry technique and, quantitative phenotypic expression of cartilage markers to reassure cartilaginous tissue formation. Growth kinetics performance of chondrocytes cultured in three (3) types of culture media from the most to least was in the following order: FD10GFs>FD2GFs>FD. Following growth kinetics analysis, we decided to use FD10GFs and FD (control) for further evaluation and 'chondrocytes-fibrin' constructs formation. Chondrocytes cultured in FD10GFs preserved the normal diploid state (2c) with no evidence of aneuploidy, haploidy or tetraploidy. Expression of cartilage-specific markers namely collagen type II, aggrecan core protein and sox-9 were significantly higher in FD10GFs when compared to control. After implantation, 'chondrocytes-fibrin' constructs exhibited firm, white, smooth and glistening cartilage-like properties. FD10GFs constructs formed better quality cartilage-like tissue than FD constructs in term of overall cartilaginous tissue formation, cells organization and extracellular matrix distribution in the specimens. Cartilaginous tissue formation was confirmed by the presence of lacunae and cartilage-isolated cells embedded within basophilic ground substance. Presence of proteoglycan was confirmed by positive Safranin O staining. Collagen type II exhibited immunopositivity at the pericellular and inter-territorial matrix area. Chondrogenic properties of the construct were further confirmed by the expression of genes encoding collagen type II, aggrecan core protein and sox9. In conclusion, FD10GFs promotes the proliferation of chondrocytes and formation of good quality 'chondrocytes-fibrin' constructs which may have potential use of matrix-induced cell implantation.

Chondrogenesis of human mesenchymal stem cells in fibrin constructs evaluated in vitro and in nude mouse and rabbit defects models

In this study, hMSCs encapsulated in a fibrin hydrogel containing heparinized NPs loaded with TGF-β3 (100 ng/ml), or TGF-β3 (100 ng/ml) alone, were subjected to growth factor release and denaturation tests at one, two and four weeks in in vitro culture systems. Additionally, stem cell differentiation was assessed via RT-PCR, real-time quantitative PCR (qPCR), histology, and immunohistochemical assays. In the in vivo studies with nude mouse, when transplanted into nude mice, hMSCs embedded in fibrin hydrogels survived and proliferated more readily in those samples containing TGF-β3-loaded NPs, or TGF-β3 alone, compared to those containing only NPs or the fibrin hydrogel alone. Additionally, RT-PCR, real-time qPCR, histology, Western blotting, and immunohistochemistry analyses revealed that chondrocyte-specific extracellular matrix (ECM) genes and their proteins were expressed at high levels by hMSCs embedded in hydrogels containing TGF-β3-loaded NPs. Finally, the results observed in the rabbit animal model treated with hMSCs embedded in a fibrin hydrogel containing TGF-β3-loaded NPs were also evaluated by the RT-PCR, real-time qPCR, histology, Western blotting, and immunohistochemistry analyses. The in vitro and in vivo results indicated that transplanted hMSCs together with TGF-β3 may constitute a clinically efficient method for the regeneration of hyaline articular cartilage.