Tissue engineering of annulus fibrosus using electrospun fibrous scaffolds with aligned polycaprolactone fibers

Development and Evaluation of Biodegradable Core-Shell Microfibrous and Nanofibrous Scaffolds for Tissue Engineering Applications

Tissue engineering scaffolds as three-dimensional substrates may serve as ideal templates for tissue regeneration by simulating the structure of the extracellular matrix (ECM). Many biodegradable synthetic polymers, either hydrophobic, like Poly-ε-caprolactone (PCL), or hydrophilic, like Poly(Vinyl Alcohol) (PVA), are widely used as candidate bioactive materials for fabricating tissue engineering scaffolds. However, a combination of good cytocompatibility of hydrophilic polymers with good biomechanical performance of hydrophobic polymers could be beneficial for the in vivo performance of the scaffolds. In this study, we aimed to fabricate biodegradable fibrous scaffolds by combining the properties of hydrophobic PCL with those of hydrophilic PVA and evaluate their properties in comparison with pristine PCL scaffolds. Therefore, single-layered PCL scaffolds, sequential tri-layered (PVA/PCL/PVA), and core-shell (PVA as shell and PCL as core) composite scaffolds were developed utilizing the electrospinning technique. The material structural and biomechanical properties of the electrospun scaffolds, before and after their hydrolytic degradation over a seven-month period following storage in phosphate-buffered saline (PBS) at 37 °C, were comprehensively compared. In addition, human embryonic kidney cells (HEK-293) were cultured on the scaffolds to investigate potential cell attachment, infiltration, and proliferation. The results demonstrated the long-term efficacy of core-shell biodegradable fibrous scaffolds in comparison to single-layers PCL and tri-layers PVA/PCL/PVA, not only due to its superior morphological characteristics and mechanical properties, but also due to its ability to promote homogeneous cell distribution and proliferation, without any external chemical or physical stimuli.

Porous biomaterial scaffolds for skeletal muscle tissue engineering

Volumetric muscle loss is a traumatic injury which overwhelms the innate repair mechanisms of skeletal muscle and results in significant loss of muscle functionality. Tissue engineering seeks to regenerate these injuries through implantation of biomaterial scaffolds to encourage endogenous tissue formation and to restore mechanical function. Many types of scaffolds are currently being researched for this purpose. Scaffolds are typically made from either natural, synthetic, or conductive polymers, or any combination therein. A major criterion for the use of scaffolds for skeletal muscle is their porosity, which is essential for myoblast infiltration and myofiber ingrowth. In this review, we summarize the various methods of fabricating porous biomaterial scaffolds for skeletal muscle regeneration, as well as the various types of materials used to make these scaffolds. We provide guidelines for the fabrication of scaffolds based on functional requirements of skeletal muscle tissue, and discuss the general state of the field for skeletal muscle tissue engineering.

Scaffold Fabrication Techniques of Biomaterials for Bone Tissue Engineering: A Critical Review

Bone tissue engineering (BTE) is a promising alternative to repair bone defects using biomaterial scaffolds, cells, and growth factors to attain satisfactory outcomes. This review targets the fabrication of bone scaffolds, such as the conventional and electrohydrodynamic techniques, for the treatment of bone defects as an alternative to autograft, allograft, and xenograft sources. Additionally, the modern approaches to fabricating bone constructs by additive manufacturing, injection molding, microsphere-based sintering, and 4D printing techniques, providing a favorable environment for bone regeneration, function, and viability, are thoroughly discussed. The polymers used, fabrication methods, advantages, and limitations in bone tissue engineering application are also emphasized. This review also provides a future outlook regarding the potential of BTE as well as its possibilities in clinical trials.

Biomechanical evaluation of a novel repair strategy for intervertebral disc herniation in an ovine lumbar spine model

Following herniation of the intervertebral disc, there is a need for advanced surgical strategies to protect the diseased tissue from further herniation and to minimize further degeneration. Accordingly, a novel tissue engineered implant for annulus fibrosus (AF) repair was fabricated via three-dimensional fiber deposition and evaluated in a large animal model. Specifically, lumbar spine kinetics were assessed for eight (n = 8) cadaveric ovine lumbar spines in three pure moment loading settings (flexion-extension, lateral bending, and axial rotation) and three clinical conditions (intact, with a defect in the AF, and with the defect treated using the AF repair implant). In ex vivo testing, seven of the fifteen evaluated biomechanical measures were significantly altered by the defect. In each of these cases, the treated spine more closely approximated the intact biomechanics and four of these cases were also significantly different to the defect. The same spinal kinetics were also assessed in a preliminary in vivo study of three (n = 3) ovine lumbar spines 12 weeks post-implantation. Similar to the ex vivo results, functional efficacy of the treatment was demonstrated as compared to the defect model at 12 weeks post-implantation. These promising results motivate a future large animal study cohort which will establish statistical power of these results further elucidate the observed outcomes, and provide a platform for clinical translation of this novel AF repair patch strategy. Ultimately, the developed approach to AF repair holds the potential to maintain the long-term biomechanical function of the spine and prevent symptomatic re-herniation.

A Biodegradable Polymeric Matrix for the Repair of Annulus Fibrosus Defects in Intervertebral Discs

BACKGROUND

Tissue defects in the annulus fibrosus (AF) due to intervertebral disc (IVD) degeneration or after nucleodiscectomy have little self-healing capacity. To prevent progressive degeneration of the IVD, the AF must be repaired. Biological closure has not yet been achieved and is a challenge for the research community. In this study, a scaffold made of absorbable poly (glycolic acid) (PGA) and hyaluronan (HA) that exhibit excellent biocompatibility and cell colonization properties was used to repair AF defects in an ovine model.

METHODS

A partial resection was performed in AF in L3/4 or L4/5 of 10 sheep and PGA-HA scaffolds were implanted on the defects (n = 5), while defects in the control group were left untreated (n = 5). Three months post-operation, the lumbar discs were sectioned and stained with hematoxylin and eosin and safranin-O/fast-green. Histological features including proteoglycan content, annular structure, cellular morphology, blood vessel ingrowth and tear/cleft formation were scored using a modified scoring scheme by 3 investigators and evaluated by a pathologist independently.

RESULTS

The treated AF exhibited significantly enhanced repair tissue structure with signs of proteoglycan formation compared to the untreated group. The median scores were 4.3 for the treated and 9.8 for the untreated group. Cystic degeneration, perivascular infiltration, inflammation and necrosis were only present in the untreated group. Blood vessel ingrowth and tear/cleft formation were increased, though not significant, in the untreated group while cell morphology was comparable in both groups.

CONCLUSION

PGA-HA scaffolds used for AF closure support repair tissue formation in an ovine lumbar disc defect model.

Biomaterials and Cell-Based Regenerative Therapies for Intervertebral Disc Degeneration with a Focus on Biological and Biomechanical Functional Repair: Targeting Treatments for Disc Herniation

Intervertebral disc (IVD) degeneration is a common cause of low back pain and most spinal disorders. As IVD degeneration is a major obstacle to the healthy life of so many individuals, it is a major issue that needs to be overcome. Currently, there is no clinical treatment for the regeneration of degenerated IVDs. However, recent advances in regenerative medicine and tissue engineering suggest the potential of cell-based and/or biomaterial-based IVD regeneration therapies. These treatments may be indicated for patients with IVDs in the intermediate degenerative stage, a point where the number of viable cells decreases, and the structural integrity of the disc begins to collapse. However, there are many biological, biomechanical, and clinical challenges that must be overcome before the clinical application of these IVD regeneration therapies can be realized. This review summarizes the basic research and clinical trials literature on cell-based and biomaterial-based IVD regenerative therapies and outlines the important role of these strategies in regenerative treatment for IVD degenerative diseases, especially disc herniation.

Recent Progress in Electrospun Nanofiber-Based Degenerated Intervertebral Disc Repair

Annulus fibrosus fissure and fibrosis of nucleus pulposus are severe morphological characteristics of intervertebral disc degeneration. Currently, surgery or drugs are used to relieve pain in such cases. Tissue engineering is a new multidisciplinary strategy with great potential for use in joint replacement and organ regeneration. Based on the natural anatomy of intervertebral discs, intervertebral disc scaffolds are fabricated by exploiting the special arrangement of extracellular matrix fibers. Electrospun nanofibers possess clear advantages in repairing degenerated intervertebral discs. This article reviews and summarizes recently developed methods for improving and fabricating electrospun nanofiber annulus fibrosus scaffolds in terms of nanofiber alignment, material selection, loading additives, and the progress made in combining other advanced technologies with electrospun nanofibers. In addition, the improvement in mechanical properties and biocompatibility of nucleus pulposus scaffolds by electrospun nanofiber-reinforced hydrogels is discussed. Finally, complete intervertebral disc scaffolds can be fabricated using the disc-like angle-ply structure and other emerging fabrication methods. Taken together, electrospun nanofiber intervertebral disc scaffolds are promising for clinical applications.

Generation of an in vitro model of the outer annulus fibrosus-cartilage interface

Current treatments for degenerative disc disease do not restore full biological functionality of the intervertebral disc (IVD). As a result, regenerative medicine approaches are being developed to generate a biological replacement that when implanted will restore form and function of the degenerated IVD. Tissue-engineered models to date have focused on the generation of nucleus pulposus and annulus fibrosus IVD components. However, these tissues need to be integrated with a cartilage endplate in order for successful implantation to occur. The purpose of this study was to generate an in vitro annulus fibrosus-cartilage interface model which would enable us to better understand the biological and biomechanical implications of such interfaces. It was hypothesized that in vitro-formed outer annulus fibrosus (OAF) and cartilage tissues would integrate in direct-contact coculture to yield an interface containing extracellular matrix with aspects resembling the native OAF-CEP interface. In vitro-formed tissues were generated using bovine OAF cell-seeded angle-ply, multi-lamellated polycarbonate urethane scaffolds and articular chondrocytes, which were then placed in direct-contact coculture. 2-week old OAF tissues integrated with 3-day old cartilage by 1 week of coculture. Immunohistochemical staining of 2-week interfaces showed that distributions of collagen type I, collagen type II, and aggrecan were similar to the native bovine interface. The apparent tensile strength of the in vitro interface increased significantly between 2 and 4 weeks of coculture. In summary, an annulus fibrosus-cartilage interface model can be formed in vitro which will facilitate the identification of conditions required to generate an entire tissue-engineered disc replacement suitable for clinical use.

Mechanical and Cytocompatibility Evaluation of UHMWPE/PCL/Bioglass® Fibrous Composite for Acetabular Labrum Implant

In this study, a fibrous composite was developed as synthetic graft for labral reconstruction treatment, comprised of ultra-high molecular weight polyethylene (UHMWPE) fabric, ultrafine fibre of polycaprolactone (PCL), and 45S5 Bioglass®. This experiment aimed to examine the mechanical performance and cytocompatibility of the composite. Electrospinning and a slurry dipping technique were applied for composite fabrication. To assess the mechanical performance of UHMWPE, tensile cyclic loading test was carried out. Meanwhile, cytocompatibility of the composite on fibroblastic cells was examined through a viability assay, as well as SEM images to observe cell attachment and proliferation. The mechanical test showed that the UHMWPE fabric had a mean displacement of 1.038 mm after 600 cycles, approximately 4.5 times greater resistance compared to that of natural labrum, based on data obtained from literature. A viability assay demonstrated the predominant occupation of live cells on the material surface, suggesting that the composite was able to provide a viable environment for cell growth. Meanwhile, SEM images exhibited cell adhesion and the formation of cell colonies on the material surface. These results indicated that the UHMWPE/PCL/Bioglass® composite could be a promising material for labrum implants.

In vitro and in vivo evaluation of an electrospun-aligned microfibrous implant for Annulus fibrosus repair

Annulus fibrosus (AF) impairment is associated with reherniation, discogenic pain, and disc degeneration after surgical partial discectomy. Due to a limited intrinsic healing capacity, defects in the AF persist over time and it is hence necessary to adopt an appropriate strategy to close and repair the damaged AF. In this study, a cell-free biodegradable scaffold made of polycaprolactone (PCL), electrospun, aligned microfibers exhibited high levels of cell colonization, alignment, and AF-like extracellular matrix deposition when evaluated in an explant culture model. The biomimetic multilayer fibrous scaffold was then assessed in an ovine model of AF impairment. After 4 weeks, no dislocation of the implants was detected, and only one sample out of six showed a partial delamination. Histological and immunohistochemical analyses revealed integration of the implant with the surrounding tissue as well as homogeneously aligned collagen fiber organization within each lamella compared to the disorganized and scarcer fibrous tissue in a randomly organized control fibrous scaffold. In conclusion, this biomimetic electrospun implant exhibited promising properties in terms of AF defect closure, with AF-like neotissue formation that fully integrated with the surrounding ovine tissue.

Modelling and Optimization of Polycaprolactone Ultrafine-Fibres Electrospinning Process Using Response Surface Methodology

Electrospun fibres have gained broad interest in biomedical applications, including tissue engineering scaffolds, due to their potential in mimicking extracellular matrix and producing structures favourable for cell and tissue growth. The development of scaffolds often involves multivariate production parameters and multiple output characteristics to define product quality. In this study on electrospinning of polycaprolactone (PCL), response surface methodology (RSM) was applied to investigate the determining parameters and find optimal settings to achieve the desired properties of fibrous scaffold for acetabular labrum implant. The results showed that solution concentration influenced fibre diameter, while elastic modulus was determined by solution concentration, flow rate, temperature, collector rotation speed, and interaction between concentration and temperature. Relationships between these variables and outputs were modelled, followed by an optimization procedure. Using the optimized setting (solution concentration of 10% w/v, flow rate of 4.5 mL/h, temperature of 45 °C, and collector rotation speed of 1500 RPM), a target elastic modulus of 25 MPa could be achieved at a minimum possible fibre diameter (1.39 ± 0.20 µm). This work demonstrated that multivariate factors of production parameters and multiple responses can be investigated, modelled, and optimized using RSM.

Surgical repair of annulus defect with biomimetic multilamellar nano/microfibrous scaffold in a porcine model

Annulus defect is associated with reherniation and disc degeneration after discectomy; currently there is no effective treatment that addresses this problem. The annulus is a hierarchical lamellar structure, where each lamella consists of aligned collagen fibres, which are parallel and tilted at 30° to the spinal axis. In this study, a biomimetic biodegradable scaffold consisting of multilamellar nano/microfibres, sharing nanotopography and microporosity similar to the native lamellar structure, was assessed in a porcine model, aided by sealing with fascia and medical glue and subsequent suture fixation. After 6- and 12-week observation, we found that this treatment restored nucleus volume and slowed down disc degeneration, as indicated by magnetic resonance imaging of T1/T2-weighted, T2-mapping, T1-ρ imaging. Histological analysis showed aligned collagen fibres organized in the scaffold and integrated with surrounding native annulus tissue. The autologous bone marrow concentrate-seeded scaffolds showed slightly earlier collagen fibre formation at 6 weeks. This novel treatment could efficiently close the annulus defect with newly formed, organized and integrated collagen fibres in a porcine model. Copyright © 2016 John Wiley & Sons, Ltd.

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.

Design Requirements for Annulus Fibrosus Repair: Review of Forces, Displacements, and Material Properties of the Intervertebral Disk and a Summary of Candidate Hydrogels for Repair

There is currently a lack of clinically available solutions to restore functionality to the intervertebral disk (IVD) following herniation injury to the annulus fibrosus (AF). Microdiscectomy is a commonly performed surgical procedure to alleviate pain caused by herniation; however, AF defects remain and can lead to accelerated degeneration and painful conditions. Currently available AF closure techniques do not restore mechanical functionality or promote tissue regeneration, and have risk of reherniation. This review determined quantitative design requirements for AF repair materials and summarized currently available hydrogels capable of meeting these design requirements by using a series of systematic PubMed database searches to yield 1500+ papers that were screened and analyzed for relevance to human lumbar in vivo measurements, motion segment behaviors, and tissue level properties. We propose a testing paradigm involving screening tests as well as more involved in situ and in vivo validation tests to efficiently identify promising biomaterials for AF repair. We suggest that successful materials must have high adhesion strength (∼0.2 MPa), match as many AF material properties as possible (e.g., approximately 1 MPa, 0. 3 MPa, and 30 MPa for compressive, shear, and tensile moduli, respectively), and have high tensile failure strain (∼65%) to advance to in situ and in vivo validation tests. While many biomaterials exist for AF repair, few undergo extensive mechanical characterization. A few hydrogels show promise for AF repair since they can match at least one material property of the AF while also adhering to AF tissue and are capable of easy implantation during surgical procedures to warrant additional optimization and validation.

Integration of 3D Printed and Micropatterned Polycaprolactone Scaffolds for Guidance of Oriented Collagenous Tissue Formation In Vivo

Scaffold design incorporating multiscale cues for clinically relevant, aligned tissue regeneration has potential to improve structural and functional integrity of multitissue interfaces. The objective of this preclinical study is to develop poly(ε-caprolactone) (PCL) scaffolds with mesoscale and microscale architectural cues specific to human ligament progenitor cells and assess their ability to form aligned bone-ligament-cementum complexes in vivo. PCL scaffolds are designed to integrate a 3D printed bone region with a micropatterned PCL thin film consisting of grooved pillars. The patterned film region is seeded with human ligament cells, fibroblasts transduced with bone morphogenetic protein-7 genes seeded within the bone region, and a tooth dentin segment positioned on the ligament region prior to subcutaneous implantation into a murine model. Results indicate increased tissue alignment in vivo using micropatterned PCL films, compared to random-porous PCL. At week 6, 30 μm groove depth significantly enhances oriented collagen fiber thickness, overall cell alignment, and nuclear elongation relative to 10 μm groove depth. This study demonstrates for the first time that scaffolds with combined hierarchical mesoscale and microscale features can align cells in vivo for oral tissue repair with potential for improving the regenerative response of other bone-ligament complexes.

Modulation of the gene expression of annulus fibrosus-derived stem cells using poly(ether carbonate urethane)urea scaffolds of tunable elasticity

Annulus fibrosus (AF) injuries commonly lead to substantial deterioration of the intervertebral disc (IVD). While tissue engineering has recently evolved into a promising approach for AF regeneration, it remains challenging due to the cellular, biochemical, and mechanical heterogeneity of AF tissue. In this study, we explored the use of AF-derived stem cells (AFSCs) to achieve diversified differentiation of cells for AF tissue engineering. Since the differentiation of stem cells relies significantly on the elasticity of the substrate, we synthesized a series of biodegradable poly(ether carbonate urethane)urea (PECUU) materials whose elasticity approximated that of native AF tissue. When AFSCs were cultured on electrospun PECUU fibrous scaffolds, the gene expression of collagen-I in the cells increased with the elasticity of scaffold material, whereas the expression of collagen-II and aggrecan genes showed an opposite trend. At the protein level, the content of collagen-I gradually increased with substrate elasticity, while collagen-II and GAG contents decreased. In addition, the cell traction forces (CTFs) of AFSCs gradually decreased with scaffold elasticity. Such substrate elasticity-dependent changes of AFSCs were similar to the gradual transition in the genetic, biochemical, and biomechanical characteristics of cells from inner to outer regions of native AF tissue. Together, findings from this study indicate that AFSCs, depending on the substrate elasticity, have strong tendencies to differentiate into various types of AF-like cells, thereby providing a solid foundation for the tissue engineering applications of AFSCs.

STATEMENT OF SIGNIFICANCE

Repairing the annulus fibrosus (AF) of intervertebral disc (IVD) is critical for the treatment of disc degeneration disease, but remains challenging due to the significant heterogeneity of AF tissue. Previously, we have identified rabbit AF-derived stem cells (AFSCs), which are AF tissue-specific and hold promise for AF regeneration. In this study, we synthesized a series of poly(ether carbonate urethane)ureas of various elasticity (or stiffness) and explored the potential of induced differentiation of AFSCs using electrospun PECUU scaffolds. This work has, for the first time, found that AFSCs are able to present different gene expression patterns simply as a result of the elasticity of scaffold material. Therefore, our findings will help supplement current knowledge of AF tissue regeneration and may benefit a diversified readership from scientific, engineering, and clinical settings whose work involves the biology and tissue engineering of IVD.

Polymeric scaffolds in tissue engineering: a literature review

The tissue engineering scaffold acts as an extracellular matrix that interacts to the cells prior to forming new tissues. The chemical and structural characteristics of scaffolds are major concerns in fabricating of ideal three-dimensional structure for tissue engineering applications. The polymer scaffolds used for tissue engineering should possess proper architecture and mechanical properties in addition to supporting cell adhesion, proliferation, and differentiation. Much research has been done on the topic of polymeric scaffold properties such as surface topographic features (roughness and hydrophilicity) and scaffold microstructures (pore size, porosity, pore interconnectivity, and pore and fiber architectures) that influence the cell-scaffold interactions. In this review, efforts were given to evaluate the effect of both chemical and structural characteristics of scaffolds on cell behaviors such as adhesion, proliferation, migration, and differentiation. This review would provide the fundamental information which would be beneficial for scaffold design in future. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 431-459, 2017.

3D-Printed ABS and PLA Scaffolds for Cartilage and Nucleus Pulposus Tissue Regeneration

Painful degeneration of soft tissues accounts for high socioeconomic costs. Tissue engineering aims to provide biomimetics recapitulating native tissues. Biocompatible thermoplastics for 3D printing can generate high-resolution structures resembling tissue extracellular matrix. Large-pore 3D-printed acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) scaffolds were compared for cell ingrowth, viability, and tissue generation. Primary articular chondrocytes and nucleus pulposus (NP) cells were cultured on ABS and PLA scaffolds for three weeks. Both cell types proliferated well, showed high viability, and produced ample amounts of proteoglycan and collagen type II on both scaffolds. NP generated more matrix than chondrocytes; however, no difference was observed between scaffold types. Mechanical testing revealed sustained scaffold stability. This study demonstrates that chondrocytes and NP cells can proliferate on both ABS and PLA scaffolds printed with a simplistic, inexpensive desktop 3D printer. Moreover, NP cells produced more proteoglycan than chondrocytes, irrespective of thermoplastic type, indicating that cells maintain individual phenotype over the three-week culture period. Future scaffold designs covering larger pore sizes and better mimicking native tissue structure combined with more flexible or resorbable materials may provide implantable constructs with the proper structure, function, and cellularity necessary for potential cartilage and disc tissue repair in vivo.

The effect of the fibre orientation of electrospun scaffolds on the matrix production of rabbit annulus fibrosus-derived stem cells

Annulus fibrosus (AF) tissue engineering has recently received increasing attention as a treatment for intervertebral disc (IVD) degeneration; however, such engineering remains challenging because of the remarkable complexity of AF tissue. In order to engineer a functional AF replacement, the fabrication of cell-scaffold constructs that mimic the cellular, biochemical and structural features of native AF tissue is critical. In this study, we fabricated aligned fibrous polyurethane scaffolds using an electrospinning technique and used them for culturing AF-derived stem/progenitor cells (AFSCs). Random fibrous scaffolds, also prepared via electrospinning, were used as a control. We compared the morphology, proliferation, gene expression and matrix production of AFSCs on aligned scaffolds and random scaffolds. There was no apparent difference in the attachment or proliferation of cells cultured on aligned scaffolds and random scaffolds. However, compared to cells on random scaffolds, the AFSCs on aligned scaffolds were more elongated and better aligned, and they exhibited higher gene expression and matrix production of collagen-I and aggrecan. The gene expression and protein production of collagen-II did not appear to differ between the two groups. Together, these findings indicate that aligned fibrous scaffolds may provide a favourable microenvironment for the differentiation of AFSCs into cells similar to outer AF cells, which predominantly produce collagen-I matrix.

Gene expression modulation in TGF-β3-mediated rabbit bone marrow stem cells using electrospun scaffolds of various stiffness

Tissue engineering has recently evolved into a promising approach for annulus fibrosus (AF) regeneration. However, selection of an ideal cell source, which can be readily differentiated into AF cells of various regions, remains challenging because of the heterogeneity of AF tissue. In this study, we set out to explore the feasibility of using transforming growth factor-β3-mediated bone marrow stem cells (tBMSCs) for AF tissue engineering. Since the differentiation of stem cells significantly relies on the stiffness of substrate, we fabricated nanofibrous scaffolds from a series of biodegradable poly(ether carbonate urethane)-urea (PECUU) materials whose elastic modulus approximated that of native AF tissue. We cultured tBMSCs on PECUU scaffolds and compared their gene expression profile to AF-derived stem cells (AFSCs), the newly identified AF tissue-specific stem cells. As predicted, the expression of collagen-I in both tBMSCs and AFSCs increased with scaffold stiffness, whereas the expression of collagen-II and aggrecan genes showed an opposite trend. Interestingly, the expression of collagen-I, collagen-II and aggrecan genes in tBMSCs on PECUU scaffolds were consistently higher than those in AFSCs regardless of scaffold stiffness. In addition, the cell traction forces (CTFs) of both tBMSCs and AFSCs gradually decreased with scaffold stiffness, which is similar to the CTF change of cells from inner to outer regions of native AF tissue. Together, findings from this study indicate that tBMSCs had strong tendency to differentiate into various types of AF cells and presented gene expression profiles similar to AFSCs, thereby establishing a rationale for the use of tBMSCs in AF tissue engineering.

Circumferentially oriented microfiber scaffold prepared by wet-spinning for tissue engineering of annulus fibrosus

Repairing damaged annulus fibrosus (AF) is one of the most challenging topics for treating intervertebral disc (IVD) disease.

Identification of rabbit annulus fibrosus-derived stem cells

Annulus fibrosus (AF) injuries can lead to substantial deterioration of intervertebral disc (IVD) which characterizes degenerative disc disease (DDD). However, treatments for AF repair/regeneration remain challenging due to the intrinsic heterogeneity of AF tissue at cellular, biochemical, and biomechanical levels. In this study, we isolated and characterized a sub-population of cells from rabbit AF tissue which formed colonies in vitro and could self-renew. These cells showed gene expression of typical surface antigen molecules characterizing mesenchymal stem cells (MSCs), including CD29, CD44, and CD166. Meanwhile, they did not express negative markers of MSCs such as CD4, CD8, and CD14. They also expressed Oct-4, nucleostemin, and SSEA-4 proteins. Upon induced differentiation they showed typical osteogenesis, chondrogenesis, and adipogenesis potential. Together, these AF-derived colony-forming cells possessed clonogenicity, self-renewal, and multi-potential differentiation capability, the three criteria characterizing MSCs. Such AF-derived stem cells may potentially be an ideal candidate for DDD treatments using cell therapies or tissue engineering approaches.

Regional variations in the cellular, biochemical, and biomechanical characteristics of rabbit annulus fibrosus

Tissue engineering of annulus fibrosus (AF), the essential load-bearing disc component, remains challenging due to the intrinsic heterogeneity of AF tissue. In order to provide a set of characterization data of AF tissue, which serve as the benchmark for constructing tissue engineered AF, we analyzed tissues and cells from various radial zones of AF, i.e., inner AF (iAF), middle AF (mAF), and outer AF (oAF), using a rabbit model. We found that a radial gradient in the cellular, biochemical, and biomechanical characteristics of rabbit AF existed. Specifically, the iAF cells (iAFCs) had the highest expression of collagen-II and aggrecan genes, while oAF cells (oAFCs) had the highest collagen-I gene expression. The contents of DNA, total collagen and collagen-I sequentially increased from iAF, mAF to oAF, while glycosaminoglycan (GAG) and collagen-II levels decreased. The cell traction forces of primary AFCs gradually decreased from iAFCs, mAFCs to oAFCs, being 336.6±155.3, 199.0±158.8, and 123.8±76.1 Pa, respectively. The storage moduli of iAF, mAF, and oAF were 0.032±0.002, 2.121±0.656, and 4.130±0.159 MPa, respectively. These measurements have established a set of reference data for functional evaluation of the efficacy of AF tissue engineering strategies using a convenient and cost-effective rabbit model, the findings of which may be further translated to human research.

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.

Biodegradable electrospun scaffolds for annulus fibrosus tissue engineering: effect of scaffold structure and composition on annulus fibrosus cells in vitro

Electrospinning technology is an attractive process for the fabrication of a scaffold with an annulus fibrosus (AF)-like architecture for tissue engineering. Oriented and nonoriented electrospun scaffolds were prepared from poly(ester-urethane) (PU) and poly(ɛ-caprolactone) (PCL) as well as corresponding homogeneous films. Scaffolds' characteristics and mechanical properties were characterized by scanning electron microscopy, static water contact measurements, and dynamic mechanical analysis, respectively. The effect of scaffold architecture and polymer composition on bovine AF cells was investigated. PU and PCL films and scaffolds supported AF cell growth and extracellular matrix production and accumulation. Electrospun scaffolds increased the retention of collagen and glycosaminoglycan compared with films. Fiber orientation of the scaffolds promoted the AF cell phenotype with a trend toward an upregulation of matrix gene expression for oriented relative to nonoriented scaffolds. The higher yield strain of an oriented electrospun PU scaffold, compared with other scaffolds, will be advantageous for AF tissue engineering under a dynamic mechanical environment.

The use of fiber-reinforced scaffolds cocultured with Schwann cells and vascular endothelial cells to repair rabbit sciatic nerve defect with vascularization

To explore the feasibility of biodegradable fiber-reinforced 3D scaffolds with satisfactory mechanical properties for the repair of long-distance sciatic nerve defect in rabbits and effects of vascularized graft in early stage on the recovery of neurological function, Schwann cells and vascular endothelial cells were cocultured in the fiber-reinforced 3D scaffolds. Experiment group which used prevascularized nerve complex for the repair of sciatic nerve defect and control group which only cultured with Schwann cells were set. The animals in both groups underwent electromyography to show the status of the neurological function recovery at 4, 8, and 16 weeks after the surgery. Sciatic nerve regeneration and myelination were observed under the light microscope and electron microscope. Myelin sheath thickness, axonal diameter, and number of myelinated nerve fiber were quantitatively analyzed using image analysis system. The recovery of foot ulcer, the velocity of nerve conduction, the number of regenerating nerve fiber, and the recovery of ultrastructure were increased in the experimental group than those in the control group. Prevascularized tissue engineered fiber-reinforced 3D scaffolds for the repair of sciatic nerve defects in rabbits can effectively promote the recovery of neurological function.

A chondroitinase-ABC and TGF-β1 treatment regimen for enhancing the mechanical properties of tissue-engineered fibrocartilage

The development of functionally equivalent fibrocartilage remains elusive despite efforts to engineer tissues such as knee meniscus, intervertebral disc and temporomandibular joint disc. Attempts to engineer these structures often fail to create tissues with mechanical properties on a par with native tissue, resulting in constructs unsuitable for clinical applications. The objective of this study was to engineer a spectrum of biomimetic fibrocartilages representative of the distinct functional properties found in native tissues. Using the self-assembly process, different co-cultures of meniscus cells and articular chondrocytes were seeded into agarose wells and treated with the catabolic agent chondroitinase-ABC (C-ABC) and the anabolic agent transforming growth factor-β1 (TGF-β1) via a two-factor (cell ratio and bioactive treatment), full factorial study design. Application of both C-ABC and TGF-β1 resulted in a beneficial or positive increase in the collagen content of treated constructs compared to controls. Significant increases in both the collagen density and fiber diameter were also seen with this treatment, increasing these values by 32 and 15%, respectively, over control values. Mechanical testing found the combined bioactive treatment to synergistically increase the Young's modulus and ultimate tensile strength of the engineered fibrocartilages compared to controls, with values reaching the lower spectrum of those found in native tissues. Together, these data demonstrate that C-ABC and TGF-β1 interact to develop a denser collagen matrix better able to withstand tensile loading. This study highlights a way to optimize the tensile properties of engineered fibrocartilage using a biochemical and a biophysical agent together to create distinct fibrocartilages with functional properties mimicking those of native tissue.

Oriented lamellar silk fibrous scaffolds to drive cartilage matrix orientation: towards annulus fibrosus tissue engineering

A novel design of silk-based scaffold is developed using a custom-made winding machine, with fiber alignment resembling the anatomical criss-cross lamellar fibrous orientation features of the annulus fibrosus of the intervertebral disc. Crosslinking of silk fibroin fibers with chondroitin sulphate (CS) was introduced to impart superior biological functionality. The scaffolds, with or without CS, instructed alignment of expanded human chondrocytes and of the deposited extracellular matrix while supporting their chondrogenic redifferentiation. The presence of CS crosslinking could not induce statistically significant changes in the measured collagen or glycosaminoglycan content, but resulted in an increased construct stiffness. By offering the combined effect of cell/matrix alignment and chondrogenic support, the silk fibroin scaffolds developed with precise fiber orientation in lamellar form represent a suitable substrate for tissue engineering of the annulus fibrosus part of the intervertebral disc.

Electrospun nanofibers in drug delivery: recent developments and perspectives

In this review article, some key challenges in drug delivery are first introduced and methods that have been applied in attempts to solve them enumerated. Particularly intractable problems are highlighted: these include issues of solubility, targeting and drug degradation. The technique of electrospinning is subsequently introduced, and the influence of processing parameters on the fibers produced discussed. The potential of electrospun nanofibers in drug delivery is then explored, with examples given from the recent literature to illustrate how fibers can be used to overcome hurdles in drug solubility, degradation and targeting. Future perspectives and challenges are also considered.

In vivo biofunctionality comparison of different topographic PLLA scaffolds

In this work, the in vivo biodegradation of, biocompatibility of, and host response to various topographic scaffolds were investigated. Randomly oriented fibrous poly(L-lactide) (PLLA) nanofibers were fabricated using the electrospinning technique. A PLLA scaffold was obtained by salt leaching. Both the electrospun PLLA nanofibers and the salt-leaching PLLA scaffolds formed three-dimensional pore structures. Cytotoxicity studies, in which rat muscle-derived stem cells (rMDSCs) were grown on electrospun PLLA nanofibers or the salt-leaching PLLA scaffolds, revealed that the rMDSCs cell count on the PLLA nanofibers was slightly higher than that on the salt-leaching PLLA scaffolds. An in vivo study was carried out by implanting the scaffolds subcutaneously into rats to test the biodegradation, biocompatibility, and host response at regular intervals over 0-4 weeks. The degradation of the PLLA nanofibers 1, 2, and 4 weeks after initial implantation was more extensive than that observed for the salt-leaching PLLA scaffolds. PLLA nanofibers seeded the growth of larger fibrous tissue masses due to in vivo cellular infiltration into the randomly oriented fibrillar structures of the PLLA nanofibers. In addition, the inflammatory cell accumulation in PLLA nanofibers was lower than that in the salt-leaching PLLA scaffolds. These results indicate that the electrospun PLLA nanofibers may serve as a good scaffold to elicit fibrous cellular infiltration, to minimize host response, and to enhance tissue-scaffold integration.