Bioengineering a multicomponent spinal motion segment construct--a 3D model for complex tissue engineering

Tailoring extracellular matrix niches: Impact of glycosaminoglycan content on multiple differentiation of human mesenchymal stem cells

Glycosaminoglycan (GAG) represents an important extracellular matrix (ECM), particularly in GAG-rich tissues such as nucleus pulposus and cartilage. The ratio of GAGs/hydroxyproline (HYP) is an indicator of the relative abundance of the space-filling GAG matrix to the fibrous collagen matrix in a particular tissue. Here, we hypothesize that ECM niche with different GAG/HYP ratios will affect the outcomes of multiple differentiation of human mesenchymal stem cells (hMSCs). Specifically, we fabricated collagen-based biomaterials with different GAG/HYP ratios, and differentiate hMSCs in these materials towards osteogenic, chondrogenic and discogenic lineages. In osteogenic differentiation, Collagen without GAG (GAG/HYP ratio 0) showed higher calcium (Ca) and phosphorus (P) deposition and Ca/P ratio, more biomimetic ultrastructure, and better osteogenic phenotypic expression. For chondrogenic differentiation, aminated collagen (aCol-GAG) with intermediate GAG content (GAG/HYP ratio 5.0:1) showed higher GAG deposition, more biomimetic ultrastructure, and better chondrogenic phenotype. In discogenic differentiation, aminated collagen-aminated hyaluronic acid (aHA)-GAG (aCol-aHA-GAG) with the highest GAG content (GAG/HYP ratio 19.8:1), showed intensive GAG deposition, biomimetic ultrastructure, and higher phenotypic marker expression. This study contributes to developing collagen-based biomimetic materials with different GAG/HYP ratios and suggests the use of tissue-specific GAG/HYP ratio as a scaffold design parameter for hMSCs-based musculoskeletal tissue engineering. (198 words).

Regenerative therapies for lumbar degenerative disc diseases: a literature review

This review aimed to summarize the recent advances and challenges in the field of regenerative therapies for lumbar disc degeneration. The current first-line treatment options for symptomatic lumbar disc degeneration cannot modify the disease process or restore the normal structure, composition, and biomechanical function of the degenerated discs. Cell-based therapies tailored to facilitate intervertebral disc (IVD) regeneration have been developed to restore the IVD extracellular matrix or mitigate inflammatory conditions. Human clinical trials on Mesenchymal Stem Cells (MSCs) have reported promising outcomes exhibited by MSCs in reducing pain and improving function. Nucleus pulposus (NP) cells possess unique regenerative capacities. Biomaterials aimed at NP replacement in IVD regeneration, comprising synthetic and biological materials, aim to restore disc height and segmental stability without compromising the annulus fibrosus. Similarly, composite IVD replacements that combine various biomaterial strategies to mimic the native disc structure, including organized annulus fibrosus and NP components, have shown promise. Furthermore, preclinical studies on regenerative medicine therapies that utilize cells, biomaterials, growth factors, platelet-rich plasma (PRP), and biological agents have demonstrated their promise in repairing degenerated lumbar discs. However, these therapies are associated with significant limitations and challenges that hinder their clinical translation. Thus, further studies must be conducted to address these challenges.

Hydrogel-Based Strategies for Intervertebral Disc Regeneration: Advances, Challenges and Clinical Prospects

Millions of people worldwide suffer from low back pain and disability associated with intervertebral disc (IVD) degeneration. IVD degeneration is highly correlated with aging, as the nucleus pulposus (NP) dehydrates and the annulus fibrosus (AF) fissures form, which often results in intervertebral disc herniation or disc space collapse and related clinical symptoms. Currently available options for treating intervertebral disc degeneration are symptoms control with therapy modalities, and/or medication, and/or surgical resection of the IVD with or without spinal fusion. As such, there is an urgent clinical demand for more effective disease-modifying treatments for this ubiquitous disorder, rather than the current paradigms focused only on symptom control. Hydrogels are unique biomaterials that have a variety of distinctive qualities, including (but not limited to) biocompatibility, highly adjustable mechanical characteristics, and most importantly, the capacity to absorb and retain water in a manner like that of native human nucleus pulposus tissue. In recent years, various hydrogels have been investigated in vitro and in vivo for the repair of intervertebral discs, some of which are ready for clinical testing. In this review, we summarize the latest findings and developments in the application of hydrogel technology for the repair and regeneration of intervertebral discs.

Textile Design of an Intervertebral Disc Replacement Device from Silk Yarn

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

A bio-inspired nano-material recapitulating the composition, ultra-structure, and function of the glycosaminoglycan-rich extracellular matrix of nucleus pulposus

The nucleus pulposus (NP) of intervertebral disc represents a soft gel consisting of glycosaminoglycans (GAGs)-rich extracellular matrix (ECM). Significant loss of GAGs and normal functions are the most prevalent changes in degenerated disc. Attempts targeted to incorporate GAGs into collagen fibrous matrices have been made but the efficiency is very low, and the resulting structures showed no similarity with native NP. Inspired by the characteristic composition and structures of the ECM of native NP, here, we hypothesize that by chemically modifying the collagen (Col) and hyaluronic acid (HA) and co-precipitating with GAGs, a bio-inspired nano-material recapitulating the composition, ultra-structure and function of the GAG-rich ECM will be fabricated. Compositionally, the bio-inspired nano-material namely Aminated Collagen-Aminated Hyaluronic Acid-GAG (aCol-aHA-GAG) shows a record high GAG/hydroxyproline ratio up to 39.1:1 in a controllable manner, out-performing that of the native NP. Ultra-structurally, the nano-material recapitulates the characteristic 'nano-beads' (25 nm) and 'bottle-brushes' (133 nm) features as those found in native NP. Functionally, the nano-material supports the viability and maintains the morphological and phenotypic markers of bovine NP cells, and shows comparable mechanical properties of native NP. This work contributes to the development of a compositionally, structurally, and functionally biomimetic nano-material for NP tissue engineering.

Importance of Matrix Cues on Intervertebral Disc Development, Degeneration, and Regeneration

Back pain is one of the leading causes of disability worldwide and is frequently caused by degeneration of the intervertebral discs. The discs’ development, homeostasis, and degeneration are driven by a complex series of biochemical and physical extracellular matrix cues produced by and transmitted to native cells. Thus, understanding the roles of different cues is essential for designing effective cellular and regenerative therapies. Omics technologies have helped identify many new matrix cues; however, comparatively few matrix molecules have thus far been incorporated into tissue engineered models. These include collagen type I and type II, laminins, glycosaminoglycans, and their biomimetic analogues. Modern biofabrication techniques, such as 3D bioprinting, are also enabling the spatial patterning of matrix molecules and growth factors to direct regional effects. These techniques should now be applied to biochemically, physically, and structurally relevant disc models incorporating disc and stem cells to investigate the drivers of healthy cell phenotype and differentiation. Such research will inform the development of efficacious regenerative therapies and improved clinical outcomes.

Innovative Biological Treatment Methods for Degenerative Disc Disease

Low back pain is the leading cause of work absences and years lived with disability, and it is often associated with degenerative disc disease. In recent years, biological treatment approaches such as the use of growth factors, cell injections, annulus fibrosus (AF) repair, nucleus pulposus replacement, and tissue-engineered discs have been explored as means for preventing or reversing degenerative disc disease. Both animal and clinical studies have shown promising results for cell-based therapy on the grounds of its regenerative potential. Clinical data also indicate that stem cell injection is safe when appropriately performed, albeit its long-term safety and efficacy are yet to be explored. Numerous challenges also remain to be overcome, such as isolating, differentiating, and preconditioning the disc cells, as well as managing the nutrient-deficient and oxygen-deficient micromilieu of the intervertebral disc (IVD). AF repair methods including devices used in clinical trials have shown success in decreasing reherniation rates and improving overall clinical outcomes. In addition, recent studies that combined AF repair and nucleus pulposus replacement have shown improved biomechanical stability in IVDs after the combined treatment. Tissue-engineered IVDs for total disc replacement are still being developed, and future studies are necessary to overcome the challenges in their delivery, efficacy, and safety.

Architecture-Promoted Biomechanical Performance-Tuning of Tissue-Engineered Constructs for Biological Intervertebral Disc Replacement

Background: Biological approaches to intervertebral disc (IVD) restoration and/or regeneration have become of increasing interest. However, the IVD comprises a viscoelastic system whose biological replacement remains challenging. The present study sought to design load-sharing two-component model systems of circular, nested, concentric elements reflecting the nucleus pulposus and annulus fibrosus. Specifically, we wanted to investigate the effect of architectural design variations on (1) model system failure loads when testing the individual materials either separately or homogeneously mixed, and (2) also evaluate the potential of modulating other mechanical properties of the model systems. Methods: Two sets of softer and harder biomaterials, 0.5% and 5% agarose vs. 0.5% agarose and gelatin, were used for fabrication. Architectural design variations were realized by varying ring geometries and amounts while keeping the material composition across designs comparable. Results: Variations in the architectural design, such as lamellar width, number, and order, combined with choosing specific biomaterial properties, strongly influenced the biomechanical performance of IVD constructs. Biomechanical characterization revealed that the single most important parameter, in which the model systems vastly exceeded those of the individual materials, was failure load. The model system failure loads were 32.21- and 84.11-fold higher than those of the agarose materials and 55.03- and 2.14-fold higher than those of the agarose and gelatin materials used for system fabrication. The compressive strength, dynamic stiffness, and viscoelasticity of the model systems were always in the range of the individual materials. Conclusions: Relevant architecture-promoted biomechanical performance-tuning of tissue-engineered constructs for biological IVD replacement can be realized by slight modifications in the design of constructs while preserving the materials’ compositions. Minimal variations in the architectural design can be used to precisely control structure–function relations for IVD constructs rather than choosing different materials. These fundamental findings have important implications for efficient tissue-engineering of IVDs and other load-bearing tissues, as potential implants need to withstand high in situ loads.

Rac1-GTPase regulates compression-induced actin protrusions (CAPs) of mesenchymal stem cells in 3D collagen micro-tissues

Cells can sense mechanical signals through cytoskeleton reorganization. We previously discovered the formation of omni-directional actin protrusions upon compression loading, namely compression-induced actin protrusions (CAPs), in human mesenchymal stem cells (MSCs) in 3D micro-tissues. Here, the regulatory roles of three RhoGTPases (CDC42, Rac1 and RhoA) in the formation of CAPs were investigated. Upon compression loading, extensive formation of CAPs was found, significantly associated with an upregulated mRNA expression of Rac1 only, but not CDC42, nor RhoA. Upon chemical inhibition of these RhoGTPase activity during compression, only Rac1 activity was significantly suppressed, associating with the reduced CAP formation. Silencing the upstream regulators of these RhoGTPase pathways including Rac1 by specific siRNA dramatically disrupted actin cytoskeleton, distorted cell morphology and aborted CAP formation. Silencing cortactin (CTTN), a downstream effector of the Rac1 pathway, induced a compensatory upregulation of Rac1, enabling the MSCs to respond to the compression loading stimulus in terms of CAP formation, although at a reduced number. The importance of Rac1 signalling in CAP formation and the corresponding upregulation of lamellipodial markers also suggest that these CAPs are lamellipodia in nature. This study delineates the mechanism of compression-induced cytoskeleton reorganization, contributing to rationalizing mechanical loading regimes for functional tissue engineering.

Overall Structure Construction of an Intervertebral Disk Based on Highly Anisotropic Wood Hydrogel Composite Materials with Mechanical Matching and Buckling Buffering

Natural intervertebral disks (IVDs) exhibit distinctive anisotropic mechanical support and dissipation performances due to their well-developed special microstructures. As the intact IVD structure degrades, the absence of function will lead to severe backache. However, the complete simulation for the characteristic structure and function of native IVD is unattainable using current methods. In this work, by overall construction of the two-phase structure of native IVD (extraction of the naturally aligned cellulose framework and in situ polymerization of the nanocomposite hydrogel), a complete wood framework IVD (WF-IVD) is manufactured containing elastic nanocomposite hydrogel-based nucleus pulposus (NP) and anisotropic wood cellulose hydrogel-based annulus fibrosus (AF). In addition to the imitation and construction of the natural structure, WF-IVD also achieves favorable mechanical matching and good biocompatibility and possesses unique mechanical buckling buffer characteristics owing to the aligned fiber bundles. This study offers a promising strategy for the mimicking and construction of complex native tissues.

Intervertebral disc degeneration and regeneration: a motion segment perspective

Back and neck pain have become primary reasons for disability and healthcare spending globally. While the causes of back pain are multifactorial, intervertebral disc degeneration is frequently cited as a primary source of pain. The annulus fibrosus (AF) and nucleus pulposus (NP) subcomponents of the disc are common targets for regenerative therapeutics. However, disc degeneration is also associated with degenerative changes to adjacent spinal tissues, and successful regenerative therapies will likely need to consider and address the pathology of adjacent spinal structures beyond solely the disc subcomponents. This review summarises the current state of knowledge in the field regarding associations between back pain, disc degeneration, and degeneration of the cartilaginous and bony endplates, the AF-vertebral body interface, the facet joints and spinal muscles, in addition to a discussion of regenerative strategies for treating pain and degeneration from a whole motion segment perspective.

Advances in Tissue Engineering for Disc Repair

Intervertebral disc (IVD) degeneration is a leading cause of chronic low back pain (LBP) that results in serious disability and significant economic burden. IVD degeneration alters the disc structure and spine biomechanics, resulting in subsequent structural changes throughout the spine. Currently, treatments of chronic LBP due to IVD degeneration include conservative treatments, such as pain medication and physiotherapy, and surgical treatments, such as removal of herniated disc without or with spinal fusion. However, none of these treatments can completely restore a degenerated disc and its function. Thus, although the exact pathogenesis of disc degeneration remains unclear, there are studies examining the effectiveness of biological approaches, such as growth factor injection, gene therapy, and cell transplantation, in promoting IVD regeneration. Furthermore, tissue engineering using a combination of cell transplantation and biomaterials has emerged as a promising new approach for repair or restoration of degenerated discs. The main purpose of this review was to provide an overview of the current status of tissue engineering applications for IVD regenerative therapy by performing literature searches using PubMed. Significant advances in tissue engineering have opened the door to a new generation of regenerative therapies for the treatment of chronic discogenic LBP.

Sacrificial Fibers Improve Matrix Distribution and Micromechanical Properties in a Tissue-Engineered Intervertebral Disc

Tissue-engineered replacement discs are an area of intense investigation for the treatment of end-stage intervertebral disc (IVD) degeneration. These living implants can integrate into the IVD space and recapitulate native motion segment function. We recently developed a multiphasic tissue-engineered disc-like angle-ply structure (DAPS) that models the micro-architectural and functional features of native tissue. While these implants resulted in functional restoration of the motion segment in rat and caprine models, we also noted deficiencies in cell infiltration and homogeneity of matrix deposition in the electrospun poly(ε-caprolactone) outer region (annulus fibrosus, AF) of the DAPS. To address this limitation, here, we incorporated a sacrificial water-soluble polymer, polyethylene oxide (PEO), as a second fiber fraction within the AF region to increase porosity of the implant. Maturation of these PEO-modified DAPS were evaluated after 5 and 10 weeks of in vitro culture in terms of AF biochemical content, MRI T2 values, overall construct mechanical properties, AF micromechanical properties and cell and matrix distribution. To assess the performance of the PEO-modified DAPS in vivo, precultured constructs were implanted into the rat caudal IVD space for 10 weeks. Results showed that matrix distribution was more homogenous in PCL/PEO DAPS, as evidenced by more robust histological staining, organized collagen deposition and micromechanical properties, compared to standard PCL-only DAPS in vitro. Cell and matrix infiltration were also improved in vivo, but no differences in macromechanical properties and a trend towards improved micromechanical properties were observed. These findings demonstrate that the inclusion of a sacrificial PEO fiber fraction in the DAPS AF region improves cellular colonization, matrix elaboration, and in vitro and in vivo function of an engineered IVD implant. STATEMENT OF SIGNIFICANCE: This work establishes a method for improving cell infiltration and matrix distribution within tissue-engineered dense fibrous scaffolds for intervertebral disc replacement. Tissue-engineered whole disc replacements are an attractive alternative to the current gold standard (mechanical disc arthroplasty or vertebral fusion) for the clinical treatment of patients with advanced disc degeneration.

Scaffold-Free tissue engineering with aligned bone marrow stromal cell sheets to recapitulate the microstructural and biochemical composition of annulus fibrosus

Current tissue engineering strategies through scaffold-based approaches fail to recapitulate the complex three-dimensional microarchitecture and biochemical composition of the native Annulus Fibrosus tissue. Considering limited access to healthy annulus fibrosus cells from patients, this study explored the potential of bone marrow stromal cells (BMSC) to fabricate a scaffold-free multilamellar annulus fibrosus-like tissue by integrating micropatterning technologies into multi-layered BMSC engineering. BMSC sheet with cells and collagen fibres aligned at ~30° with respect to their longitudinal dimension were developed on a microgroove-patterned PDMS substrate. Two sheets were then stacked together in alternating directions to form an angle-ply bilayer tissue, which was rolled up, sliced to form a multi-lamellar angle-ply tissue and cultured in a customized medium. The development of the annulus fibrosus-like tissue was further characterized by histological, gene expression and microscopic and mechanical analysis. We demonstrated that the engineered annulus fibrosus-like tissue with aligned BMSC sheet showed parallel collagen fibrils, biochemical composition and microstructures that resemble the native disk. Furthermore, aligned cell sheet showed enhanced expression of annulus fibrosus associated extracellular matrix markers and higher mechanical strength than that of the non-aligned cell sheet. The present study provides a new strategy in annulus fibrosus tissue engineering methodology to develop a scaffold-free annulus fibrosus-like tissue that resembles the microarchitecture and biochemical attributes of a native tissue. This can potentially lead to a promising avenue for advancing BMSC-mediated annulus fibrosus regeneration towards future clinical applications.

Biomaterials-Induced Stem Cells Specific Differentiation Into Intervertebral Disc Lineage Cells

Stem cell therapy, which promotes stem cells differentiation toward specialized cell types, increases the resident population and production of extracellular matrix, and can be used to achieve intervertebral disc (IVD) repair, has drawn great attention for the development of IVD-regenerating materials. Many materials that have been reported in IVD repair have the ability to promote stem cells differentiation. However, due to the limitations of mechanical properties, immunogenicity and uncontrollable deviations in the induction of stem cells differentiation, there are few materials that can currently be translated into clinical applications. In addition to the favorable mechanical properties and biocompatibility of IVD materials, maintaining stem cells activity in the local niche and increasing the ability of stem cells to differentiate into nucleus pulposus (NP) and annulus fibrosus (AF) cells are the basis for promoting the application of IVD-regenerating materials in clinical practice. The purpose of this review is to summarize IVD-regenerating materials that focus on stem cells strategies, analyze the properties of these materials that affect the differentiation of stem cells into IVD-like cells, and then present the limitations of currently used disc materials in the field of stem cell therapy and future research perspectives.

Engineering a biomimetic integrated scaffold for intervertebral disc replacement

Tissue engineering technology provides a promising alternative to restore physiological functionality of damaged intervertebral disc (IVD). Advanced tissue engineering strategies for IVD have increasingly focused on engineering IVD regions combined the inner nucleus pulposus (NP) and surrounding annulus fibrosus (AF) tissue. However, simulating the cellular and matrix structures and function of the complex structure of IVD is still a critical challenge. Toward this goal, this study engineered a biomimetic AF-NP composite with circumferentially oriented poly(ε-caprolactone) microfibers seeded with AF cells, with an alginate hydrogel encapsulating NP cells as a core. Fluorescent imaging and histological analysis showed that AF cells spread along the circumferentially oriented PCL microfibers and NP cells colonized in the alginate hydrogel similar to native IVD, without obvious migration and mixing between the AF and NP region. Engineered IVD implants showed progressive tissue formation over time after subcutaneous implantation in nude mice, which were indicated by deposition and organization of extracellular matrix and enhanced mechanical properties. In terms of form and function of IVD-like tissue, our engineered biomimetic AF-NP composites have potential application for IVD replacement.

Biomaterials for intervertebral disc regeneration: Current status and looming challenges

A biomaterial-based strategy is employed to regenerate the degenerated intervertebral disc, which is considered a major generator of neck and back pain. Although encouraging enhancements in the anatomy and kinematics of the degenerative disc have been gained by biomaterials with various formulations in animals, the number of biomaterials tested in humans is rare. At present, most studies that involve the use of newly developed biomaterials focus on regeneration of the degenerative disc, but not pain relief. In this review, we summarise the current state of the art in the field of biomaterial-based regeneration or repair for the nucleus pulposus, annulus fibrosus, and total disc transplantation in animals and humans, and we then provide essential suggestions for the development and clinical translation of biomaterials for disc regeneration. It is important for researchers to consider the commonly neglected issues instead of concentrating solely on biomaterial development and fabrication.

Promise, progress, and problems in whole disc tissue engineering

Intervertebral disc degeneration is frequently implicated as a cause of back and neck pain, which are pervasive musculoskeletal complaints in modern society. For the treatment of end stage disc degeneration, replacement of the disc with a viable, tissue-engineered construct that mimics native disc structure and function is a promising alternative to fusion or mechanical arthroplasty techniques. Substantial progress has been made in the field of whole disc tissue engineering over the past decade, with a variety of innovative designs characterized both in vitro and in vivo in animal models. However, significant barriers to clinical translation remain, including construct size, cell source, culture technique, and the identification of appropriate animal models for preclinical evaluation. Here we review the clinical need for disc tissue engineering, the current state of the field, and the outstanding challenges that will need to be addressed by future work in this area.

Current strategies for treatment of intervertebral disc degeneration: substitution and regeneration possibilities

Background

Intervertebral disc degeneration has an annual worldwide socioeconomic impact masked as low back pain of over 70 billion euros. This disease has a high prevalence over the working age class, which raises the socioeconomic impact over the years. Acute physical trauma or prolonged intervertebral disc mistreatment triggers a biochemical negative tendency of catabolic-anabolic balance that progress to a chronic degeneration disease. Current biomedical treatments are not only ineffective in the long-run, but can also cause degeneration to spread to adjacent intervertebral discs. Regenerative strategies are desperately needed in the clinics, such as: minimal invasive nucleus pulposus or annulus fibrosus treatments, total disc replacement, and cartilaginous endplates decalcification.

Main body

Herein, it is reviewed the state-of-the-art of intervertebral disc regeneration strategies from the perspective of cells, scaffolds, or constructs, including both popular and unique tissue engineering approaches. The premises for cell type and origin selection or even absence of cells is being explored. Choice of several raw materials and scaffold fabrication methods are evaluated. Extensive studies have been developed for fully regeneration of the annulus fibrosus and nucleus pulposus, together or separately, with a long set of different rationales already reported. Recent works show promising biomaterials and processing methods applied to intervertebral disc substitutive or regenerative strategies. Facing the abundance of studies presented in the literature aiming intervertebral disc regeneration it is interesting to observe how cartilaginous endplates have been extensively neglected, being this a major source of nutrients and water supply for the whole disc.

Conclusion

Several innovative avenues for tackling intervertebral disc degeneration are being reported – from acellular to cellular approaches, but the cartilaginous endplates regeneration strategies remain unaddressed. Interestingly, patient-specific approaches show great promise in respecting patient anatomy and thus allow quicker translation to the clinics in the near future.

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.

Biomaterials for intervertebral disc regeneration and repair

The intervertebral disc contributes to motion, weight bearing, and flexibility of the spine, but is susceptible to damage and morphological changes that contribute to pathology with age and injury. Engineering strategies that rely upon synthetic materials or composite implants that do not interface with the biological components of the disc have not met with widespread use or desirable outcomes in the treatment of intervertebral disc pathology. Here we review bioengineering advances to treat disc disorders, using cell-supplemented materials, or acellular, biologically based materials, that provide opportunity for cell-material interactions and remodeling in the treatment of intervertebral disc disorders. While a field still in early development, bioengineering-based strategies employing novel biomaterials are emerging as promising alternatives for clinical treatment of intervertebral disc disorders.

Stiffness-Controlled Thermoresponsive Hydrogels for Cell Harvesting with Sustained Mechanical Memory

Most mechanobiological investigations focused on in situ mechanical regulation of cells on stiffness-controlled substrates with few downstream applications, as it is still challenging to harvest and expand mechanically primed cells by enzymatic digestion (e.g., trypsin) without interrupting cellular mechanical memory between passages. This study develops thermoresponsive hydrogels with controllable stiffness to generate mechanically primed cells with intact mechanical memory for augmented wound healing. No significant cellular property alteration of the fibroblasts primed on thermoresponsive hydrogels with varied stiffness has been observed through thermoresponsive harvesting. When reseeding the harvested cells for further evaluation, softer hydrogels are proven to better sustain the mechanical priming effects compared to rigid tissue culture plate, which indicates that both the stiffness-controlled substrate and thermoresponsive harvesting are required to sustain cellular mechanical memory between passages. Moreover, epigenetics analysis reveals that thermoresponsive harvesting could reduce the rearrangement and loss of chromatin proteins compared to that of trypsinization. In vivo wound healing using mechanically primed fibroblasts shows featured epithelium and sebaceous glands, which indicates augmented skin recovery compared with trypsinized fibroblasts. Thus, the thermoresponsive hydrogel-based cell harvesting system offers a powerful tool to investigate mechanobiology between cell passages and produces abundant cells with tailored mechanical priming properties for cell-based applications.

Construction Strategy and Progress of Whole Intervertebral Disc Tissue Engineering

Degenerative disc disease (DDD) is the major cause of low back pain, which usually leads to work absenteeism, medical visits and hospitalization. Because the current conservative procedures and surgical approaches to treatment of DDD only aim to relieve the symptoms of disease but not to regenerate the diseased disc, their long-term efficiency is limited. With the rapid developments in medical science, tissue engineering techniques have progressed markedly in recent years, providing a novel regenerative strategy for managing intervertebral disc disease. However, there are as yet no ideal methods for constructing tissue-engineered intervertebral discs. This paper reviews published reports pertaining to intervertebral disc tissue engineering and summarizes data concerning the seed cells and scaffold materials for tissue-engineered intervertebral discs, construction of tissue-engineered whole intervertebral discs, relevant animal experiments and effects of mechanics on the construction of tissue-engineered intervertebral disc and outlines the existing problems and future directions. Although the perfect regenerative strategy for treating DDD has not yet been developed, great progress has been achieved in the construction of tissue-engineered intervertebral discs. It is believed that ongoing research on intervertebral disc tissue engineering will result in revolutionary progress in the treatment of DDD.

In vivo performance of an acellular disc-like angle ply structure (DAPS) for total disc replacement in a small animal model

Total intervertebral disc replacement with a biologic engineered disc may be an alternative to spinal fusion for treating end-stage disc disease. In previous work, we developed disc-like angle ply structures (DAPS) that replicate the structure and function of the native disc and a rat tail model to evaluate DAPS in vivo. Here, we evaluated a strategy in which, after in vivo implantation, endogenous cells could colonize the acellular DAPS and form an extracellular matrix organized by the DAPS topographical template. To do so, acellular DAPS were implanted into the caudal spines of rats and evaluated over 12 weeks by mechanical testing, histology, and microcomputed tomography. An external fixation device was used to stabilize the implant site and various control groups were included to evaluate the effect of immobilization. There was robust tissue formation within the DAPS after implantation and compressive mechanical properties of the implant matched that of the native motion segment. Immobilization provided a stable site for fibrous tissue formation after either a discectomy or a DAPS implantation, but bony fusion eventually resulted, with segments showing intervertebral bridging after long-term implantation, a process that was accelerated by the implanted DAPS. Thus, while compressive mechanical properties were replicated after DAPS implantation, methods to actively prevent fusion must be developed. Future work will focus on limiting fusion by remobilizing the motion segment after a period of integration, delivering pro-chondrogenic factors, and pre-seeding DAPS with cells prior to implantation. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:23-31, 2017.

Stem Cells in Skeletal Tissue Engineering: Technologies and Models

This review surveys the use of pluripotent and multipotent stem cells in skeletal tissue engineering. Specific emphasis is focused on evaluating the function and activities of these cells in the context of development in vivo, and how technologies and methods of stem cell-based tissue engineering for stem cells must draw inspiration from developmental biology. Information on the embryonic origin and in vivo differentiation of skeletal tissues is first reviewed, to shed light on the persistence and activities of adult stem cells that remain in skeletal tissues after embryogenesis. Next, the development and differentiation of pluripotent stem cells is discussed, and some of their advantages and disadvantages in the context of tissue engineering are presented. The final section highlights current use of multipotent adult mesenchymal stem cells, reviewing their origin, differentiation capacity, and potential applications to tissue engineering.

A Structurally and Functionally Biomimetic Biphasic Scaffold for Intervertebral Disc Tissue Engineering

Tissue engineering offers high hopes for the treatment of intervertebral disc (IVD) degeneration. Whereas scaffolds of the disc nucleus and annulus have been extensively studied, a truly biomimetic and mechanically functional biphasic scaffold using naturally occurring extracellular matrix is yet to be developed. Here, a biphasic scaffold was fabricated with collagen and glycosaminoglycans (GAGs), two of the most abundant extracellular matrix components in the IVD. Following fabrication, the scaffold was characterized and benchmarked against native disc. The biphasic scaffold was composed of a collagen-GAG co-precipitate making up the nucleus pulposus-like core, and this was encapsulated in multiple lamellae of photochemically crosslinked collagen membranes comprising the annulus fibrosus-like lamellae. On mechanical testing, the height of our engineered disc recovered by ~82-89% in an annulus-independent manner, when compared with the 99% recovery exhibited by native disc. The annulus-independent nature of disc height recovery suggests that the fluid replacement function of the engineered nucleus pulposus core might mimic this hitherto unique feature of native disc. Biphasic scaffolds comprised of 10 annulus fibrosus-like lamellae had the best overall mechanical performance among the various designs owing to their similarity to native disc in most aspects, including elastic compliance during creep and recovery, and viscous compliance during recovery. However, the dynamic mechanical performance (including dynamic stiffness and damping factor) of all the biphasic scaffolds was similar to that of the native discs. This study contributes to the rationalized design and development of a biomimetic and mechanically viable biphasic scaffold for IVD tissue engineering.