In vitro and in vivo co-culture of chondrocytes and bone marrow stem cells in photocrosslinked PCL-PEG-PCL hydrogels enhances cartilage formation

Redifferentiation of genetically modified dedifferentiated chondrocytes in a microcavitary hydrogel

OBJECTIVES

We genetically modified dedifferentiated chondrocytes (DCs) using lentiviral vectors and adenoviral vectors encoding TGF-β3 (referred to as transgenic groups below) and encapsulated these DCs in the microcavitary hydrogel and investigated the combinational effect on redifferentiation of the genetically manipulated DCs.

RESULTS

The Cell Counting Kit-8 data indicated that both transgenic groups exhibited significantly higher cell viability in the first week but inferior cell viability in the subsequent timepoints compared with those of the control group. Real-time polymerase chain reaction and western blot analysis results demonstrated that both transgenic groups had a better effect on redifferentiation to some extent, as evidenced by higher expression levels of chondrogenic genes, suggesting the validity of combination with transgenic DCs and the microcavitary hydrogel on redifferentiation. Although transgenic DCs with adenoviral vectors presented a superior extent of redifferentiation, they also expressed greater levels of the hypertrophic gene type X collagen. It is still worth further exploring how to deliver TGF-β3 more efficiently and optimizing the appropriate parameters, including concentration and duration.

CONCLUSIONS

The results demonstrated the better redifferentiation effect of DCs with the combinational use of transgenic TGF-β3 and a microcavitary alginate hydrogel and implied that DCs would be alternative seed cells for cartilage tissue engineering due to their easily achieved sufficient cell amounts through multiple passages and great potential to redifferentiate to produce cartilaginous extracellular matrix.

Biomimetic Injectable Hydrogel Based on Methacrylate-Modified Silk Fibroin Embedded with Kartogenin for Superficial Cartilage Regeneration

The weak regeneration ability of chondrocytes is one of the main reasons that limit the therapeutic effect of clinical cartilage injury. Injectable hydrogels are potential scaffolds for cartilage tissue engineering with advantages such as minimally invasive surgery, porous structure, and drug sustained-release ability. At present, many biomaterials have been developed for the repair of deep cartilage defects. However, cartilage injury often begins on the surface, which requires us to propose a treatment strategy suitable for superficial cartilage injury repair. In this study, we fabricated a biomimetic injectable hydrogel based on methacrylate-modified silk fibroin (SilMA) embedded with kartogenin (KGN). The SilMA/KGN hydrogels have good biohistocompatibility and the ability to promote cartilage differentiation. In addition, SEM results show that it has a porous structure conducive to cell adhesion and proliferation. Most importantly, it has demonstrated remarkable superficial cartilage repair ability in vivo, showing potential in cartilage tissue engineering.

In situ Hydrogels Prepared by Photo-initiated Crosslinking of Acrylated Polymers for Local Delivery of Antitumor Drugs

A triblock copolymer was synthesized by ring opening polymerization of ε-caprolactone in the presence of poly(ethylene glycol) (PEG). The resulted PCL-PEG-PCL triblock copolymer, PEG and monomethoxy (MPEG) were functionalized by end group acrylation. NMR and FT-IR analyses evidenced the successful synthesis and functionalization of polymers. A series of photo-crosslinked hydrogels composed of acrylated PEG-PCL-Acr and MPEG-Acr or PEG-Acr were prepared by exposure to visible light using lithium phenyl-2,4,6-trimethylbenzoylphosphinate as initiator. The hydrogels present a porous and interconnected structure as shown by SEM. The swelling performance of hydrogels is closely related to the crosslinking density and hydrophilic content. Addition of MPEG or PEG results in increase in water absorption capacity of hydrogels. In vitro degradation of hydrogels was realized in the presence of a lipase from porcine pancreas. Various degradation rates were obtained which mainly depend on the hydrogel composition. MTT assay confirmed the good biocompatibility of hydrogels. Importantly, in situ gelation was achieved by irradiation of a precursor solution injected in the abdomen of mice. Doxorubicin (DOX) was selected as a model antitumor drug to evaluate the potential of hydrogels in cancer therapy. Drug-loaded hydrogels were prepared by in situ encapsulation. In vitro drug release studies showed a sustained release during 28 days with small burst release. DOX-loaded hydrogels exhibit antitumor activity against A529 lung cancer cells comparable to free drug, suggesting that injectable in situ hydrogel with tunable properties could be most promising for local drug delivery in cancer therapy.

Thermosensitive and biodegradable PCL-based hydrogels: potential scaffolds for cartilage tissue engineering

Due to a lack of sufficient blood supply and unique physicochemical properties, the treatment of injured cartilage is laborious and needs an efficient strategy. Unfortunately, most of the current therapeutic approaches are, but not completely, unable to restore the function of injured cartilage. Tissue engineering-based modalities are an alternative option to reconstruct the injured tissue. Considering the unique structure and consistency of cartilage tissue (osteochondral junction), it is mandatory to apply distinct biomaterials with unique properties slightly different from scaffolds used for soft tissues. PCL is extensively used for the fabrication of fine therapeutic scaffolds to accelerate the restorative process. Thermosensitive PCL hydrogels with distinct chemical compositions have paved the way for sophisticated cartilage regeneration. This review aimed to collect recent findings regarding the application of PCL in hydrogels blended with natural, synthetic materials in the context of cartilage healing.

Preparation of porous PCL-PEG-PCL scaffolds using supercritical carbon dioxide

In this study, the Supercritical Carbon Dioxide (scCO₂) gas foaming procedure was used in the preparation of scaffolds containing the model drug dexamethasone (DXMT). The method used did not include an organic solvent thus making it a safe method. The ring-opening polymerization of PCL-PEG-PCL (PCEC) triblock was conducted using an organocatalyst [1,8 diazabicyclo [5.4.0] undec-7-ene (DBU)]. After mixing 5.0 g of DXMT with 50.0 g of PCEC, hydraulic pressure was applied to compress the mixed powder into disc-like tablets. The tablet-like scaffold of the triblock containing DXMT was inserted into a scCO₂ gas-foaming device. The peak porosity percentage of the synthesized triblock was found to be 55.58 %. Pressure, temperature, soaking time and the time required to depressurize were recorded as 198 bar, 50 °C, 2.0 h, and 28 min respectively. After treatment with scCO₂, the scaffolds experienced an almost full release of DXMT in vitro after 30 days (83.74 ± 1.54 % vs 52.24 ± 2.03 % before scCO₂ treatment). In conclusion, the results proved that the scCO₂ gas foaming procedure could be employed for constructing modifiable PCEC scaffolds with plausible porosity and structural and morphological features which can manipulate drug release.

Preparation and properties of a photocrosslinked MCl n -doped PDMA-g-PSMA hydrogel

To treat damaged joint areas, photocrosslinked hydrophobically associating PDMA-g-PSMA hydrogels can act as mild and easily regulated materials due to their rich pore structure, which have been widely applied in articular cartilage replacement research. In this study, the effects of ADS-MCl _n (ADS-NaCl, ADS-MgCl₂ and ADS-CaCl₂) doping systems on the micro morphology, mechanical, self-healing, and friction properties and cytotoxicity of PDMA-g-PSMA hydrogels were studied. The results showed that the solubilization behavior of the ADS-MCl _n ionic micelles affected the hydrophobic association stability, thereby changing the toughness, self-healing and friction properties of the hydrogel. Ca²⁺-doping resulted in the crystallization and precipitation of the anionic surfactants, destroying the solubilization ability of the ionic micelles for the hydrophobic units, and thus hydrogels with high hardness, low toughness and no self-healing function were obtained. Doping with Na⁺ greatly improved the dissolving power of the ADS micelles for SMA, yielding PDMA-g-PSMA hydrogels with good mechanical strength and good self-healing ability. However, in this case, a drawback is that the Na⁺-doped system will lose its components during the swelling process, leading to the degradation of its self-healing performance. Interestingly, Mg²⁺ doping resulted in the formation of highly stable ADS micellar aggregates, and then PDMA-g-PSMA hydrogels with a lower friction coefficient (0.023), less wear (35.0 mg), higher elongation at break and 100% self-healing efficiency were obtained. The hydrogel products obtained from the three doping systems all exhibited good biocompatibility. Our research provides important guidelines for the design and preparation of anti-friction artificial articular cartilage.

Lithium-Containing Biomaterials Stimulate Cartilage Repair through Bone Marrow Stromal Cells-Derived Exosomal miR-455-3p and Histone H3 Acetylation

The repair of damaged cartilage still remains a great challenge in clinic. It is demonstrated that bone marrow stromal cells (BMSCs)-chondrocytes communication is of great significance for cartilage repair. Moreover, BMSCs have been confirmed to enhance biological function of chondrocytes via exosome-mediated paracrine pathway. Lithium-containing scaffolds have been reported to effectively promote cartilage regeneration; however, whether lithium-containing biomaterial could facilitate cartilage regeneration through regulating BMSCs-derived exosomes has not been illustrated. In the study, the model lithium-substituted bioglass ceramic (Li-BGC) is selected and regulatory effects of BMSCs-derived exosomes after Li-BGC treatment (Li-BGC-Exo) are systemically evaluated. The data reveal that Li-BGC-Exo notably promotes chondrogenesis, which attributes to the upregulated exosomal miR-455-3p transfer, consequently leads to suppression of histone deacetylase 2 (HDAC2) and enhanced histone H3 acetylation in chondrocytes. Notably, BMSCs-derived exosomes after LiCl treatment (LiCl-Exo) exhibits the similar regulatory effect with Li-BGC-Exo, indicating that the pro-chondrogenesis capability of them is mainly owing to the lithium ions. Furthermore, the in vivo study proves that LiCl-Exo remarkably facilitates cartilage regeneration. The research may provide novel possibility for the intrinsic mechanism of chondrogenesis trigged by lithium-containing biomaterials, and suggests that application of lithium-containing scaffolds may be a promising strategy for cartilage regeneration.

Exosomes derived from hypoxia preconditioned mesenchymal stem cells laden in a silk hydrogel promote cartilage regeneration via the miR-205-5p/PTEN/AKT pathway

Tissue engineering has promising prospects for cartilage regeneration. However, there remains an urgent need to harvest high quality seed cells. Bone marrow mesenchymal cells (BMSCs), and in particular their exosomes, might promote the function of articular chondrocytes (ACs) via paracrine mechanisms. Furthermore, preconditioned BMSCs could provide an enhanced therapeutic effect. BMSCs naturally exist in a relatively hypoxic environment (1%-5% O₂); however, they are usually cultured under higher oxygen concentrations (21% O₂). Herein, we hypothesized that hypoxia preconditioned exosomes (H-Exos) could improve the quality of ACs and be more conducive to cartilage repair. In our study, we compared the effects of exosomes derived from BMSCs preconditioned with hypoxia and normoxia (N-Exos) on ACs, demonstrating that H-Exos significantly promoted the proliferation, migration, anabolism and anti-inflammation effects of ACs. Furthermore, we confirmed that hypoxia preconditioning upregulated the expression of miR-205-5p in H-Exos, suggesting that ACs were promoted via the miR-205-5p/PTEN/AKT pathway. Finally, an injectable silk fibroin (SF) hydrogel containing ACs and H-Exos (SF/ACs/H-Exos) was utilized to repair cartilage defects and effectively promote cartilage regeneration in vivo. The application of SF/ACs/H-Exos hydrogel in cartilage regeneration therefore has promising prospects. STATEMENT OF SIGNIFICANCE: Cartilage tissue engineering (CTE) has presented a promising prospect. However, the quality of seed cells is an important factor affecting the repair efficiency. Our study demonstrates for the first time that the exosomes derived from hypoxia preconditioned BMSCs (H-Exos) effectively promote the proliferation, migration and anabolism of chondrocytes and inhibit inflammation through miR-205-5p/PTEN/AKT pathway. Furthermore, we fabricated an injectable silk fibrion (SF) hydrogel to preserve and sustained release H-Exos. A complex composed of SF hydrogel, H-Exos and chondrocytes can effectively promote the regeneration of cartilage defects. Therefore, this study demonstrates that hypoxia pretreatment could optimize the therapeutic effects of BMSCs-derived exosomes, and the combination of exosomes and SF hydrogel could be a promising therapeutic method for cartilage regeneration.

Tissue Engineering for Tracheal Replacement: Strategies and Challenges

The critical feature in trachea replacement is to provide a hollow cylindrical framework that is laterally stable and longitudinally flexible, facilitating cartilage and epithelial tissue formation. Despite advanced techniques and sources of materials used, most inherent challenges are related to the complexity of its anatomy. Limited blood supply leads to insufficient regenerative capacity for cartilage and epithelium. Natural and synthetic scaffolds, different types of cells, and growth factors are part of tissue engineering approaches with varying outcomes. Pre-vascularization remains one of the crucial factors to expedite the regenerative process in tracheal reconstruction. This review discusses the challenges and strategies used in tracheal tissue engineering, focusing on scaffold implantation in clinical and preclinical studies conducted in recent decades.

[Research progress of different cell seeding densities and cell ratios in cartilage tissue engineering]

OBJECTIVE

To review the research progress of different cell seeding densities and cell ratios in cartilage tissue engineering.

METHODS

The literature about tissue engineered cartilage constructed with three-dimensional scaffold was extensively reviewed, and the seeding densities and ratios of most commonly used seed cells were summarized.

RESULTS

Articular chondrocytes (ACHs) and bone marrow mesenchymal stem cells (BMSCs) are the most commonly used seed cells, and they can induce hyaline cartilage formation in vitro and in vivo. Cell seeding density and cell ratio both play important roles in cartilage formation. Tissue engineered cartilage with good quality can be produced when the cell seeding density of ACHs or BMSCs reaches or exceeds that in normal articular cartilage. Under the same culture conditions, the ability of pure BMSCs to build hyaline cartilage is weeker than that of pure ACHs or co-culture of both.

CONCLUSION

Due to the effect of scaffold materials, growth factors, and cell passages, optimal cell seeding density and cell ratio need further study.

A Single-Cell Culture System for Dissecting Microenvironmental Signaling in Development and Disease of Cartilage Tissue

Cartilage tissue is comprised of extracellular matrix and chondrocytes, a cell type with very low cellular turnover in adults, providing limited capacity for regeneration. However, in development a significant number of chondrocytes actively proliferate and remodel the surrounding matrix. Uncoupling the microenvironmental influences that determine the balance between clonogenic potential and terminal differentiation of these cells is essential for the development of novel approaches for cartilage regeneration. Unfortunately, most of the existing methods are not applicable for the analysis of functional properties of chondrocytes at a single cell resolution. Here we demonstrate that a novel 3D culture method provides a long-term and permissive in vitro niche that selects for highly clonogenic, colony-forming chondrocytes which maintain cartilage-specific matrix production, thus recapitulating the in vivo niche. As a proof of concept, clonogenicity of Sox9^IRES–EGFP mouse chondrocytes is almost exclusively found in the highest GFP⁺ fraction known to be enriched for chondrocyte progenitor cells. Although clonogenic chondrocytes are very rare in adult cartilage, we have optimized this system to support large, single cell-derived chondrogenic organoids with complex zonal architecture and robust chondrogenic phenotype from adult pig and human articular chondrocytes. Moreover, we have demonstrated that growth trajectory and matrix biosynthesis in these organoids respond to a pro-inflammatory environment. This culture method offers a robust, defined and controllable system that can be further used to interrogate the effects of various microenvironmental signals on chondrocytes, providing a high throughput platform to assess genetic and environmental factors in development and disease.

Vascularization of tissue engineered cartilage - Sequential in vivo MRI display functional blood circulation

Establishing functional circulation in bioengineered tissue after implantation is vital for the delivery of oxygen and nutrients to the cells. Native cartilage is avascular and thrives on diffusion, which in turn depends on proximity to circulation. Here, we investigate whether a gridded three-dimensional (3D) bioprinted construct would allow ingrowth of blood vessels and thus prove a functional concept for vascularization of bioengineered tissue. Twenty 10 × 10 × 3-mm 3Dbioprinted nanocellulose constructs containing human nasal chondrocytes or cell-free controls were subcutaneously implanted in 20 nude mice. Over the next 3 months, the mice were sequentially imaged with a 7 T small-animal MRI system, and the diffusion and perfusion parameters were analyzed. The chondrocytes survived and proliferated, and the shape of the constructs was well preserved. The diffusion coefficient was high and well preserved over time. The perfusion and diffusion patterns shown by MRI suggested that blood vessels develop over time in the 3D bioprinted constructs; the vessels were confirmed by histology and immunohistochemistry. We conclude that 3D bioprinted tissue with a gridded structure allows ingrowth of blood vessels and has the potential to be vascularized from the host. This is an essential step to take bioengineered tissue from the bench to clinical practice.

Gradient chondroitin sulfate/poly (γ-glutamic acid) hydrogels inducing differentiation of stem cells for cartilage tissue engineering

Based on the gradient distribution of structure and composition in biological cartilage tissue, we designed a gradient hydrogel scaffold by the moving photomask, using chondroitin sulfate and poly (γ-glutamic acid) as crude materials. The hydrogel scaffold had a gradient distribution of cross-linking density, which can be verified from the results of SEM and swelling behavior. Besides, the hydrogel exhibited great viscoelastic, toughness (70% strain), and strength properties (600 kPa). Additionally, the gradient hydrogel's superior cell compatibility was proved through the MTT, live/dead staining assays, and 3D cell culture experiments. Remarkably, the results of in vitro stem cell differentiation experiments showed that the duration of light directly affected the differentiation extent of stem cells, demonstrating that the gradient hydrogel scaffold can better simulate the function of natural cartilage than the homogeneous one. Due to these outstanding characteristics, this gradient hydrogel is a potential scaffold for cartilage tissue engineering.

Exquisite design of injectable Hydrogels in Cartilage Repair

Cartilage damage is still a threat to human beings, yet there is currently no treatment available to fully restore the function of cartilage. Recently, due to their unique structures and properties, injectable hydrogels have been widely studied and have exhibited high potential for applications in therapeutic areas, especially in cartilage repair. In this review, we briefly introduce the properties of cartilage, some articular cartilage injuries, and now available treatment strategies. Afterwards, we propose the functional and fundamental requirements of injectable hydrogels in cartilage tissue engineering, as well as the main advantages of injectable hydrogels as a therapy for cartilage damage, including strong plasticity and excellent biocompatibility. Moreover, we comprehensively summarize the polymers, cells, and bioactive molecules regularly used in the fabrication of injectable hydrogels, with two kinds of gelation, i.e., physical and chemical crosslinking, which ensure the excellent design of injectable hydrogels for cartilage repair. We also include novel hybrid injectable hydrogels combined with nanoparticles. Finally, we conclude with the advances of this clinical application and the challenges of injectable hydrogels used in cartilage repair.

3D Extracellular Matrix Mimics: Fundamental Concepts and Role of Materials Chemistry to Influence Stem Cell Fate

Synthetic 3D extracellular matrices (ECMs) find application in cell studies, regenerative medicine, and drug discovery. While cells cultured in a monolayer may exhibit unnatural behavior and develop very different phenotypes and genotypes than in vivo, great efforts in materials chemistry have been devoted to reproducing in vitro behavior in in vivo cell microenvironments. This requires fine-tuning the biochemical and structural actors in synthetic ECMs. This review will present the fundamentals of the ECM, cover the chemical and structural features of the scaffolds used to generate ECM mimics, discuss the nature of the signaling biomolecules required and exploited to generate bioresponsive cell microenvironments able to induce a specific cell fate, and highlight the synthetic strategies involved in creating functional 3D ECM mimics.

Biomaterials for Personalized Cell Therapy

Cell therapy has already had an important impact on healthcare and provided new treatments for previously intractable diseases. Notable examples include mesenchymal stem cells for tissue regeneration, islet transplantation for diabetes treatment, and T cell delivery for cancer immunotherapy. Biomaterials have the potential to extend the therapeutic impact of cell therapies by serving as carriers that provide 3D organization and support cell viability and function. With the growing emphasis on personalized medicine, cell therapies hold great potential for their ability to sense and respond to the biology of an individual patient. These therapies can be further personalized through the use of patient-specific cells or with precision biomaterials to guide cellular activity in response to the needs of each patient. Here, the role of biomaterials for applications in tissue regeneration, therapeutic protein delivery, and cancer immunotherapy is reviewed, with a focus on progress in engineering material properties and functionalities for personalized cell therapies.

Advances of injectable hydrogel-based scaffolds for cartilage regeneration

Articular cartilage is an important load-bearing tissue distributed on the surface of diarthrodial joints. Due to its avascular, aneural and non-lymphatic features, cartilage has limited self-regenerative properties. To date, the utilization of biomaterials to aid in cartilage regeneration, especially through the use of injectable scaffolds, has attracted considerable attention. Various materials, therapeutics and fabrication approaches have emerged with a focus on manipulating the cartilage microenvironment to induce the formation of cartilaginous structures that have similar properties to the native tissues. In particular, the design and fabrication of injectable hydrogel-based scaffolds have advanced in recent years with the aim of enhancing its therapeutic efficacy and improving its ease of administration. This review summarizes recent progress in these efforts, including the structural improvement of scaffolds, network cross-linking techniques and strategies for controlled release, which present new opportunities for the development of injectable scaffolds for cartilage regeneration.

Matrix Production in Chondrocytes Transfected with Sex Determining Region Y-Box 9 and Telomerase Reverse Transcriptase Genes: An In Vitro Evaluation from Monolayer Culture to Three-Dimensional Culture

Background

This study aimed to observe the cartilaginous matrix production in SRY (sex determining region Y)-box 9 (SOX9)- and/or telomerase reverse transcriptase (TERT)-transfected chondrocytes from monolayer to three-dimensional (3D) culture.

Methods

The genes were transferred into chondrocytes at passage-1 (P1) via lipofection. The post-transfected chondrocytes (SOX9-, TERT- and SOX9/TERT) were analysed at P1, P2 and P3. The non-transfected group was used as control. The 3D culture was established using the chondrocytes seeded in a disc-shaped PLGA/fibrin and PLGA scaffolds. The resulting 3D "cells-scaffolds" constructs were analysed at week-1, -2 and -3. The histoarchitecture was evaluated using haematoxylin and eosin, alcian blue and safranin o stains. The quantitative sulphated glycosaminoglycan (sGAG) content was measured using biochemical assay. The cartilage-specific markers expression were analysed via real-time polymerase chain reaction.

Results

All monolayer cultured chondrocytes showed flattened, fibroblast-like appearance throughout passages. Proteoglycan and sGAG were not detected at the pericellular matrix region of the chondrocytes. The sGAG content assay indicated the matrix production depletion in the culture. The cartilage-specific markers, COL2A1 and ACAN, were downregulated. However, the dedifferentiation marker, COL1A1 was upregulated. In 3D "cells-scaffolds" constructs, regardless of transfection groups, chondrocytes seeded in PLGA/fibrin showed a more uniform distribution and produced denser matrix than the PLGA group especially at week-3. Both sGAG and proteoglycan were clearly visualised in the constructs, supported by the increment of sGAG content, quantitatively. Both COL2A1 and ACAN were upregulated in SOX9/TERT-PLGA and SOX9/TERT-PLGA/fibrin respectively. While, COL1A1 was downregulated in SOX9/TERT-PLGA.

Conclusion

These findings indicated that the SOX9/TERT-transfected chondrocytes incorporation into 3D scaffolds facilitates the cartilage regeneration which is viable structurally and functionally.

A review of emerging bone tissue engineering via PEG conjugated biodegradable amphiphilic copolymers

Defects in bones can be caused by a plethora of reasons, such as trauma or illness, and in many cases, it poses challenges to the current treatment approaches for bone repair. With increasing demand of bone bioengineering in tissue transplant, there is a need to source for sustainable solutions to induce bone regeneration. Polymeric biomaterials have been identified as a promising approach due to its excellent biocompatibility and controllable biodegradability. Specifically, poly(ethylene glycol) (PEG) is one of the most commonly investigated polymer for use in bio-related application due to its bioinertness and versatility. Furthermore, the hydrophilic nature enables it to be incorporated with hydrophobic but biodegradable polymers like, polylactide (PLA) and polycaprolactone (PCL), to create an amphiphilic polymer. This article reviews the recent synthetic strategies available for the construction of PEG conjugated polymeric system, analysis of PEG influence on the material properties, and provides an overview of its application in bone engineering.

Progress of co-culture systems in cartilage regeneration

INTRODUCTION

Cartilage tissue engineering has rapidly developed in recent decades, exhibiting promising potential to regenerate and repair cartilage. However, the origin of a large amount of a suitable seed cell source is the major bottleneck for the further clinical application of cartilage tissue engineering. The use of a monoculture of passaged chondrocytes or mesenchymal stem cells results in undesired outcomes, such as fibrocartilage formation and hypertrophy. In the last two decades, co-cultures of chondrocytes and a variety of mesenchymal stem cells have been intensively investigated in vitro and in vivo, shedding light on the perspective of co-culture in cartilage tissue engineering.

AREAS COVERED

We summarize the recent literature on the application of heterologous cell co-culture systems in cartilage tissue engineering and compare the differences between direct and indirect co-culture systems as well as discuss the underlying mechanisms.

EXPERT OPINION

Co-culture system is proven to address many issues encountered by monocultures in cartilage tissue engineering, including reducing the number of chondrocytes needed and alleviating the dedifferentiation of chondrocytes. With the further development and knowledge of biomaterials, cartilage tissue engineering that combines the co-culture system and advanced biomaterials is expected to solve the difficult problem regarding the regeneration of functional cartilage.

Multilineage Constructs for Scaffold-Based Tissue Engineering: A Review of Tissue-Specific Challenges

There is a growing interest in the regeneration of tissue in interfacial regions, where biological, physical, and chemical attributes vary across tissue type. The simultaneous use of distinct cell lineages can help in developing in vitro structures, analogous to native composite tissues. This literature review gathers the recent reports that have investigated multiple cell types of various sources and lineages in a coculture system for tissue-engineered constructs. Such studies aim at mimicking the native organization of tissues and their interfaces, and/or to improve the development of complex tissue substitutes. This paper thus distinguishes itself from those focusing on technical aspects of coculturing for a single specific tissue. The first part of this review is dedicated to variables of cocultured tissue engineering such as scaffold, cells, and in vitro culture environment. Next, tissue-specific coculture methods and approaches are covered for the most studied tissues. Finally, cross-analysis is performed to highlight emerging trends in coculture principles and to discuss how tissue-specific challenges can inspire new approaches for regeneration of different interfaces to improve the outcomes of various tissue engineering strategies.

Chondrocytes and stem cells in 3D-bioprinted structures create human cartilage in vivo

Cartilage repair and replacement is a major challenge in plastic reconstructive surgery. The development of a process capable of creating a patient-specific cartilage framework would be a major breakthrough. Here, we described methods for creating human cartilage in vivo and quantitatively assessing the proliferative capacity and cartilage-formation ability in mono- and co-cultures of human chondrocytes and human mesenchymal stem cells in a three-dimensional (3D)-bioprinted hydrogel scaffold. The 3D-bioprinted constructs (5 × 5 × 1.2 mm) were produced using nanofibrillated cellulose and alginate in combination with human chondrocytes and human mesenchymal stem cells using a 3D-extrusion bioprinter. Immediately following bioprinting, the constructs were implanted subcutaneously on the back of 48 nude mice and explanted after 30 and 60 days, respectively, for morphological and immunohistochemical examination. During explantation, the constructs were easy to handle, and the majority had retained their macroscopic grid appearance. Constructs consisting of human nasal chondrocytes showed good proliferation ability, with 17.2% of the surface areas covered with proliferating chondrocytes after 60 days. In constructs comprising a mixture of chondrocytes and stem cells, an additional proliferative effect was observed involving chondrocyte production of glycosaminoglycans and type 2 collagen. This clinically highly relevant study revealed 3D bioprinting as a promising technology for the creation of human cartilage.

Injectable and thermosensitive TGF-β1-loaded PCEC hydrogel system for in vivo cartilage repair

Chondral defects pose a great challenge for clinicians to manage owing to the limited capacity for self-healing. Various traditional approaches have been adopted for the repair of these defects with unsatisfactory results. Cartilage tissue engineering techniques have emerged as promising strategies to enhance regeneration and overcome these traditional shortcomings. The cell-homing based technique is considered the most promising owing to its unique advantages. Thermosensitive hydrogels have been applied as scaffolds for biomedical applications with smart sol-gel response for altering environmental temperature. Transforming growth factor (TGF)-β1 is considered to be capable of promoting chondrogenesis. In this study, a novel TGF-β1-loaded poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) (PCEC) hydrogel was fabricated using simple procedures. Hydrogel characterization, rheological testing, component analysis, and assessment of sol-gel transition, in vitro degradation, and TGF-β1 release confirmed that this material possesses a porous microstructure with favorable injectability and sustained drug release. Full-thickness cartilage defects were induced on rat knees for in vivo cartilage repair for eight weeks. Micro-CT and histological evaluation provided further evidence of the optimal capacity of this novel hydrogel for cartilage regeneration with respect to that of other methods. Moreover, our results demonstrated that the cell-free hydrogel is thermosensitive, injectable, biodegradable, and capable of in vivo cartilage repair and possesses high potential and benefits for acellular cartilage tissue engineering and clinical application in the future.

Cell-laden hydrogels for osteochondral and cartilage tissue engineering

Despite tremendous advances in the field of regenerative medicine, it still remains challenging to repair the osteochondral interface and full-thickness articular cartilage defects. This inefficiency largely originates from the lack of appropriate tissue-engineered artificial matrices that can replace the damaged regions and promote tissue regeneration. Hydrogels are emerging as a promising class of biomaterials for both soft and hard tissue regeneration. Many critical properties of hydrogels, such as mechanical stiffness, elasticity, water content, bioactivity, and degradation, can be rationally designed and conveniently tuned by proper selection of the material and chemistry. Particularly, advances in the development of cell-laden hydrogels have opened up new possibilities for cell therapy. In this article, we describe the problems encountered in this field and review recent progress in designing cell-hydrogel hybrid constructs for promoting the reestablishment of osteochondral/cartilage tissues. Our focus centers on the effects of hydrogel type, cell type, and growth factor delivery on achieving efficient chondrogenesis and osteogenesis. We give our perspective on developing next-generation matrices with improved physical and biological properties for osteochondral/cartilage tissue engineering. We also highlight recent advances in biomanufacturing technologies (e.g. molding, bioprinting, and assembly) for fabrication of hydrogel-based osteochondral and cartilage constructs with complex compositions and microarchitectures to mimic their native counterparts.

STATEMENT OF SIGNIFICANCE

Despite tremendous advances in the field of regenerative medicine, it still remains challenging to repair the osteochondral interface and full-thickness articular cartilage defects. This inefficiency largely originates from the lack of appropriate tissue-engineered biomaterials that replace the damaged regions and promote tissue regeneration. Cell-laden hydrogel systems have emerged as a promising tissue-engineering platform to address this issue. In this article, we describe the fundamental problems encountered in this field and review recent progress in designing cell-hydrogel constructs for promoting the reestablishment of osteochondral/cartilage tissues. Our focus centers on the effects of hydrogel composition, cell type, and growth factor delivery on achieving efficient chondrogenesis and osteogenesis. We give our perspective on developing next-generation hydrogel/inorganic particle/stem cell hybrid composites with improved physical and biological properties for osteochondral/cartilage tissue engineering. We also highlight recent advances in biomanufacturing and bioengineering technologies (e.g. 3D bioprinting) for fabrication of hydrogel-based osteochondral and cartilage constructs.

Articular cartilage generation applying PEG-LA-DM/PEGDM copolymer hydrogels

Background

Injuries to the human native cartilage tissue are particularly problematic because cartilage has little to no ability to heal or regenerate itself. Employing a tissue engineering strategy that combines suitable cell sources and biomimetic hydrogels could be a promising alternative to achieve cartilage regeneration. However, the weak mechanical properties may be the major drawback to use fully degradable hydrogels. Besides, most of the fully degradable hydrogels degrade too fast to permit enough extracellular matrix (ECM) production for neocartilage formation. In this study, we demonstrated the feasibility of neocartilage regeneration using swine articular chondrocytes photoencapsualted into poly (ethylene glycol) dimethacrylate (PEGDM) copolymer hydrogels composed of different degradation profiles: degradable (PEG-LA-DM) and nondegradable (PEGDM) macromers in molar ratios of 50/50, 60/40, 70/30, 80/20, and 90/10.

Methods

Articular chondrocytes were isolated enzymatically from juvenile Yorkshire swine cartilage. 6 × 10⁷ cells cells were added to each milliliter of macromer/photoinitiator (I2959) solution. Nonpolymerized gel containing the cells (100 μL) was placed in cylindrical molds (4.5 mm diameter × 6.5 mm in height). The macromer/photoinitiator/chondrocyte solutions were polymerized using ultraviolet (365 nm) light at 10 mW/cm² for 10 mins. Also, an articular cartilaginous ring model was used to examine the capacity of the engineered cartilage to integrate with native cartilage. Samples in the pilot study were collected at 6 weeks. Samples in the long-term experimental groups (60/40 and 70/30) were implanted into nude mice subcutaneously and harvested at 6, 12 and 18 weeks. Additionally, cylindrical constructs that were not implanted used as time zero controls. All of the harvested specimens were examined grossly and analyzed histologically and biochemically.

Results

Histologically, the neocartilage formed in the photochemically crosslinked gels resembled native articular cartilage with chondrocytes in lacunae and surrounded by new ECM. Increases in total DNA, glycosaminoglycan, and hydroxyproline were observed over the time periods studied. The neocartilage integrated with existing native cartilage.

Conclusions

Articular cartilage generation was achieved using swine articular chondrocytes photoencapsulated in copolymer PEGDM hydrogels, and the neocartilage tissue had the ability to integrate with existing adjacent native cartilage.

Cell factory-derived bioactive molecules with polymeric cryogel scaffold enhance the repair of subchondral cartilage defect in rabbits

We have explored the potential of cell factory-derived bioactive molecules, isolated from conditioned media of primary goat chondrocytes, for the repair of subchondral cartilage defects. Enzyme-linked immunosorbent assay (ELISA) confirms the presence of transforming growth factor-β1 in an isolated protein fraction (12.56 ± 1.15 ng/mg protein fraction). These bioactive molecules were used alone or with chitosan-agarose-gelatin cryogel scaffolds, with and without chondrocytes, to check whether combined approaches further enhance cartilage repair. To evaluate this, an in vivo study was conducted on New Zealand rabbits in which a subchondral defect (4.5 mm wide × 4.5 mm deep) was surgically created. Starting after the operation, bioactive molecules were injected at the defect site at regular intervals of 14 days. Histopathological analysis showed that rabbits treated with bioactive molecules alone had cartilage regeneration after 4 weeks. However, rabbits treated with bioactive molecules along with scaffolds, with or without cells, showed cartilage formation after 3 weeks; 6 weeks after surgery, the cartilage regenerated in rabbits treated with either bioactive molecules alone or in combinations showed morphological similarities to native cartilage. No systemic cytotoxicity or inflammatory response was induced by any of the treatments. Further, ELISA was done to determine systemic toxicity, which showed no difference in concentration of tumour necrosis factor-α in blood serum, before or after surgery. In conclusion, intra-articular injection with bioactive molecules alone may be used for the repair of subchondral cartilage defects, and bioactive molecules along with chondrocyte-seeded scaffolds further enhance the repair. Copyright © 2015 John Wiley & Sons, Ltd.

Beneficial effects of coculturing synovial derived mesenchymal stem cells with meniscus fibrochondrocytes are mediated by fibroblast growth factor 1: increased proliferation and collagen synthesis

Meniscus reconstruction is in great need for orthopedic surgeons. Meniscal fibrochondrocytes transplantation was proposed to regenerate functional meniscus, with limited donor supply. We hypothesized that coculture of synovial mesenchymal stem cells (SSC) with meniscal fibrochondrocytes (me-CH) can support matrix production of me-CH, thus reducing the number of me-CH needed for meniscus reconstruction. A pellet coculture system of human SSC and me-CH was used in this study. Enhanced glycosaminoglycans (GAG) in coculture pellets were demonstrated by Alcian blue staining and GAG quantification, when compared to monoculture. More collagen synthesis was shown in coculture pellets by hydroxyproline assay. Increased proliferation of me-CH was observed in coculture. Data from BrdU staining and ELISA demonstrated that conditioned medium of SSCs enhanced the proliferation and collagen synthesis of me-CH, and this effect was blocked by neutralizing antibody against fibroblast growth factor 1 (FGF1). Western blot showed that conditioned medium of SSCs can activate mitogen-activated protein kinase (MAPK) signaling pathways by increasing the phosphorylation of mitogen-activated regulated protein kinase 1/2 (MEK) and extracellular-signal-regulated kinases 1/2 (ERK). Overall, this study provided evidence that synovial MSCs can support proliferation and collagen synthesis of fibrochondrocytes, by secreting FGF1. Coimplantation of SSC and me-CH could be a useful strategy for reconstructing meniscus.