Tissue engineering strategies applied in the regeneration of the human intervertebral disk

Current Therapeutic Strategies of Intervertebral Disc Regenerative Medicine

Intervertebral disc degeneration (IDD) is one of the most frequent causes of low back pain. No treatment is currently available to delay the progression of IDD. Conservative treatment or surgical interventions is only used to target the symptoms of IDD rather than treat the underlying cause. Currently, numerous potential therapeutic strategies are available, including molecular therapy, gene therapy, and cell therapy. However, the hostile environment of degenerated discs is a major problem that has hindered the clinical applicability of such approaches. In this regard, the design of drugs using alternative delivery systems (macro-, micro-, and nano-sized particles) may resolve this problem. These can protect and deliver biomolecules along with helping to improve the therapeutic effect of drugs via concentrating, protecting, and prolonging their presence in the degenerated disc. This review summarizes the research progress of diagnosis and the current options for treating IDD.

Comparative Analysis and Regeneration Strategies for Three Types of Cartilage

Cartilage tissue, encompassing hyaline cartilage, fibrocartilage, and elastic cartilage, plays a pivotal role in the human body because of its unique composition, structure, and biomechanical properties. However, the inherent avascularity and limited regenerative capacity of cartilage present significant challenges to its healing following injury. This review provides a comprehensive analysis of the current state of cartilage tissue engineering, focusing on the critical components of cell sources, scaffolds, and growth factors tailored to the regeneration of each cartilage type. We explore the similarities and differences in the composition, structure, and biomechanical properties of the three cartilage types and their implications for tissue engineering. A significant emphasis is placed on innovative strategies for cartilage regeneration, including the potential for in situ transformation of cartilage types through microenvironmental manipulation, which may offer novel avenues for repair and rehabilitation. The review underscores the necessity of a nuanced approach to cartilage tissue engineering, recognizing the distinct requirements of each cartilage type while exploring the potential of transforming one cartilage type into another as a flexible and adaptive repair strategy. Through this detailed examination, we aim to broaden the understanding of cartilage tissue engineering and inspire further research and development in this promising field.

Mechanical Basis of Lumbar Intervertebral Disk Degeneration

The etiology of degenerative disk disease (DDD) is multifactorial. Among the various factors, mechanical processes contributing to endplate or discal injuries have been discussed as the initiating events in the degenerative cascade. DDD encompasses the multitudinous changes undergone by the different structures of the spinal segment, namely intervertebral disk (IVD), facet joints, vertebral end plate (VEP), adjoining marrow (Modic changes), and vertebral body. It has been etiologically linked to a complex interplay of diverse mechanisms. Mechanically, two different mechanisms have been proposed for intervertebral disk degeneration (IVDD): endplate-driven, especially in upper lumbar levels, and annulus-driven degeneration. VEP is the weakest link of the lumbar spine, and fatigue damage can be inflicted upon them under physiological loads, leading to the initiation of DDD. Disk calcification has been put forth as another initiator of inflammation, stiffening, and abnormal stresses across the IVD. The initial mechanical disruption leads to secondary IVDD through unfavorable loading of the nucleus pulposus and annulus fibrosis. The final degenerative cascade is then propagated through a combination of biological, inflammatory, autoimmune, or metabolic pathways (impaired transport of metabolites or nutrients). Abnormal spinopelvic alignment, especially pelvic incidence, also significantly impacts the degenerative process. Hence, the etiology of DDD is multifactorial. Mechanical pathways, including VEP injuries, increased disk stiffness, and abnormal spinopelvic alignment, play a significant role in the initiation of IVDD.

Decellularized Nucleus Pulposus Matrix/Chitosan Hybrid Hydrogels for Nucleus Pulposus Tissue Engineering

STUDY DESIGN

Basic research.

OBJECTIVE

To prepared 3 DNPM/chitosan hybrid hydrogels and chose the best DNPM/chitosan hybrid hydrogel for NP tissue engineering.

METHODS

Three DNPM/chitosan hybrid hydrogels were fabricated by changing the ratio of the decellularized NP matrix to chitosan and crosslinking with genipin. NP stem cells (NPSCs) were cultured on the hybrid hydrogels and their proliferation, morphology, and gene expression were evaluated. Finally, an in vivo experiment was performed to evaluate the immune response to the hydrogels.

RESULTS

The adhered NPSCs proliferated well on the hybrid hydrogel. The gene expression of NP-related collagen type II, aggrecan, and Sox-9 from NPSCs cultured on DNPM/chitosan hybrid hydrogel-1 was greater than from cells cultured on DNPM/chitosan hybrid hydrogel-2 and DNPM/chitosan hybrid hydrogel-3. Few inflammatory cells were observed during the in vivo experiment with DNPM/chitosan hybrid hydrogel-1.

CONCLUSIONS

DNPM/chitosan hybrid hydrogel-1 is a potential candidate scaffold for NP tissue engineering.

Combining biomimetic collagen/hyaluronan hydrogels with discogenic growth factors promotes mesenchymal stroma cell differentiation into Nucleus Pulposus like cells

Based on stem cell injection into degenerated Nucleus Pulposus (NP), novel treatments for intervertebral disc (IVD) regeneration were disappointing because of cell leakage or inappropriate cell differentiation. In this study, we hypothesized that mesenchymal stromal cells encapsulated within injectable hydrogels possessing adequate physico-chemical properties would differentiate into NP like cells. Composite hydrogels consisting of type I collagen and tyramine-substituted hyaluronic acid (THA) were prepared to mimic the NP physico-chemical properties. Human bone marrow derived mesenchymal stromal cells (BM-MSCs) were encapsulated within hydrogels and cultivated in proliferation medium (supplemented with 10% fetal bovine serum) or differentiation medium (supplemented with GDF5 and TGFβ1) over 28 days. Unlike pure collagen, collagen/THA composite hydrogels were stable over 28 days in culture. In proliferation medium, the cell viability within pure collagen hydrogels was high, whereas that in composite and pure THA hydrogels was lower due to the weaker cell adhesion. Nonetheless, BM-MSCs proliferated in all hydrogels. In composite hydrogels, cells exhibited a rounded morphology similar to NP cells. The differentiation medium did not impact the hydrogel stability and cell morphology but negatively impacted the cell viability in pure collagen hydrogels. A high THA content within hydrogels promoted the gene expression of NP markers such as collagen II, aggrecan, SOX9 and cytokeratin 18 at day 28. The differentiation medium potentialized this effect with an earlier and higher expression of these NP markers. Taken together, these results show that the physico-chemical properties of collagen/THA composite hydrogels and GDF5/TGFβ1 act in synergy to promote the differentiation of BM-MSCs into NP like cells.

Can extracellular vesicles be considered as a potential frontier in the treatment of intervertebral disc disease?

As a global public health problem, low back pain (LBP) caused by intervertebral disc degeneration (IDD) seriously affects patients' quality of life. In addition, the prevalence of IDD tends to be younger, which brings a huge burden to individuals and society economically. Current treatments do not delay or reverse the progression of IDD. The emergence of biologic therapies has brought new hope for the treatment of IDD. Among them, extracellular vesicles (EVs), as nanoscale bioactive substances that mediate cellular communication, have now produced many surprising results in the research of the treatment of IDD. This article reviews the mechanisms and roles of EVs in delaying IDD and describes the prospects and challenges of EVs.

Gelatin-modified 3D printed PGS elastic hierarchical porous scaffold for cartilage regeneration

Regenerative cartilage replacements are increasingly required in clinical settings for various defect repairs, including bronchial cartilage deficiency, articular cartilage injury, and microtia reconstruction. Poly (glycerol sebacate) (PGS) is a widely used bioelastomer that has been developed for various regenerative medicine applications because of its excellent elasticity, biodegradability, and biocompatibility. However, because of inadequate active groups, strong hydrophobicity, and limited ink extrusion accuracy, 3D printed PGS scaffolds may cause insufficient bioactivity, inefficient cell inoculation, and inconsistent cellular composition, which seriously hinders its further cartilage regenerative application. Here, we combined 3D printed PGS frameworks with an encapsulated gelatin hydrogel to fabricate a PGS@Gel composite scaffold. PGS@Gel scaffolds have a controllable porous microstructure, with suitable pore sizes and enhanced hydrophilia, which could significantly promote the cells' penetration and adhesion for efficient chondrocyte inoculation. Furthermore, the outstanding elasticity and fatigue durability of the PGS framework enabled the regenerated cartilage built by the PGS@Gel scaffolds to resist the dynamic in vivo environment and maintain its original morphology. Importantly, PGS@Gel scaffolds increased the rate of cartilage regeneration concurrent with scaffold degradation. The scaffold was gradually degraded and integrated to form uniform, dense, and mature regenerated cartilage tissue with little scaffold residue.

A Study Based on Network Pharmacology Decoding the Multi-Target Mechanism of Duhuo Jisheng Decoction for the Treatment of Intervertebral Disc Degeneration

Intervertebral disc degeneration (IDD) poses a grim public health impact. Duhuo Jisheng Decoction (DJD), a traditional Chinese medicine formula, has recently received significant attention for its efficacy and safety in treating IDD. However, the pathological processes of IDD in which DJD interferes and molecular mechanism involved are poorly understood, which brings difficulties to the clinical practice of DJD for the treatment of IDD. This study systematically investigated the underlying mechanism of DJD treatment of IDD. Network pharmacology approaches were employed, integrating molecular docking and random walk with restart (RWR) algorithm, to identify key compounds and targets for DJD in the treatment of IDD. Bioinformatics approaches were used to further explore the biological insights in DJD treatment of IDD. The analysis identifies AKT1, PIK3R1, CHUK, ALB, TP53, MYC, NR3C1, IL1B, ERBB2, CAV1, CTNNB1, AR, IGF2, and ESR1 as key targets. Responses to mechanical stress, oxidative stress, cellular inflammatory responses, autophagy, and apoptosis are identified as the critical biological processes involved in DJD treatment of IDD. The regulation of DJD targets in extracellular matrix components, ion channel regulation, transcriptional regulation, synthesis and metabolic regulation of reactive oxygen products in the respiratory chain and mitochondria, fatty acid oxidation, the metabolism of Arachidonic acid, and regulation of Rho and Ras protein activation are found to be potential mechanisms in disc tissue response to mechanical stress and oxidative stress. MAPK, PI3K/AKT, and NF-κB signaling pathways are identified as vital signaling pathways for DJD to treat IDD. Quercetin and Kaempferol are assigned a central position in the treatment of IDD. This study contributes to a more comprehensive understanding of the mechanism of DJD in treating IDD. It provides a reference for applying natural products to delay the pathological process of IDD.

Injectable and tissue adhesive EGCG-laden hyaluronic acid hydrogel depot for treating oxidative stress and inflammation

Oxidative stress and inflammation are common pathological mechanisms for the progression of tissue degeneration. Epigallocatechin-3-gallate (EGCG) features antioxidant and anti-inflammatory properties, which is a promising drug for the treatment of tissue degeneration. Herein, we utilize the phenylborate ester reaction of EGCG and phenylboronic acid (PBA) to fabricate an injectable and tissue adhesive EGCG-laden hydrogel depot (EGCG HYPOT), which can achieve anti-inflammatory and antioxidative effects via smart delivery of EGCG. Specifically, the phenylborate ester bonds, formed by EGCG and PBA-modified methacrylated hyaluronic acid (HAMA-PBA), endow EGCG HYPOT injectability, shape adaptation and efficient load of EGCG. After photo-crosslinking, EGCG HYPOT exhibits good mechanical properties, tissue adhesion and sustained acid-responsive release of EGCG. EGCG HYPOT can scavenge oxygen and nitrogen free radicals. Meanwhile, EGCG HYPOT can scavenge intracellular reactive oxygen species (ROS) and suppress the expression of pro-inflammatory factors. EGCG HYPOT may provide a new idea for alleviation of inflammatory disturbance.

Elastic Fibers in the Intervertebral Disc: From Form to Function and toward Regeneration

Despite extensive efforts over the past 40 years, there is still a significant gap in knowledge of the characteristics of elastic fibers in the intervertebral disc (IVD). More studies are required to clarify the potential contribution of elastic fibers to the IVD (healthy and diseased) function and recommend critical areas for future investigations. On the other hand, current IVD in-vitro models are not true reflections of the complex biological IVD tissue and the role of elastic fibers has often been ignored in developing relevant tissue-engineered scaffolds and realistic computational models. This has affected the progress of IVD studies (tissue engineering solutions, biomechanics, fundamental biology) and translation into clinical practice. Motivated by the current gap, the current review paper presents a comprehensive study (from the early 1980s to 2022) that explores the current understanding of structural (multi-scale hierarchy), biological (development and aging, elastin content, and cell-fiber interaction), and biomechanical properties of the IVD elastic fibers, and provides new insights into future investigations in this domain.

Fucoidan-loaded nanofibrous scaffolds promote annulus fibrosus repair by ameliorating the inflammatory and oxidative microenvironments in degenerative intervertebral discs

Tissue engineering holds potential in the treatment of intervertebral disc degeneration (IDD). However, implantation of tissue engineered constructs may cause foreign body reaction and aggravate the inflammatory and oxidative microenvironment of the degenerative intervertebral disc (IVD). In order to ameliorate the adverse microenvironment of IDD, in this study, we prepared a biocompatible poly (ether carbonate urethane) urea (PECUU) nanofibrous scaffold loaded with fucoidan, a natural marine bioactive polysaccharide which has great anti-inflammatory and antioxidative functions. Compared with pure PECUU scaffold, the fucoidan-loaded PECUU nanofibrous scaffold (F-PECUU) decreased the gene and protein expression related to inflammation and the oxidative stress in the lipopolysaccharide (LPS) induced annulus fibrosus cells (AFCs) significantly (p<0.05). Especially, gene expression of Ill 6 and Ptgs2 was decreased by more than 50% in F-PECUU with 3.0 wt% fucoidan (HF-PECUU). Moreover, the gene and protein expression related to the degradation of extracellular matrix (ECM) were reduced in a fucoidan concentration-dependent manner significantly, with increased almost 3 times gene expression of Col1a2 and Acan in HF-PECUU. Further, in a 'box' defect model, HF-PECUU decreased the expression of COX-2 and deposited more ECM between scaffold layers when compared with pure PECUU. The disc height and nucleus pulposus hydration of repaired IVD reached up to 75% and 85% of those in the sham group. In addition, F-PECUU helped to maintain an integrate tissue structure with a similar compression modulus to that in sham group. Taken together, the F-PECUU nanofibrous scaffolds showed promising potential to promote AF repair in IDD treatment by ameliorating the harsh degenerative microenvironment. STATEMENT OF SIGNIFICANCE: Annulus fibrosus (AF) tissue engineering holds potential in the treatment of intervertebral disc degeneration (IDD), but is restricted by the inflammatory and oxidative microenvironment of degenerative disc. This study developed a biocompatible polyurethane scaffold (F-PECUU) loaded with fucoidan, a marine bioactive polysaccharide, for ameliorating IDD microenvironment and promoting disc regeneration. F-PECUU alleviated the inflammation and oxidative stress caused by lipopolysaccharide and prevented extracellular matrix (ECM) degradation in AF cells. In vivo, it promoted ECM deposition to maintain the height, water content and mechanical property of disc. This work has shown the potential of marine polysaccharides-containing functional scaffolds in IDD treatment by ameliorating the harsh microenvironment accompanied with disc degeneration.

Evaluation of Tissue Engineering Approaches for Intervertebral Disc Regeneration in Relevant Animal Models

Translation of tissue engineering strategies for the regeneration of intervertebral disc (IVD) requires a strong understanding of pathophysiology through the relevant animal model. There is no relevant animal model due to differences in disc anatomy, cellular composition, extracellular matrix components, disc physiology, and mechanical strength from humans. However, available animal models if used correctly could provide clinically relevant information for the translation into humans. In this review, we have investigated different types of strategies for the development of clinically relevant animal models to study biomaterials, cells, biomolecular or their combination in developing tissue engineering-based treatment strategies. Tissue engineering strategies that utilize various animal models for IVD regeneration are summarized and outcomes have been discussed. The understanding of animal models for the validation of regenerative approaches is employed to understand and treat the pathophysiology of degenerative disc disease (DDD) before proceeding for human trials. These animal models play an important role in building a therapeutic regime for IVD tissue regeneration, which can serve as a platform for clinical applications.

Damage to the human lumbar cartilage endplate and its clinical implications

The cartilaginous endplate (CEP) is a thin layer of hyaline cartilage, and plays an important role in the diffusion of nutrients into the intervertebral discs. Its damage may seriously affect the disc degeneration, and result in low back pain (LBP). However, the structural features of damaged CEPs have not been well characterized, and this hinders our understanding of the etiology of disc degeneration and pain. To present the structural features of micro-damaged CEPs in patients with disc degeneration and LBP that might even be regarded as an initial factor for disc degeneration, we performed a histological study of micro-damaged CEPs harvested from human lumbar intervertebral discs and analyzed its clinical implications. Human lumbar CEPs were excised from 35 patients (mean age 60.91 years) who had disc degeneration and LBP. Control tissue was obtained from 15 patients (mean age 54.67 years) with lumbar vertebral burst fractures. LBP and disability were assessed clinically, and all patients underwent anterior vertebral body fusion surgery. CEPs together with some adjacent nucleus pulposus (NP) were sectioned at 4 µm, and stained using H&E, Safranin O/Fast Green, and Alcian Blue. Immunostaining and PCR were used to identify various markers of degeneration, innervation, and inflammation. Histology demonstrated physical micro-damage in 14/35 CEPs from the disc degeneration group. Six major types of damage could be distinguished: fissure, traumatic nodes, vascular mimicry, incorporation of NP tissue within the CEP, incorporation of bone within the CEP, and incorporation of NP and bone within the CEP. Pain and disability scores (ODI: p = 0.0190; JOA: p = 0.0205; JOABPEQ: p = 0.0034) were significantly higher in those with micro-damaged CEPs (N = 14) than in those with non-damaged CEPs (N = 21). CEP damage was significantly associated with elevated MMP3 (p = 0.043), MMP13 (p = 0.0191), ADAMTS5 (p = 0.0253), TNF-α (p = 0.0011), and Substance P (p = 0.0028), and with reduced Sox9 (p = 0.0212), aggrecan (p = 0.0127), and type II collagen (p = 0.0139). In conclusion, we presented a new classification of human lumbar micro-damaged CEPs. Furthermore, we verify disc degeneration, innervation, and discogenic pain in micro-damaged CEPs.

Current Progress in the Endogenous Repair of Intervertebral Disk Degeneration Based on Progenitor Cells

Intervertebral disk (IVD) degeneration is one of the most common musculoskeletal disease. Current clinical treatment paradigms for IVD degeneration cannot completely restore the structural and biomechanical functions of the IVD. Bio-therapeutic techniques focused on progenitor/stem cells, especially IVD progenitor cells, provide promising options for the treatment of IVD degeneration. Endogenous repair is an important self-repair mechanism in IVD that can allow the IVD to maintain a long-term homeostasis. The progenitor cells within IVD play a significant role in IVD endogenous repair. Improving the adverse microenvironment in degenerative IVD and promoting progenitor cell migration might be important strategies for implementation of the modulation of endogenous repair of IVD. Here, we not only reviewed the research status of treatment of degenerative IVD based on IVD progenitor cells, but also emphasized the concept of endogenous repair of IVD and discussed the potential new research direction of IVD endogenous repair.

Advanced Strategies for the Regeneration of Lumbar Disc Annulus Fibrosus

Damage to the annulus fibrosus (AF), the outer region of the intervertebral disc (IVD), results in an undesirable condition that may accelerate IVD degeneration causing low back pain. Despite intense research interest, attempts to regenerate the IVD have failed so far and no effective strategy has translated into a successful clinical outcome. Of particular significance, the failure of strategies to repair the AF has been a major drawback in the regeneration of IVD and nucleus replacement. It is unlikely to secure regenerative mediators (cells, genes, and biomolecules) and artificial nucleus materials after injection with an unsealed AF, as IVD is exposed to significant load and large deformation during daily activities. The AF defects strongly change the mechanical properties of the IVD and activate catabolic routes that are responsible for accelerating IVD degeneration. Therefore, there is a strong need to develop effective therapeutic strategies to prevent or reconstruct AF damage to support operational IVD regenerative strategies and nucleus replacement. By the way of this review, repair and regenerative strategies for AF reconstruction, their current status, challenges ahead, and future outlooks were discussed.

The Traceability of Mesenchymal Stromal Cells After Injection Into Degenerated Discs in Patients with Low Back Pain

Low back pain is a major health issue and one main cause to this condition is believed to be intervertebral disc (IVD) degeneration. Stem cell therapy for degenerated discs using mesenchymal stromal cells (MSCs) has been suggested. The aim of the study was to investigate the presence and distribution pattern of autologous MSCs transplanted into degenerated IVDs in patients and explanted posttransplantation. IVD tissues from four patients (41, 45, 47, and 47 years of age) participating in a clinical feasibility study on MSC transplantation to degenerative discs were investigated. Three patients decided to undergo fusion surgery at time points 8 months and one patient at 28 months posttransplantation. Pretransplantation, MSCs from bone marrow aspirate were isolated by centrifugation in FICOLL^® test tubes and cultured (passage 1). Before transplantation, MSCs were labeled with 1 mg/mL iron sucrose (Venofer^®) and 1 × 10⁶ MSCs were transplanted into degenerated IVDs. At the time point of surgery, IVD tissues were collected. IVD tissue samples were fixated, embedded in paraffin, and sections prepared. IVD samples were stained with Prussian Blue, by which iron deposits are visualized and examined (light microscopy). Immunohistochemistry (IHC), including SOX9 (sex determining region Y box 9), Coll2A1 (collagen 2A1), and cell viability (TUNEL) were performed. Cells positive for iron deposits were observed in IVD tissues (3/4 patients). The cells/iron deposits were observed in clusters and/or as solitary cells in regions in IVD tissue samples [regions of interest (ROIs)]. By IHC, SOX9- and Coll2A1-positive cells were detected in the same regions as the detected cells/iron deposits. A few nonviable cells were detected by TUNEL assay in ROIs. Results demonstrated that MSCs, labeled with iron sucrose, transplanted into degenerated IVDs were detectable 8 months posttransplantation. The detected cellular activity indicates that MSCs have differentiated into chondrocyte-like cells and that the injected MSCs and/or their progeny have survived since the cells were found in large cluster and as solitary cells which were distributed at different parts of the IVD.

Identification of Aberrantly Expressed Genes during Aging in Rat Nucleus Pulposus Cells

Nucleus pulposus cells (NPCs) play a vital role in maintaining the homeostasis of the intervertebral disc (IVD). Previous studies have discovered that NPCs exhibited malfunction due to cellular senescence during disc aging and degeneration; this might be one of the key factors of IVD degeneration. Thus, we conducted this study in order to investigate the altered biofunction and the underlying genes and pathways of senescent NPCs. We isolated and identified NPCs from the tail discs of young (2 months) and old (24 months) SD rats and confirmed the senescent phenotype through SA-β-gal staining. CCK-8 assay, transwell assay, and cell scratch assay were adopted to detect the proliferous and migratory ability of two groups. Then, a rat Gene Chip Clariom™ S array was used to detect differentially expressed genes (DEGs). After rigorous bioinformatics analysis of the raw data, totally, 1038 differentially expressed genes with a fold change > 1.5 were identified out of 23189 probes. Among them, 617 were upregulated and 421 were downregulated. Furthermore, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were conducted and revealed numerous number of enriched GO terms and signaling pathways associated with senescence of NPCs. A protein-protein interaction (PPI) network of the DEGs was constructed using the Search Tool for the Retrieval of Interacting Genes (STRING) database and Cytoscape software. Module analysis was conducted for the PPI network using the MCODE plugin in Cytoscape. Hub genes were identified by the CytoHubba plugin in Cytoscape. Derived 5 hub genes and most significantly up- or downregulated genes were further verified by real-time PCR. The present study investigated underlying mechanisms in the senescence of NPCs on a genome-wide scale. The illumination of molecular mechanisms of NPCs senescence may assist the development of novel biological methods to treat degenerative disc diseases.

Intervertebral disc tissue engineering: A brief review

Intervertebral disc (IVD) degeneration (IDD) is associated with low back pain and significantly affects the patient's quality of life. Degeneration of the IVD alters disk height and the mechanics of the spine, leading to chronic segmental spinal instability. The pathophysiology of IVD disease is still not well understood. Current therapies for IDD include conservative and invasive approaches, but none of those treatments are able to restore the disc structure and function. Recently, tissue engineering techniques emerged as a possible approach to treat IDD, by replacing a damaged IVD with scaffolds and appropriate cells. Advances in manufacturing techniques, material processing and development, surface functionalization, drug delivery systems and cell incorporation furthered the development of tissue engineering therapies. In this review, biomaterial scaffolds and cell-based therapies for IVD regeneration are briefly discussed.

The establishment and biological assessment of a whole tissue-engineered intervertebral disc with PBST fibers and a chitosan hydrogel in vitro and in vivo

Intervertebral disc (IVD) degeneration (IDD) is the main cause of low back pain in the clinic. In the advanced stage of IDD, both cell transplantation and gene therapy have obvious limitations. At this stage, tissue-engineered IVDs (TE-IVDs) provide new hope for the treatment of this disease. We aimed to construct a TE-IVD with a relatively complete structure. The inner annulus fibrosus (AF) was constructed using poly (butylene succinate-co-terephthalate) copolyester (PBST) electrospun fibers, and the outer AF consisted of solid PBST. The nucleus pulposus (NP) scaffold was constructed using a chitosan hydrogel, as reported in our previous research. The three components were assembled in vitro, and the mechanical properties were analyzed. AF and NP cells were implanted on the corresponding scaffolds. Then, the cell-seeded scaffolds were implanted subcutaneously in nude mice and cultured for 4 weeks; then they were removed and implanted into New Zealand white rabbits. After 4 weeks, their properties were analyzed. The PBST outer AF provided mechanical support for the whole TE-IVD. The electrospun film and chitosan hydrogel simulated the natural structure of the IVD well. Its mechanical property could meet the requirement of the normal IVD. Four weeks later, X-ray and MR imaging examination results suggested that the height of the intervertebral space was retained. The cells on the TE-IVD expressed extracellular matrix, which indicated that the cells maintained their biological function. Therefore, we conclude that the whole TE-IVD has biological and biomechanical properties to some extent, which is a promising candidate for IVD replacement therapies. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 2305-2316, 2019.

Mechanisms of endogenous repair failure during intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is frequently associated with Low back pain (LBP), which can severely reduce the quality of human life and cause enormous economic loss. However, there is a lack of long-lasting and effective therapies for IVD degeneration at present. Recently, stem cell based tissue engineering techniques have provided novel and promising treatment for the repair of degenerative IVDs. Numerous studies showed that stem/progenitor cells exist naturally in IVDs and could migrate from their niche to the IVD to maintain the quantity of nucleus pulposus (NP) cells. Unfortunately, these endogenous repair processes cannot prevent IVD degeneration as effectively as expected. Therefore, theoretical basis for regeneration of the NP in situ can be obtained from studying the mechanisms of endogenous repair failure during IVD degeneration. Although there have been few researches to study the mechanism of cell death and migration of stem/progenitor cells in IVD so far, studies demonstrated that the major inducing factors (compression and hypoxia) of IVD degeneration could decrease the number of NP cells by regulating apoptosis, autophagy, and necroptosis, and the particular chemokines and their receptors played a vital role in the migration of mesenchymal stem cells (MSCs). These studies provide a clue for revealing the mechanisms of endogenous repair failure during IVD degeneration. This article reviewed the current research situation and progress of the mechanisms through which IVD stem/progenitor cells failed to repair IVD tissues during IVD degeneration. Such studies provide an innovative research direction for endogenous repair and a new potential treatment strategy for IVD degeneration.

In vitro efficacy of silk sericin microparticles and platelet lysate for intervertebral disk regeneration

Intervertebral disk degeneration is an oxidative and inflammatory pathological condition that induces viability and functionality reduction of Nucleus Pulposus cells (NPs). Cellular therapies were previously proposed to repair and substitute the herniated disk but low proliferative index and pathological conditions of NPs dramatically reduced the efficacy of this approach. To overcome these problems we proposed, for the first time, a therapeutic system based on the association of silk sericin microparticles and platelet-derived products. Silk sericin (SS) is a bioactive protein with marked antioxidant properties, while platelet lysate (PL) and platelet poor plasma (PPP) represent a source of growth factors able to support cell viability and to promote tissue regeneration. We demonstrated that the mixture PL + PPP promoted NPs proliferation with a significant reduction of cellular doubling time. SS microparticles, alone or in combination with PPP, presented the higher ROS-scavenging activity while, SS microparticles and PL resulted as the best association able to protect NPs against oxidative stress induce by hydroxide peroxide. Based on these results, the authors are confident that, with the ever increasing need of efficacious tools for regenerative medicine purposes, SS microparticles and PL + PPP association could represent an effective approach for the development of low impact and non-invasive therapies.

An injectable nucleus pulposus cell-modified decellularized scaffold: biocompatible material for prevention of disc degeneration

We developed a nucleus pulposus cell (NPC)-modulated decellularized small intestinal submucosa (SIS) scaffold, and assessed the ability of this material to prevent Intervertebral disc degeneration (IVD) degeneration. Decellularized porcine SIS was squashed into particles and the biological safety and efficiency of these particles were evaluated. Next, SIS particles were seeded with rabbit NPCs, cultured for two months in vitro, decellularized again and suspended for intervertebral injection. We demonstrated that use of the decellularization protocol resulted in the removal of cellular components with maximal retention of extracellular matrix. The xenogeneic decellularized SIS did not display cytotoxicity in vitro and its application prevented NPC degradation. Furthermore, the xenogeneic SIS microparticles were effective in preventing IVD progression in vivo in a rabbit disc degeneration model. In conclusion, our study describes an optimized method for decellularized SIS preparation and demonstrated that the material is safe and effective for treating IVD degeneration.

Fabrication of a novel whole tissue-engineered intervertebral disc for intervertebral disc regeneration in the porcine lumbar spine

Tissue-engineered intervertebral discs (IVDs) have been proposed as a useful therapeutic strategy for the treatment of intervertebral disc degeneration (IDD). However, most studies have focused on fabrication and assessment of tissue-engineered IVDs in small animal models and the mechanical properties of the scaffolds are far below those of native human IVDs. The aim of this study was to produce a novel tissue-engineered IVD for IDD regeneration in the porcine lumbar spine. Firstly, a novel whole tissue-engineered IVD scaffold was fabricated using chitosan hydrogel to simulate the central nucleus pulposus (NP) structure, surrounded with a poly(butylene succinate-co-terephthalate) (PBST) fiber film for inner annulus fibrosus (IAF). And, a poly(ether ether ketone) (PEEK) ring was used to stimulate the outer annulus fibrosus (OAF). Then, the scaffolds were seeded with IVD cells and the cell-scaffold hybrids were transplanted into the porcine damaged spine and harvested at 4 and 8 weeks. In vitro cell experiments showed that IVD cells distributed and grew well in the scaffolds including porous hydrogel and PBST fibers. After implantation into pigs, radiographic and MRI images indicated that the tissue-engineered IVD construct could preserve the disc height in the case of discectomy as the normal disc height and maintain a large extracellular matrix and water content in the NP. Combined with the histological and gene expression results, it was concluded that the tissue-engineered IVD had similar morphological and histological structure to the natural IVD. Moreover, after implantation for 8 weeks, the tissue-engineered IVD showed a good compressive stress and elastic moduli, approaching those of natural porcine IVD. Therefore, the prepared tissue-engineered IVD construct had similar morphological and biofunctional properties to the native tissue. Also, the tissue-engineered IVD construct with excellent biocompatibility and mechanical properties provides a promising candidate for human IDD regeneration.

A novel whole tissue-engineered IVD consisting of a triphasic scaffold demonstrated excellent biocompatibility and mechanical properties in the porcine lumbar spine.

Decellularized allogeneic intervertebral disc: natural biomaterials for regenerating disc degeneration

Intervertebral disc degeneration is associated with back pain and disc herniation. This study established a modified protocol for intervertebral disc (IVD) decellularization and prepared its extracellular matrix (ECM). By culturing mesenchymal stem cells (MSCs)(3, 7, 14 and 21 days) and human degenerative IVD cells (7 days) in the ECM, implanting it subcutaneously in rabbit and injecting ECM microparticles into degenerative disc, the biological safety and efficacy of decellularized IVD was evaluated both in vitro and in vivo. Here, we demonstrated that cellular components can be removed completely after decellularization and maximally retain the structure and biomechanics of native IVD. We revealed that allogeneic ECM did not evoke any apparent inflammatory reaction in vivo and no cytotoxicity was found in vitro. Moreover, IVD ECM can induce differentiation of MSCs into IVD-like cells in vitro. Furthermore, allogeneic ECM microparticles are effective on the treatment of rabbit disc degeneration in vivo. In conclusion, our study developed an optimized method for IVD decellularization and we proved decellularized IVD is safe and effective for the treatment of degenerated disc diseases.

Nanocellulose reinforced gellan-gum hydrogels as potential biological substitutes for annulus fibrosus tissue regeneration

Intervertebral disc (IVD) degeneration is associated with both structural damage and aging related degeneration. Annulus fibrosus (AF) defects such as annular tears, herniation and discectomy require novel tissue engineering strategies to functionally repair AF tissue. An ideal construct will repair the AF by providing physical and biological support, facilitating regeneration. The presented strategy herein proposes a gellan gum-based construct reinforced with cellulose nanocrystals (nCell) as a biological self-gelling AF substitute. Nanocomposite hydrogels were fabricated and characterized with respect to hydrogel swelling capacity, degradation rate in vitro and mechanical properties. Rheological evaluation on the nanocomposites demonstrated the GGMA reinforcement with nCell promoted matrix entanglement with higher scaffold stiffness observed upon ionic crosslinking. Compressive mechanical tests demonstrated compressive modulus values close to those of the human AF tissue. Furthermore, cell culture studies with encapsulated bovine AF cells indicated that nanocomposite constructs promoted cell viability and a physiologically relevant cell morphology for up to fourteen days in vitro.

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.

An interpenetrating network-strengthened and toughened hydrogel that supports cell-based nucleus pulposus regeneration

Hydrogel is a suitable scaffold for the nucleus pulposus (NP) regeneration. However, its unmatched mechanical properties lead to implant failure in late-stage disc degeneration because of structural failure and implant extrusion after long-term compression. In this study, we evaluated an interpenetrating network (IPN)-strengthened and toughened hydrogel for NP regeneration, using dextran and gelatin as the primary network while poly (ethylene glycol) as the secondary network. The aim of this study was to realize the NP regeneration using the hydrogel. To achieve this, we optimized its properties by adjusting the mass ratios of the secondary/primary networks and determining the best preparation conditions for NP regeneration in a series of biomechanical, cytocompatibility, tissue engineering, and in vivo study. We found the optimal formulation of the IPN hydrogel, at a secondary/primary network ratio of 1:4, exhibited high toughness (the compressive strain reached 86%). The encapsulated NP cells showed increasing proliferation, cell clustering and matrix deposition. Furthermore, the hydrogel could support long-term cell retention and survival in the rat IVDs. It facilitated rehydration and regeneration of porcine degenerative NPs. In conclusion, this study demonstrates the tough IPN hydrogel could be a promising candidate for functional disc regeneration in future.

Icariin Prevents H2O2-Induced Apoptosis via the PI3K/Akt Pathway in Rat Nucleus Pulposus Intervertebral Disc Cells

Icariin is a prenylated flavonol glycoside derived from the Chinese herb Epimedium sagittatum. This study investigated the mechanism by which icariin prevents H₂O₂-induced apoptosis in rat nucleus pulposus (NP) cells. NP cells were isolated from the rat intervertebral disc and they were divided into five groups after 3 passages: (A) blank control; (B) 200 μM H₂O₂; (C) 200 μM H₂O₂ + 20 μM icariin; (D) 20 μM icariin + 200 μM H₂O₂ + 25 μM LY294002; (E) 200 μM H₂O₂ + 25 μM LY294002. LY294002 is a selective inhibitor of the phosphoinositide 3-kinase (PI3K)/Akt signaling pathway. NP cell viability, apoptosis rate, intracellular reactive oxygen species levels, and the expression of AKT, p-AKT, p53, Bcl-2, Bax, caspase-3 were estimated. The results show that, compared with the control group, H₂O₂ significantly increased NP cell apoptosis and the level of intracellular ROS. Icariin pretreatment significantly decreased H₂O₂-induced apoptosis and intracellular ROS and upregulated p-Akt and BCL-2 and downregulated caspase-3 and Bax. LY294002 abolished the protective effects of icariin. Our results show that icariin can attenuate H2O2-induced apoptosis in rat nucleus pulposus cells and PI3K/AKT pathway is at least partly included in this protection effect.

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.

3D segmentation of intervertebral discs: from concept to the fabrication of patient-specific scaffolds

Aim: To develop a methodology for producing patient-specific scaffolds that mimic the annulus fibrosus (AF) of the human intervertebral disc by means of combining MRI and 3D bioprinting. Methods: In order to obtain the AF 3D model from patient's volumetric MRI dataset, the RheumaSCORE segmentation software was used. Polycaprolactone scaffolds with three different internal architectures were fabricated by 3D bioprinting, and characterized by microcomputed tomography. Results: The demonstrated methodology of a geometry reconstruction pipeline enabled us to successfully obtain an accurate AF model and 3D print patient-specific scaffolds with different internal architectures. Conclusion: The results guide us toward patient-specific intervertebral disc tissue engineering as demonstrated by a way of manufacturing personalized scaffolds using patient's MRI data.

Photocrosslinked tyramine-substituted hyaluronate hydrogels with tunable mechanical properties improve immediate tissue-hydrogel interfacial strength in articular cartilage

Articular cartilage lacks the ability to self-repair and a permanent solution for cartilage repair remains elusive. Hydrogel implantation is a promising technique for cartilage repair; however for the technique to be successful hydrogels must interface with the surrounding tissue. The objective of this study was to investigate the tunability of mechanical properties in a hydrogel system using a phenol-substituted polymer, tyramine-substituted hyaluronate (TA-HA), and to determine if the hydrogels could form an interface with cartilage. We hypothesized that tyramine moieties on hyaluronate could crosslink to aromatic amino acids in the cartilage extracellular matrix. Ultraviolet (UV) light and a riboflavin photosensitizer were used to create a hydrogel by tyramine self-crosslinking. The gel mechanical properties were tuned by varying riboflavin concentration, TA-HA concentration, and UV exposure time. Hydrogels formed with a minimum of 2.5 min of UV exposure. The compressive modulus varied from 5 to 16 kPa. Fluorescence spectroscopy analysis found differences in dityramine content. Cyanine-3 labelled tyramide reactivity at the surface of cartilage was dependent on the presence of riboflavin and UV exposure time. Hydrogels fabricated within articular cartilage defects had increasing peak interfacial shear stress at the cartilage-hydrogel interface with increasing UV exposure time, reaching a maximum shear stress 3.5× greater than a press-fit control. Our results found that phenol-substituted polymer/riboflavin systems can be used to fabricate hydrogels with tunable mechanical properties and can interface with the surface tissue, such as articular cartilage.

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.

Joint analysis of IVD herniation and degeneration by rat caudal needle puncture model

Intervertebral disc (IVD) degeneration is responsible for various spine pathologies and present clinical treatments are insufficient. Concurrently, the mechanisms behind IVD degeneration are still not completely understood, so as to allow development of efficient tissue engineering approaches. A model of rat IVD degeneration directly coupled to herniation is here proposed in a pilot study. Disc injury is induced by needle puncture, using two different needles gauges: a low caliber 25-G needle and a high caliber 21-G needle. Histological, biochemical, and radiographic degeneration was evaluated at 2 and 6 weeks post-injury. We show that the larger caliber needle results in a more extended histological and radiographic degeneration within the IVD, compared to the smaller one. TUNEL quantification indicates also increased cell death in the 21-G group. Analyses of collagen type I (Picrosirius red staining), collagen type II (immunofluorescence), and GAG content (Blyscan assay) indicate that degeneration features spontaneously recover from 2 to 6 weeks, for both needle types. Moreover, we show the occurrence of hernia proportional to the needle gauge. The number of CD68+ macrophages present, as well as cell apoptosis within the herniated tissue are both proportional to hernia volume. Moreover, hernias formed after lesion tend to spontaneously diminish in volume after 6 weeks. Finally, MMP3 is increased in the hernia in the 21-G group at 2 weeks. This model, by uniquely combining IVD degeneration and IVD herniation in the same animal, may help to understand mechanisms behind IVD pathophysiology, such as hernia formation and spontaneous regression. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:258-268, 2017.

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.

Cell-based therapies for intervertebral disc and cartilage regeneration- Current concepts, parallels, and perspectives

Lower back pain from degenerative disc disease represents a global health burden, and presents a prominent opportunity for regenerative therapeutics. While current regenerative therapies such as autologous disc chondrocyte transplantation (ADCT), allogeneic juvenile chondrocyte implantation (NuQu®), and immunoselected allogeneic adipose derived precursor cells (Mesoblast) show exciting clinical potential, limitations remain. The heterogeneity of preclinical approaches and the paucity of clinical guidance have limited translational outcomes in disc repair, lagging almost a decade behind cartilage repair. Advances in cartilage repair have evolved to single step approaches with improved orthopedic repair and regeneration. Elements from cartilage regeneration endeavors could be adopted and applied to harness translatable approaches and deliver a clinically and economically feasible regenerative surgery for back pain. In this article, we trace the developments behind the translational success of cartilage repair, examine elements to consider in achieving disc regeneration, and the need for surgical redesign. We further discuss clinical parameters, objectives, and coordination required to deliver improved regenerative surgery. Cell source, processing, and delivery modalities are key issues to be addressed in considering surgical redesign. Advances in biomanufacturing, tissue cryobanking, and point of care cell processing technology may enable intraoperative solutions for single step procedures. To maximize translational success a triad partnership between clinicians, industry, and researchers will be critical in providing instructive clinical guidelines for design as well as practical and economic considerations. This will allow a consensus in research ventures and add regenerative surgery into the algorithm in managing and treating a debilitating condition such as back pain. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:8-22, 2017.

Systemic Delivery of Bone Marrow Mesenchymal Stem Cells for In Situ Intervertebral Disc Regeneration

Cell therapies for intervertebral disc (IVD) regeneration presently rely on transplantation of IVD cells or stem cells directly to the lesion site. Still, the harsh IVD environment, with low irrigation and high mechanical stress, challenges cell administration and survival. In this study, we addressed systemic transplantation of allogeneic bone marrow mesenchymal stem cells (MSCs) intravenously into a rat IVD lesion model, exploring tissue regeneration via cell signaling to the lesion site. MSC transplantation was performed 24 hours after injury, in parallel with dermal fibroblasts as a control; 2 weeks after transplantation, animals were killed. Disc height index and histological grading score indicated less degeneration for the MSC‐transplanted group, with no significant changes in extracellular matrix composition. Remarkably, MSC transplantation resulted in local downregulation of the hypoxia responsive GLUT‐1 and in significantly less herniation, with higher amounts of Pax5+ B lymphocytes and no alterations in CD68+ macrophages within the hernia. The systemic immune response was analyzed in the blood, draining lymph nodes, and spleen by flow cytometry and in the plasma by cytokine array. Results suggest an immunoregulatory effect in the MSC‐transplanted animals compared with control groups, with an increase in MHC class II+ and CD4+ cells, and also upregulation of the cytokines IL‐2, IL‐4, IL‐6, and IL‐10, and downregulation of the cytokines IL‐13 and TNF‐α. Overall, our results indicate a beneficial effect of systemically transplanted MSCs on in situ IVD regeneration and highlight the complex interplay between stromal cells and cells of the immune system in achieving successful tissue regeneration. Stem Cells Translational Medicine2017;6:1029–1039

Electrospun scaffold containing TGF-β1 promotes human mesenchymal stem cell differentiation towards a nucleus pulposus-like phenotype under hypoxia

The study was aimed at evaluating the effect of electrospun scaffold containing TGF-β1 on promoting human mesenchymal stem cells (MSCs) differentiation towards a nucleus pulposus-like phenotype under hypoxia. Two kinds of nanofibrous scaffolds containing TGF-β1 were fabricated using uniaxial electrospinning (Group I) and coaxial electrospinning (Group II). Human MSCs were seeded on both kinds of scaffolds and cultured in a hypoxia chamber (2% O2), and then the scaffolds were characterised. Cell proliferation and differentiation were also evaluated after 3 weeks of cell culture. Results showed that both kinds of scaffolds shared similar diameter distributions and protein release. However, Group I scaffolds were more hydrophilic than that of Group II. Both kinds of scaffolds induced the MSCs to differentiate towards the nucleus pulposus-type phenotype in vitro. In addition, the expression of nucleus pulposus-associated genes (aggrecan, type II collagen, HIF-1α and Sox-9) in Group I increased more than that of Group II. These results indicate that electrospinning nanofibrous scaffolds containing TGF-β1 supports the differentiation of MSCs towards the pulposus-like phenotype in a hypoxia chamber, which would be a more appropriate choice for nucleus pulposus regeneration.

Collagen-low molecular weight hyaluronic acid semi-interpenetrating network loaded with gelatin microspheres for cell and growth factor delivery for nucleus pulposus regeneration

Intervertebral disc (IVD) degeneration is one of the main causes of low back pain. Current surgical treatments are complex and generally do not fully restore spine mobility. Development of injectable extracellular matrix-based hydrogels offers an opportunity for minimally invasive treatment of IVD degeneration. Here we analyze a specific formulation of collagen-low molecular weight hyaluronic acid (LMW HA) semi-interpenetrating network (semi-IPN) loaded with gelatin microspheres as a potential material for tissue engineering of the inner part of the IVD, the nucleus pulposus (NP). The material displayed a gel-like behavior, it was easily injectable as demonstrated by suitable tests and did not induce cytotoxicity or inflammation. Importantly, it supported the growth and chondrogenic differentiation potential of mesenchymal stem cells (MSC) and nasal chondrocytes (NC) in vitro and in vivo. These properties of the hydrogel were successfully combined with TGF-β3 delivery by gelatin microspheres, which promoted the chondrogenic phenotype. Altogether, collagen-LMW HA loaded with gelatin microspheres represents a good candidate material for NP tissue engineering as it combines important rheological, functional and biological features.

Biomaterials for Bone Regenerative Engineering

Strategies for bone tissue regeneration have been continuously evolving for the last 25 years since the introduction of the "tissue engineering" concept. The convergence of the life, physical, and engineering sciences has brought in several advanced technologies available to tissue engineers and scientists. This resulted in the creation of a new multidisciplinary field termed as "regenerative engineering". In this article, the role of biomaterials in bone regenerative engineering is systematically reviewed to elucidate the new design criteria for the next generation of biomaterials for bone regenerative engineering. The exemplary design of biomaterials harnessing various materials characteristics towards successful bone defect repair and regeneration is highlighted. Particular attention is given to the attempts of incorporating advanced materials science, stem cell technologies, and developmental biology into biomaterials design to engineer and develop the next generation bone grafts.

Injectable microcryogels reinforced alginate encapsulation of mesenchymal stromal cells for leak-proof delivery and alleviation of canine disc degeneration

In situ crosslinked thermo-responsive hydrogel applied for minimally invasive treatment of intervertebral disc degeneration (IVDD) may not prevent extrusion of cell suspension from injection site due to high internal pressure of intervertebral disc (IVD), causing treatment failure or osteophyte formation. In this study, mesenchymal stromal cells (MSCs) were encapsulated in alginate precursor and loaded into previously developed macroporous PGEDA-derived microcryogels (PMs) to form three-dimensional (3D) microscale cellular niches, enabling non-thermo-responsive alginate hydrogel to be injectable. The PMs reinforced alginate hydrogel showed superior elasticity compared to alginate hydrogel alone and could well protect encapsulated cells through injection. Chondrogenic committed MSCs in the injectable microniches expressed higher level of nucleus pulposus (NP) cell markers compared to 2D cultured cells. In an ex vivo organ culture model, injection of MSCs-laden PMs into NP tissue prevented cell leakage, improved cell retention and survival compared to free cell injection. In canine IVDD models, alleviated degeneration was observed in MSCs-laden PMs treated group after six months which was superior to other treated groups. Our results provide in-depth demonstration of injectable alginate hydrogel reinforced by PMs as a leak-proof cell delivery system for augmented regenerative therapy of IVDD in canine models.

Natural-based nanocomposites for bone tissue engineering and regenerative medicine: a review

Tissue engineering and regenerative medicine has been providing exciting technologies for the development of functional substitutes aimed to repair and regenerate damaged tissues and organs. Inspired by the hierarchical nature of bone, nanostructured biomaterials are gaining a singular attention for tissue engineering, owing their ability to promote cell adhesion and proliferation, and hence new bone growth, compared with conventional microsized materials. Of particular interest are nanocomposites involving biopolymeric matrices and bioactive nanosized fillers. Biodegradability, high mechanical strength, and osteointegration and formation of ligamentous tissue are properties required for such materials. Biopolymers are advantageous due to their similarities with extracellular matrices, specific degradation rates, and good biological performance. By its turn, calcium phosphates possess favorable osteoconductivity, resorbability, and biocompatibility. Herein, an overview on the available natural polymer/calcium phosphate nanocomposite materials, their design, and properties is presented. Scaffolds, hydrogels, and fibers as biomimetic strategies for tissue engineering, and processing methodologies are described. The specific biological properties of the nanocomposites, as well as their interaction with cells, including the use of bioactive molecules, are highlighted. Nanocomposites in vivo studies using animal models are also reviewed and discussed.