Low-density cultures of bovine chondrocytes: effects of scaffold material and culture system

Chemokines in Cartilage Regeneration and Degradation: New Insights

Cartilage plays a crucial role in the human body by forming long bones during development and growth to bear loads on joints and intervertebral discs. However, the increasing prevalence of cartilage degenerative disorders is a growing public health concern, especially due to the poor innate regenerative capacity of cartilage. Chondrocytes are a source of several inflammatory mediators that play vital roles in the pathogenesis of cartilage disorders. Among these mediators, chemokines have been explored as potential contributors to cartilage degeneration and regeneration. Our review focuses on the progress made during the last ten years in identifying the regulators and roles of chemokines and their receptors in different mechanisms related to chondrocytes and cartilage. Recent findings have demonstrated that chemokines influence cartilage both positively and negatively. Their induction and involvement in either process depends on the local molecular environment and is both site- and time-dependent. One of the challenges in defining the role of chemokines in cartilage pathology or regeneration is the apparent redundancy in the interaction of chemokines with their receptors. Hence, it is crucial to determine, for each situation, whether targeting specific chemokines or their receptors will help in developing effective therapeutic strategies for cartilage repair.

Biomanufacturing of 3D Tissue Constructs in Microgravity and their Applications in Human Pathophysiological Studies

The growing interest in bioengineering in-vivo-like 3D functional tissues has led to novel approaches to the biomanufacturing process as well as expanded applications for these unique tissue constructs. Microgravity, as seen in spaceflight, is a unique environment that may be beneficial to the tissue-engineering process but cannot be completely replicated on Earth. Additionally, the expense and practical challenges of conducting human and animal research in space make bioengineered microphysiological systems an attractive research model. In this review, published research that exploits real and simulated microgravity to improve the biomanufacturing of a wide range of tissue types as well as those studies that use microphysiological systems, such as organ/tissue chips and multicellular organoids, for modeling human diseases in space are summarized. This review discusses real and simulated microgravity platforms and applications in tissue-engineered microphysiological systems across three topics: 1) application of microgravity to improve the biomanufacturing of tissue constructs, 2) use of tissue constructs fabricated in microgravity as models for human diseases on Earth, and 3) investigating the effects of microgravity on human tissues using biofabricated in vitro models. These current achievements represent important progress in understanding the physiological effects of microgravity and exploiting their advantages for tissue biomanufacturing.

Fabrication of chondrocytes/chondrocyte-microtissues laden fibrin gel auricular scaffold for microtia reconstruction

Fibrin gel-based scaffolds have promising potential for microtia reconstruction. Autologous chondrocytes and chondrocyte cell sheets are frequently used seed cell sources for cartilage tissue engineering. However, the aesthetic outcome of chondrocyte-based microtia reconstruction is still not satisfactory. In this study, we aimed to fabricate the chondrocytes/chondrocyte-microtissues laden fibrin gel auricular scaffold for microtia reconstruction. We designed a unique auricular mold that could fabricate a fibrin gel scaffold resembling human auricle anatomy. Primary chondrocytes were harvested from rabbit auricular cartilage, and chondrocyte cell sheets were developed. Chondrocyte-microtissues were prepared from the cell sheets. The mixture of chondrocytes/chondrocyte-microtissues was laden in fibrin gel during the auricular scaffold fabrication. The protrusions and recessed structure in the auricular scaffold surface were still clearly distinguishable. After a one-week in vitro culture, the 3 D structure and auricular anatomy of the scaffold were retained. And followed by eight-week subcutaneous implantation, cartilaginous tissue was regenerated in the artificial auricular structure as indicated by the results of H&E, Toluidine blue, Safranin O, and type II collagen (immunohistochemistry) staining. Protrusions and depressions of the auricular scaffold were slightly deformed, but the overall auricular anatomy was maintained after 8-week in vivo implantation. Extracellular matrix components content were similar in artificial auricular cartilage and rabbit native auricular cartilage. In conclusion, the mixture of chondrocytes/chondrocyte-microtissues laden fibrin gel auricular scaffold showed a promising potential for cartilaginous tissue regeneration, suggesting this as an effective approach for autologous chondrocyte-based microtia reconstruction.

* Constrained Cage Culture Improves Engineered Cartilage Functional Properties by Enhancing Collagen Network Stability

When cultured with sufficient nutrient supply, engineered cartilage synthesizes proteoglycans rapidly, producing an osmotic swelling pressure that destabilizes immature collagen and prevents the development of a robust collagen framework, a hallmark of native cartilage. We hypothesized that mechanically constraining the proteoglycan-induced tissue swelling would enhance construct functional properties through the development of a more stable collagen framework. To test this hypothesis, we developed a novel "cage" growth system to mechanically prevent tissue constructs from swelling while ensuring adequate nutrient supply to the growing construct. The effectiveness of constrained culture was examined by testing constructs embedded within two different scaffolds: agarose and cartilage-derived matrix hydrogel (CDMH). Constructs were seeded with immature bovine chondrocytes and cultured under free swelling (FS) conditions for 14 days with transforming growth factor-β before being placed into a constraining cage for the remainder of culture. Controls were cultured under FS conditions throughout. Agarose constructs cultured in cages did not expand after the day 14 caging while FS constructs expanded to 8 × their day 0 weight after 112 days of culture. In addition to the physical differences in growth, by day 56, caged constructs had higher equilibrium (agarose: 639 ± 179 kPa and CDMH: 608 ± 257 kPa) and dynamic compressive moduli (agarose: 3.4 ± 1.0 MPa and CDMH 2.8 ± 1.0 MPa) than FS constructs (agarose: 193 ± 74 kPa and 1.1 ± 0.5 MPa and CDMH: 317 ± 93 kPa and 1.8 ± 1.0 MPa for equilibrium and dynamic properties, respectively). Interestingly, when normalized to final day wet weight, cage and FS constructs did not exhibit differences in proteoglycan or collagen content. However, caged culture enhanced collagen maturation through the increased formation of pyridinoline crosslinks and improved collagen matrix stability as measured by α-chymotrypsin solubility. These findings demonstrate that physically constrained culture of engineered cartilage constructs improves functional properties through improved collagen network maturity and stability. We anticipate that constrained culture may benefit other reported engineered cartilage systems that exhibit a mismatch in proteoglycan and collagen synthesis.

Chondrogenic differentiation of ChM-I gene transfected rat bone marrow-derived mesenchymal stem cells on 3-dimensional poly (L-lactic acid) scaffold for cartilage engineering

We have explored the role of Chondromodulin-I (ChM-I) in chondrogenesis of bone marrow-derived mesenchymal stem cells (BMSCs) in 3-dimensional (3D) scaffold for cartilage tissue engineering. BMSCs of Sprague Dawley (SD) rats were cultured on poly-(L-lactic acid) [PLLA] scaffolds with different pore sizes (80-200 μm, 200-450 μm) with or without surface modification by chitosan. Cell viability, proliferation, and morphology were measured using confocal microscope and the CCK-8 method. Untransfected BMSCs, BMSCs expressing pcDNA3.1(+), BMSCs expressing plasmid pcDNA3.1 (+)/ChM-I were cultured on 3D scaffolds in standard growth medium or transforming growth factor-β1 (TGF-β1) supplemented chondrogenic induction medium in vitro for 3 weeks and the expression of collagen type II was determined. Cell-scaffolds constructs were implanted subcutaneously for 3 months in vivo. BMSCs had a higher viability and proliferation in PLLA scaffolds of pore size 200-450 μm than that of 80-200 μm, and surface modification with chitosan did not enhance cell attachment. The ChM-I gene enhanced chondrogenesis and increased collagen type II synthesis. Immunohistochemistry from in vivo study showed enhanced cartilage regeneration in BMSCs expressing pcDNA3.1 (+)/ChM-I on 3D PLLA scaffolds. It also demonstrated that TGF-β1 might promote chondrogenesis of rat BMSCs by synergizing with the ChM-I gene. ChM-I could be beneficial to future applications in cartilage repair.

Introduction to cell-hydrogel mechanosensing

The development of hydrogel-based biomaterials represents a promising approach to generating new strategies for tissue engineering and regenerative medicine. In order to develop more sophisticated cell-seeded hydrogel constructs, it is important to understand how cells mechanically interact with hydrogels. In this paper, we review the mechanisms by which cells remodel hydrogels, the influence that the hydrogel mechanical and structural properties have on cell behaviour and the role of mechanical stimulation in cell-seeded hydrogels. Cell-mediated remodelling of hydrogels is directed by several cellular processes, including adhesion, migration, contraction, degradation and extracellular matrix deposition. Variations in hydrogel stiffness, density, composition, orientation and viscoelastic characteristics all affect cell activity and phenotype. The application of mechanical force on cells encapsulated in hydrogels can also instigate changes in cell behaviour. By improving our understanding of cell-material mechano-interactions in hydrogels, this should enable a new generation of regenerative medical therapies to be developed.

Culture of equine fibroblast-like synoviocytes on synthetic tissue scaffolds towards meniscal tissue engineering: a preliminary cell-seeding study

Introduction. Tissue engineering is a new methodology for addressing meniscal injury or loss. Synovium may be an ideal source of cells for in vitro meniscal fibrocartilage formation, however, favorable in vitro culture conditions for synovium must be established in order to achieve this goal. The objective of this study was to determine cellularity, cell distribution, and extracellular matrix (ECM) formation of equine fibroblast-like synoviocytes (FLS) cultured on synthetic scaffolds, for potential application in synovium-based meniscal tissue engineering. Scaffolds included open-cell poly-L-lactic acid (OPLA) sponges and polyglycolic acid (PGA) scaffolds cultured in static and dynamic culture conditions, and PGA scaffolds coated in poly-L-lactic (PLLA) in dynamic culture conditions.

Materials and Methods. Equine FLS were seeded on OPLA and PGA scaffolds, and cultured in a static environment or in a rotating bioreactor for 12 days. Equine FLS were also seeded on PGA scaffolds coated in 2% or 4% PLLA and cultured in a rotating bioreactor for 14 and 21 days. Three scaffolds from each group were fixed, sectioned and stained with Masson’s Trichrome, Safranin-O, and Hematoxylin and Eosin, and cell numbers and distribution were analyzed using computer image analysis. Three PGA and OPLA scaffolds from each culture condition were also analyzed for extracellular matrix (ECM) production via dimethylmethylene blue (sulfated glycosaminoglycan) assay and hydroxyproline (collagen) assay. PLLA coated PGA scaffolds were analyzed using double stranded DNA quantification as areflection of cellularity and confocal laser microscopy in a fluorescent cell viability assay.

Results. The highest cellularity occurred in PGA constructs cultured in a rotating bioreactor, which also had a mean sulfated glycosaminoglycan content of 22.3 µg per scaffold. PGA constructs cultured in static conditions had the lowest cellularity. Cells had difficulty adhering to OPLA and the PLLA coating of PGA scaffolds; cellularity was inversely proportional to the concentration of PLLA used. PLLA coating did not prevent dissolution of the PGA scaffolds. All cell scaffold types and culture conditions produced non-uniform cellular distribution.

Discussion/Conclusion. FLS-seeding of PGA scaffolds cultured in a rotating bioreactor resulted in the most optimal cell and matrix characteristics seen in this study. Cells grew only in the pores of the OPLA sponge, and could not adhere to the PLLA coating of PGA scaffold, due to the hydrophobic property of PLA. While PGA culture in a bioreactor produced measureable GAG, no culture technique produced visible collagen. For this reason, and due to the dissolution of PGA scaffolds, the culture conditions and scaffolds described here are not recommended for inducing fibrochondrogenesis in equine FLS for meniscal tissue engineering.

The influence of scaffold material on chondrocytes under inflammatory conditions

Cartilage tissue engineering aims to repair damaged cartilage tissue in arthritic joints. As arthritic joints have significantly higher levels of pro-inflammatory cytokines (such as IL-1β and TNFα that cause cartilage destruction, it is critical to engineer stable cartilage in an inflammatory environment. Biomaterial scaffolds constitute an important component of the microenvironment for chondrocytes in engineered cartilage. However, it remains unclear how the scaffold material influences the response of chondrocytes seeded in these scaffolds under inflammatory stimuli. Here we have compared the responses of articular chondrocytes seeded within three different polymeric scaffolding materials (silk, collagen and polylactic acid (PLA)) to IL-1β and TNFα. These scaffolds have different physical characteristics and yielded significant differences in the expression of genes associated with cartilage matrix production and degradation, cell adhesion and cell death. The silk and collagen scaffolds released pro-inflammatory cytokines faster and had higher uptake water abilities than PLA scaffolds. Correspondingly, chondrocytes cultured in silk and collagen scaffolds maintained higher levels of cartilage matrix than those in PLA, suggesting that these biophysical properties of scaffolds may regulate gene expression and the response to inflammatory stimuli in chondrocytes. Based on this study we conclude that selecting the proper scaffold material will aid in the engineering of more stable cartilage tissues for cartilage repair, and that silk and collagen are better scaffolds in terms of supporting the stability of three-dimensional cartilage under inflammatory conditions.

Mechanical properties of native and tissue-engineered cartilage depend on carrier permeability: a bioreactor study

The implantation of osteochondral constructs-tissue-engineered (TE) cartilage on a bone substitute carrier-is a promising method to treat defects in articular cartilage. Currently, however, the TE cartilage's mechanical properties are clearly inferior to those of native cartilage. Their improvement has been the subject of various studies, mainly focusing on growth factors and physical loading during cultivation. With the approach of osteochondral constructs another aspect arises: the permeability of the carrier materials. The purpose of this study was to investigate whether and how the permeability of the subchondral bone influences the properties of native cartilage and whether the bone substitute carrier's permeability influences the TE cartilage of osteochondral constructs accordingly. Consequently, the influence of the subchondral bone's permeability on native cartilage was determined: Native porcine cartilage-bone cylinders were cultivated for 2 weeks in a bioreactor under mechanical loading with and without restricted permeability of the bone. For the TE cartilage these two permeability conditions were investigated using permeable and impermeable tricalciumphosphate carriers under equivalent cultivation conditions. All specimens were evaluated mechanically, biochemically, and histologically. The restriction of the bone's permeability significantly decreased the Young's modulus of native cartilage in vitro. No biochemical differences were found. This finding was confirmed for TE cartilage: While the biochemical parameters were not affected, a permeable carrier improved the cell morphology and mechanical properties in comparison to an impermeable one. In conclusion, the carrier permeability was identified as a determining factor for the mechanical properties of TE cartilage of osteochondral constructs.

Numerical simulation of fluid field and in vitro three-dimensional fabrication of tissue-engineered bones in a rotating bioreactor and in vivo implantation for repairing segmental bone defects

In this paper, two-dimensional flow field simulation was conducted to determine shear stresses and velocity profiles for bone tissue engineering in a rotating wall vessel bioreactor (RWVB). In addition, in vitro three-dimensional fabrication of tissue-engineered bones was carried out in optimized bioreactor conditions, and in vivo implantation using fabricated bones was performed for segmental bone defects of Zelanian rabbits. The distribution of dynamic pressure, total pressure, shear stress, and velocity within the culture chamber was calculated for different scaffold locations. According to the simulation results, the dynamic pressure, velocity, and shear stress around the surface of cell-scaffold construction periodically changed at different locations of the RWVB, which could result in periodical stress stimulation for fabricated tissue constructs. However, overall shear stresses were relatively low, and the fluid velocities were uniform in the bioreactor. Our in vitro experiments showed that the number of cells cultured in the RWVB was five times higher than those cultured in a T-flask. The tissue-engineered bones grew very well in the RWVB. This study demonstrates that stress stimulation in an RWVB can be beneficial for cell/bio-derived bone constructs fabricated in an RWVB, with an application for repairing segmental bone defects.

The role of environmental factors in regulating the development of cartilaginous grafts engineered using osteoarthritic human infrapatellar fat pad-derived stem cells

Engineering functional cartilaginous grafts using stem cells isolated from osteoarthritic human tissue is of fundamental importance if autologous tissue engineering strategies are to be used in the treatment of diseased articular cartilage. It has previously been demonstrated that human infrapatellar fat pad (IFP)-derived stem cells undergo chondrogenesis in pellet culture; however, the ability of such cells to generate functional cartilaginous grafts has not been adequately addressed. The objective of this study was to explore how environmental conditions regulate the functional development of cartilaginous constructs engineered using diseased human IFP-derived stem cells (FPSCs). FPSCs were observed to display a diminished chondrogenic potential upon encapsulation in a three-dimensional hydrogel compared with pellet culture, synthesizing significantly lower levels of glycosaminoglycan and collagen on a per cell basis. To engineer more functional cartilaginous grafts, we next explored whether additional biochemical and biophysical stimulations would enhance chondrogenesis within the hydrogels. Serum stimulation was observed to partially recover the diminished chondrogenic potential within hydrogel culture. Over 42 days, stem cells that had first been expanded in a low-oxygen environment proliferated extensively on the outer surface of the hydrogel in response to serum stimulation, assembling a dense type II collagen-positive cartilaginous tissue resembling that formed in pellet culture. The application of hydrostatic pressure did not further enhance extracellular matrix synthesis within the hydrogels, but did appear to alter the spatial accumulation of extracellular matrix leading to the formation of a more compact tissue with superior mechanically functionality. Further work is required in order to recapitulate the environmental conditions present during pellet culture within scaffolds or hydrogels in order to engineer more functional cartilaginous grafts using human osteoarthritic FPSCs.

Engineering an in-vitro model of rodent cartilage

OBJECTIVES

The purpose of this study was to identify a cell source, scaffold substrate and culture environment suitable for use in engineering an in-vitro model of rodent cartilage.

METHODS

The chondrogenic activity and stability of cells isolated at Day 18 of gestation was assessed under normoxia and hypoxia using a cytokine stimulation assay and gene expression analysis. The ability of the selected cells seeded in fibrous electrospun scaffolds to form cartilaginous tissue during longterm static and dynamic culture was assessed using immunocytochemistry and biochemical analysis.

KEY FINDINGS

Rodent fetal chondrocytes appear to have enhanced phenotypic stability compared with other cell sources. Following 16 weeks under static culture, the engineered constructs were found to have greater cellularity and collagen content that native rodent cartilage.

CONCLUSIONS

A cell source, scaffold and culture environment have been identified that support the generation of in-vitro rodent cartilage. In future work, cytokine treatment of the engineered tissues will take place to generate in-vitro osteoarthritis models.

Design of a high-throughput flow perfusion bioreactor system for tissue engineering

Flow perfusion culture is used in many areas of tissue engineering and offers several key advantages. However, one challenge to these cultures is the relatively low-throughput nature of perfusion bioreactors. Here, a flow perfusion bioreactor with increased throughput was designed and built for tissue engineering. This design uses an integrated medium reservoir and flow chamber in order to increase the throughput, limit the volume of medium required to operate the system, and simplify the assembly and operation.

Cell bricks-enriched platelet-rich plasma gel for injectable cartilage engineering - an in vivo experiment in nude mice

Clinical application of platelet-rich plasma (PRP)-based injectable tissue engineering is limited by weak mechanical properties and a rapid fibrinolytic rate. We proposed a new strategy, a cell bricks-stabilized PRP injectable system, to engineer and regenerate cartilage with stable morphology and structure in vivo. Chondrocytes from the auricular cartilage of rabbits were isolated and cultured to form cell bricks (fragmented cell sheet) or cell expansions. Fifteen nude mice were divided evenly (n = 5) into cells-PRP (C-P), cell bricks-PRP (CB-P) and cell bricks-cells-PRP (CB-C-P) groups. Cells, cell bricks or a cell bricks/cells mixture were suspended in PRP and were injected subcutaneously in animals. After 8 weeks, all the constructs were replaced by white resilient tissue; however, specimens from the CB-P and CB-C-P groups were well maintained in shape, while the C-P group appeared distorted, with a compressed outline. Histologically, all groups presented lacuna-like structures, glycosaminoglycan-enriched matrices and positive immunostaining of collagen type II. Different from the uniform structure presented in CB-C-P samples, CB-P presented interrupted, island-like chondrogenesis and contracted structure; fibrous interruption was shown in the C-P group. The highest percentage of matrix was presented in CB-C-P samples. Collagen and sGAG quantification confirmed that the CB-C-P constructs had statistically higher amounts than the C-P and CB-P groups; statistical differences were also found among the groups in terms of biomechanical properties and gene expression. We concluded that cell bricks-enriched PRP gel sufficiently enhanced the morphological stability of the constructs, maintained chondrocyte phenotypes and favoured chondrogenesis in vivo, which suggests that such an injectable, completely biological system is a suitable cell carrier for cell-based cartilage repair.

Hydrogels for biomedical applications

Hydrogels are swollen, crosslinked networks that have great potential for use in biomedicine. Their softness, biocompatibility and ability for rapid diffusion of molecules make them useful for drug delivery, cell culture, wound healing and sensing applications. The chemical functionality of the gels can be easily modified to provide signalling and growth factors for cell proliferation. To allow the ingress of large cells, either porosity of the substrate can be controlled, or the gel can be made biodegradable. One ultimate goal is the growth of entire organs in the laboratory for eventual transplantation. Gels can be used as drug-delivery vehicles, either as implantable depots, or as microgels in blood-based delivery systems. One expanding area is the use of gels as surgical aides to prevent bleeding, infection and post-operative complications.

High amplitude direct compressive strain enhances mechanical properties of scaffold-free tissue-engineered cartilage

Adult cartilage has a limited healing capacity. Damages resulting from disease or injury increase over time and cause severe pain. One approach to reinstate the cartilage function is tissue engineering (TE). However, the generation of TE cartilage is time consuming and expensive and its properties are so far suboptimal. As in vivo cartilage is subject to loading, it is assumed that mechanical stimulation may enhance the quality of TE cartilage. In this study the short-term influence of variable compressive strain amplitudes on mechanical and biochemical properties of scaffold-free TE cartilage was investigated. Primary porcine chondrocytes were isolated, proliferated, redifferentiated, and transferred onto hydroxyapatite carriers, resulting in scaffold-free cartilage-carrier constructs. These constructs were placed in a custom-made bioreactor. Compression amplitudes of 5%, 10%, and 20% were applied. In each experiment four constructs were loaded with dynamic compression (3000 cycles/day, 1 Hz) for 14 days and four constructs served as unloaded control. The cartilage was evaluated biochemically, histological, and mechanically. No difference in glycosaminoglycan or collagen content between the loaded and the control groups was found. However, a positive correlation between compression amplitude and normalized Young's modulus was detected (R(2)=0.59, p<0.001). The highest compression amplitude of 20% had the strongest positive effect on the mechanical properties of the TE cartilage (Young's modulus increase of 241±28% compared to unloaded control). The data presented suggest that preconditioning with higher load amplitudes might be an attractive way of generating stiffer tissue and may help accelerating the cultivation of mechanically competent TE cartilage.

Dynamic culture enhances stem cell infiltration and modulates extracellular matrix production on aligned electrospun nanofibrous scaffolds

Electrospun nanofibrous scaffolds have become widely investigated for tissue engineering applications, owing to their ability to replicate the scale and organization of many fiber-reinforced soft tissues such as the knee meniscus, the annulus fibrosus of the intervertebral disc, tendon, and cartilage. However, due to their small pore size and dense packing of fibers, cellular ingress into electrospun scaffolds is limited. Progress in the application of electrospun scaffolds has therefore been hampered, as limited cell infiltration results in heterogeneous deposition of extracellular matrix and mechanical properties that remain below native benchmarks. In the present study, dynamic culture conditions dramatically improved the infiltration of mesenchymal stem cells into aligned nanofibrous scaffolds. While dynamic culture resulted in a reduction of glycosaminoglycan content, removal from dynamic culture to free-swelling conditions after 6 weeks resulted recovery of glycosaminoglycan content. Dynamic culture significantly increased collagen content, and collagen was more uniformly distributed throughout the scaffold thickness. While mechanical function was assessed and tensile modulus increased with culture duration, dynamic culture did not result in any additional improvement beyond free-swelling culture. Transient dynamic (6 weeks dynamic followed by 6 weeks free-swelling) culture significantly enhanced cell infiltration while permitting GAG accumulation. In this study, we demonstrated that a simple modification to standard in vitro culture conditions effectively improves cellular ingress into electrospun scaffolds, resolving a challenge which has until now limited the utility of these materials for various tissue engineering applications.

Self-assembly-peptide hydrogels as tissue-engineering scaffolds for three-dimensional culture of chondrocytes in vitro

The promising potential of a RAD-16 self-assembly-peptide hydrogel as a scaffold for tissue-engineered cartilage was investigated. Within 3 weeks of in vitro culture, chondrocytes within the hydrogel produced a high amount of GAG and type-II collagen, which are the components of cartilage-specific extracellular matrix (ECM). With the culture time increased, toluidine-blue staining for GAG and immuno-histochemistry staining for type-II collagen of the chondrocytes-hydrogel composites became more intense. Analysis of the gene expression of the ECM molecules also confirmed the chondrocytes in the peptide hydrogel maintained their phenotype within 3 weeks of in vitro culture.

Spontaneous redifferentiation of dedifferentiated human articular chondrocytes on hydrogel surfaces

Chondrocytes rapidly dedifferentiate into a more fibroblastic phenotype on a two-dimensional polystyrene substratum. This impedes fundamental research on these cells as well as their clinical application. This study investigated the redifferentiation behavior of dedifferentiated chondrocytes on a hydrogel substratum. Dedifferentiated normal human articular chondrocyte-knee (NHAC-kn) cells were released from the sixth-passage monolayer cultured on a polystyrene surface. These cells were then subcultured on a chemically crosslinked copolymer hydrogel, that is, poly(NaAMPS-co-DMAAm), and the cells thus obtained were used as the seventh-passage cultivation. Copolymer gels were synthesized from a negatively charged monomer, the sodium salt of 2-acrylamido-2-methyl-1-propanesulfonic acid (NaAMPS), and a neutral monomer, N,N-dimethylacrylamide (DMAAm). These gels were of different compositions because the molar fraction (F) of NaAMPS was varied (F = 0, 0.2, 0.4, 0.6, 0.8, and 1.0). The dedifferentiated NHAC-kn cells spontaneously redifferentiated to normal NHAC-kn cells on neutral (F = 0) and poly(NaAMPS-co-DMAAm) hydrogels of low charge density (F = 0.2). This was deduced from the cell morphology and expression of cartilage-specific genes and proteins. These results should enable us to establish a simple and efficient method for preparing large amounts of chondrocytes by cultivation on the surfaces of neutral and low-charge-density hydrogels.

Characterization of ex vivo-generated bovine and human cartilage by immunohistochemical, biochemical, and magnetic resonance imaging analyses

Osteoarthritis (OA) is a prevalent age-associated disease involving altered chondrocyte homeostasis and cartilage degeneration. The avascular nature of cartilage and the altered chondrocyte phenotype characteristic of OA severely limit the capacity for in vivo tissue regeneration. Cell- and tissue-based repair has the potential to revolutionize treatment of OA, but those approaches have exhibited limited clinical success to date. In this study, we test the hypothesis that bovine and human chondrocytes in a collagen type I scaffold will form hyaline cartilage ex vivo with immunohistochemical, biochemical, and magnetic resonance (MR) endpoints similar to the original native cartilage. Chondrocytes were isolated from 1- to 3-week-old calf knee cartilage or from cartilage obtained from human total knee arthroplasties, suspended in 2.7 mg/mL collagen I, and plated as 300 microL spot cultures with 5 x 10(6) each. Medium formulations were varied, including the amount of serum, the presence or absence of ascorbate, and treatments with cytokines. Bovine chondrocytes generated metachromatic territorial and interstitial matrix and accumulated type II collagen over time. Type VI collagen was confined primarily to the pericellular region. The ex vivo-formed bovine cartilage contained more chondroitin sulfate per dry weight than native cartilage. Human chondrocytes remained viable and generated metachromatic territorial matrix, but were unable to support interstitial matrix accumulation. MR analysis of ex vivo-formed bovine cartilage revealed evidence of progressively maturing matrix, but MR-derived indices of tissue quality did not reach those of native cartilage. We conclude that the collagen-spot culture model supports formation and maturation of three-dimensional hyaline cartilage from active bovine chondrocytes. Future studies will focus on determining the capacity of human chondrocytes to show comparable tissue formation.

Bioactive polymer/extracellular matrix scaffolds fabricated with a flow perfusion bioreactor for cartilage tissue engineering

In this study, electrospun poly(ɛ-caprolactone) (PCL) microfiber scaffolds, coated with cartilaginous extracellular matrix (ECM), were fabricated by first culturing chondrocytes under dynamic conditions in a flow perfusion bioreactor and then decellularizing the cellular constructs. The decellularization procedure yielded acellular PCL/ECM composite scaffolds containing glycosaminoglycan and collagen. PCL/ECM composite scaffolds were evaluated for their ability to support the chondrogenic differentiation of mesenchymal stem cells (MSCs) in vitro using serum-free medium with or without the addition of transforming growth factor-β1 (TGF-β1). PCL/ECM composite scaffolds supported chondrogenic differentiation induced by TGF-β1 exposure, as evidenced in the up-regulation of aggrecan (11.6 ± 3.8 fold) and collagen type II (668.4 ± 317.7 fold) gene expression. The presence of cartilaginous matrix alone reduced collagen type I gene expression to levels observed with TGF-β1 treatment. Cartilaginous matrix further enhanced the effects of growth factor treatment on MSC chondrogenesis as evidenced in the higher glycosaminoglycan synthetic activity for cells cultured on PCL/ECM composite scaffolds. Therefore, flow perfusion culture of chondrocytes on electrospun microfiber scaffolds is a promising method to fabricate polymer/extracellular matrix composite scaffolds that incorporate both natural and synthetic components to provide biological signals for cartilage tissue engineering applications.

Comparative chondrogenesis of human cells in a 3D integrated experimental-computational mechanobiology model

We present an integrated experimental-computational mechanobiology model of chondrogenesis. The response of human articular chondrocytes to culture medium perfusion, versus perfusion associated with cyclic pressurisation, versus non-perfused culture, was compared in a pellet culture model, and multiphysic computation was used to quantify oxygen transport and flow dynamics in the various culture conditions. At 2 weeks of culture, the measured cell metabolic activity and the matrix content in collagen type II and aggrecan were greatest in the perfused+pressurised pellets. The main effects of perfusion alone, relative to static controls, were to suppress collagen type I and GAG contents, which were greatest in the non-perfused pellets. All pellets showed a peripheral layer of proliferating cells, which was thickest in the perfused pellets, and most pellets showed internal gradients in cell density and matrix composition. In perfused pellets, the computed lowest oxygen concentration was 0.075 mM (7.5% tension), the maximal oxygen flux was 477.5 nmol/m(2)/s and the maximal fluid shear stress, acting on the pellet surface, was 1.8 mPa (0.018 dyn/cm(2)). In the non-perfused pellets, the lowest oxygen concentration was 0.003 mM (0.3% tension) and the maximal oxygen flux was 102.4 nmol/m(2)/s. A local correlation was observed, between the gradients in pellet properties obtained from histology, and the oxygen fields calculated with multiphysic simulation. Our results show up-regulation of hyaline matrix protein production by human chondrocytes in response to perfusion associated with cyclic pressurisation. These results could be favourably exploited in tissue engineering applications.

Novel synthesis strategies for natural polymer and composite biomaterials as potential scaffolds for tissue engineering

Recent developments in tissue engineering approaches frequently revolve around the use of three-dimensional scaffolds to function as the template for cellular activities to repair, rebuild and regenerate damaged or lost tissues. While there are several biomaterials to select as three-dimensional scaffolds, it is generally agreed that a biomaterial to be used in tissue engineering needs to possess certain material characteristics such as biocompatibility, suitable surface chemistry, interconnected porosity, desired mechanical properties and biodegradability. The use of naturally derived polymers as three-dimensional scaffolds has been gaining widespread attention owing to their favourable attributes of biocompatibility, low cost and ease of processing. This paper discusses the synthesis of various polysaccharide-based, naturally derived polymers, and the potential of using these biomaterials to serve as tissue engineering three-dimensional scaffolds is also evaluated. In this study, naturally derived polymers, specifically cellulose, chitosan, alginate and agarose, and their composites, are examined. Single-component scaffolds of plain cellulose, plain chitosan and plain alginate as well as composite scaffolds of cellulose-alginate, cellulose-agarose, cellulose-chitosan, chitosan-alginate and chitosan-agarose are synthesized, and their suitability as tissue engineering scaffolds is assessed. It is shown that naturally derived polymers in the form of hydrogels can be synthesized, and the lyophilization technique is used to synthesize various composites comprising these natural polymers. The composite scaffolds appear to be sponge-like after lyophilization. Scanning electron microscopy is used to demonstrate the formation of an interconnected porous network within the polymeric scaffold following lyophilization. It is also established that HeLa cells attach and proliferate well on scaffolds of cellulose, chitosan or alginate. The synthesis protocols reported in this study can therefore be used to manufacture naturally derived polymer-based scaffolds as potential biomaterials for various tissue engineering applications.

Can low frequency electromagnetic field help cartilage tissue engineering?

To understand whether a low-frequency pulsed electromagnetic field (EMF) could help cartilage tissue repair in the scope of tissue engineering, we tested how EMF affected collagen gel properties and the behaviors of chondrocyte cells embedded in collagen constructs. Collagen gel and primary chondrocytes embedded in collagen were exposed to EMF for 24 h. Gel and cells that were not exposed to EMF served as controls. Collagen gel exposed to EMF was more hydrophobic and less susceptible to enzymatic degradation (both p < 0.05) than control. Three weeks after EMF exposure, chondrocytes showed higher proliferation and lower glycosaminoglycan (GAG) production (both p < 0.05) than control. By the end of the third week, aggrecan, type I, II, and X collagen mRNA expressions in the EMF group were 1.8 times higher (p < 0.05), except for type II collagen) than control. The increase in gene expression did not show up in aggrecan histological staining and type II and type X collagen immunohistochemical staining. Cells from both groups kept a normal polygonal shape through out the test period. Our results suggested that one-time EMF exposure could promote collagen-embedded chondrocytes proliferation and their gene expressions. It also promoted short-term (week 1) GAG production and lacuna formation. No apparent GAG and type II collagen production was seen in histological staining three weeks after the EMF exposure.

Cartilage engineering using cell-derived extracellular matrix scaffold in vitro

A cell-derived extracellular matrix (ECM) scaffold was constructed using cultured porcine chondrocytes via a freeze-drying method, and its ability to promote cartilage formation was evaluated in vitro. Scanning electron microscope (SEM) revealed that the scaffold had highly uniform porous microstructure. Then, rabbit chondrocytes were seeded dynamically on ECM scaffold and cultured for 2 days, 1, 2, and 4 weeks in vitro for analysis. Polyglycolic acid (PGA) scaffold was used as a control. On gross observation of neocartilage tissue, a silvery white cartilage-like tissue was observed after 1 week of culture in ECM scaffold, while similar morphology was seen only after 4 weeks in PGA scaffold. The volume of neocartilage-like tissue was significantly increased in both ECM and PGA groups. The compressive strength was gradually increased with time in ECM group, while gradually decreased in PGA group. DNA, glycosaminoglycan (GAG) and collagen contents also increased gradually with time in both groups, but showed more significant increase in ECM group. Histological staining for GAG (Safranin O staining) and type II collagen (immunohistochemistry) showed sustained accumulation of ECM molecules with time, which gradually and uniformly filled porous space in ECM scaffold. On the contrary, they accumulated only at the peripheral area of PGA scaffold. These results suggest that a novel cell-derived ECM scaffold can provide a promising environment for generating a high quality cartilage in vitro.

Biomaterial effects in articular cartilage tissue engineering using polyglycolic acid, a novel marine origin biomaterial, IGF-I, and TGF-beta 1

Bovine articular chondrocytes were seeded on either polyglycolic acid (PGA) non-woven mesh scaffolds or a biomatrix from the species Porites lutea (POR). These constructs were cultured for 6 weeks in the presence of insulin-like growth factor (IGF)-I (10 ng/ml or 100 ng/ml) or transforming growth factor (TGF)-beta 1 (5 ng/ml or 30 ng/ml) to determine the in-vitro articular cartilage regeneration capacity of each. Histology, deoxyribonucleic acid content, collagen I and II (immunohistochemistry and enzyme-linked immunosorbent assay), and glycosaminoglycan (GAG) contents were measured at 0 weeks, 2 weeks, and 6 weeks to assess the characteristics of chondrogenesis. Both scaffolds supported the maintenance of the chondrocytic phenotype, as evidenced by the predominance of collagen II and the presence of rounded chondrocytes embedded in lacunae. Regardless of growth factor treatment, cells cultured on PGA scaffolds produced more collagen type II than those cultured on POR. Conversely, by 6 weeks, cells cultured on POR scaffolds produced more GAG than those cultured on PGA scaffolds, again regardless of the growth factor used. Across the two groups, 100 ng/ml of IGF-I had the greatest overall effect in GAG content. This work indicates that PGA and the POR scaffolds are both effective growth matrices for articular cartilage, with each scaffold exhibiting different yet desirable profiles of articular cartilage growth.

Gelatin/chitosan/hyaluronan scaffold integrated with PLGA microspheres for cartilage tissue engineering

Poly(lactide-co-glycotide) (PLGA) microspheres integrated into gelatin/chitosan/hyaluronan scaffolds were fabricated by freeze-drying and crosslinking with 1-ethyl-3-(3-dimethyl aminopropyl)carbodiimide. The effects of the microspheres on porosity, density, compressive modulus, phosphate-buffered saline uptake ratio and weight loss of the scaffolds were evaluated. Generally, a scaffold with a higher PLGA content had a lower porosity and weight loss, and a medium uptake ratio, but a larger apparent density and compressive modulus. When the PLGA content was lower than 50%, the PLGA-integrated scaffolds had a similar pore size (approximately 200microm) as that of the control, and as much as 90% of their porosity could be preserved. In vitro chondrocyte culture in the 50% PLGA-integrated scaffold demonstrated that the cells could proliferate and secrete extracellular matrix at the same level as in the control gelatin/chitosan/hyaluronan scaffold.

Cell-nanofiber-based cartilage tissue engineering using improved cell seeding, growth factor, and bioreactor technologies

Biodegradable nanofibrous scaffolds serving as an extracellular matrix substitute have been shown to be applicable for cartilage tissue engineering. However, a key challenge in using nanofibrous scaffolds for tissue engineering is that the small pore size limits the infiltration of cells, which may result in uneven cell distribution throughout the scaffold. This study describes an effective method of chondrocyte loading into nanofibrous scaffolds, which combines cell seeding, mixing, and centrifugation to form homogeneous, packed cell-nanofiber composites (CNCs). When the effects of different growth factors are compared, CNCs cultured in medium containing a combination of insulin-like growth factor-1 and transforming growth factor-beta1 express the highest mRNA levels of collagen type II and aggrecan. Radiolabeling analyses confirm the effect on collagen and sulfated-glycosaminoglycans (sGAG) production. Histology reveals chondrocytes with typical morphology embedded in lacuna-like space throughout the entire structure of the CNC. Upon culturing using a rotary wall vessel bioreactor, CNCs develop into a smooth, glossy cartilage-like tissue, compared to a rough-surface tissue when maintained in a static environment. Bioreactor-grown cartilage constructs produce more total collagen and sGAG, resulting in greater gain in net tissue weight, as well as express cartilage-associated genes, including collagen types II and IX, cartilage oligomeric matrix protein, and aggrecan. In addition, dynamic culture enhances the mechanical property of the engineered cartilage. Taken together, these results indicate the applicability of nanofibrous scaffolds, combined with efficient cell loading and bioreactor technology, for cell-based cartilage tissue engineering.

Engineering cartilage tissue

Cartilage tissue engineering is emerging as a technique for the regeneration of cartilage tissue damaged due to disease or trauma. Since cartilage lacks regenerative capabilities, it is essential to develop approaches that deliver the appropriate cells, biomaterials, and signaling factors to the defect site. The objective of this review is to discuss the approaches that have been taken in this area, with an emphasis on various cell sources, including chondrocytes, fibroblasts, and stem cells. Additionally, biomaterials and their interaction with cells and the importance of signaling factors on cellular behavior and cartilage formation will be addressed. Ultimately, the goal of investigators working on cartilage regeneration is to develop a system that promotes the production of cartilage tissue that mimics native tissue properties, accelerates restoration of tissue function, and is clinically translatable. Although this is an ambitious goal, significant progress and important advances have been made in recent years.

Room temperature synthesis of agarose/sol-gel glass pieces with tailored interconnected porosity

An original shaping technique has been applied to prepare porous bodies at room temperature. Agarose, a biodegradable polysaccharide, was added as binder of a sol-gel glass in powder form, yielding an easy to mold paste. Interconnected tailored porous bodies can be straightforwardly prepared by pouring the slurry into a polymeric scaffold, previously designed by stereolitography, which is subsequently eliminated by alkaline dissolution at room temperature. The so obtained pieces behave like a hydrogel with an enhanced consistency that makes them machinable and easy to manipulate. These materials generate an apatite-like layer when immersed in a simulated body fluid, indicating a potential in vivo bioactivity. The proposed method can be applied to different powdered materials to produce pieces, at room temperature, with various shapes and sizes and with tailored interconnected porosity.

Characterizing the viscoelastic properties of thin hydrogel-based constructs for tissue engineering applications

We present a novel indentation method for characterizing the viscoelastic properties of alginate and agarose hydrogel based constructs, which are often used as a model system of soft biological tissues. A sensitive long working distance microscope was used for measuring the time-dependent deformation of the thin circular hydrogel membranes under a constant load. The deformation of the constructs was measured laterally. The elastic modulus as a function of time can be determined by a large deformation theory based on Mooney-Rivlin elasticity. A viscoelastic theory, Zener model, was applied to correlate the time-dependent deformation of the constructs with various gel concentrations, and the creep parameters can therefore be quantitatively estimated. The value of Young's modulus was shown to increase in proportion with gel concentration. This finding is consistent with other publications. Our results also showed the great capability of using the technique to measure gels with incorporated corneal stromal cells. This study demonstrates a novel and convenient technique to measure mechanical properties of hydrogel in a non-destructive, online and real-time fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.

Effects of Ascorbic Acid Concentration on the Tissue Engineering of the Temporomandibular Joint Disc

The temporomandibular joint (TMJ) disc is a specialized fibrocartilaginous tissue. When the disc becomes an obstacle and becomes damaged, surgeons have no choice but to perform a discectomy. Tissue engineering may provide a novel treatment modality for TMJ disorder patients who undergo discectomy. No studies have been conducted on the most favourable media for TMJ disc cells. The objective of the current study was to examine the effects on biochemical and biomechanical properties of varying ascorbic acid concentrations (0, 25, or 50 μg/ml) on TMJ disc cells seeded on non-woven PGA scaffolds. The ascorbic acid concentration of the 25 μg/ml group resulted in more effective cell seeding of the scaffolds, with 1.53 million cells per construct, by comparison with the 0 and 50 μg/ml groups which had 1.20 million and 1.32 million cells per scaffold respectively. At week 4, the 25 μg/ml group had a higher collagen content than the 0 μg/ml group, with 30.4 ± 2.7 and 24.9 ± 3.3 μg of collagen per construct respectively. The 25 μg/ml group had a higher aggregate modulus than the 50 μg/ml group, with values of 6.1 ± 1.3 and 4.0 ± 0.9 kPa respectively at week 4. The results of this study indicate that the use of 25 μg/ml of ascorbic acid in culture media is effective for the tissue engineering of the TMJ disc, significantly outperforming media without or with 50 μg/ml of ascorbic acid.

Influence of perfusion on metabolism and matrix production by bovine articular chondrocytes in hydrogel scaffolds

Articular cartilage has a limited capacity for self-repair after damage. Engineered cartilage is a promising treatment to replace or repair damaged tissue. The growth of engineered cartilage is sensitive to the extracellular culture environment. Chondrocytes were seeded into alginate beads and agarose scaffolds at 4 millions/mL, and the response to static and perfusion culture was examined over period of up to 12 days. For both types of scaffolds, the chondrocytes kept their differentiated morphology over 12 days in all culture conditions. In alginate beads, more glycosaminoglycans (GAGs) were produced in perfusion culture than in static conditions. GAG distribution in alginate constructs was more uniform in perfusion culture than in static culture. However, in agarose constructs there was no significant difference in GAG production between static culture and perfusion culture. Under perfusion culture, the retention rate of GAG in alginate was higher than in agarsoe. It is suggested that the positive effect of perfusion culture only can be achieved by an appropriate choice of other factors such as scaffold materials.

Characterizing the viscoelastic properties of thin hydrogel-based constructs for tissue engineering applications

We present a novel indentation method for characterizing the viscoelastic properties of alginate and agarose hydrogel based constructs, which are often used as a model system of soft biological tissues. A sensitive long working distance microscope was used for measuring the time-dependent deformation of the thin circular hydrogel membranes under a constant load. The deformation of the constructs was measured laterally. The elastic modulus as a function of time can be determined by a large deformation theory based on Mooney-Rivlin elasticity. A viscoelastic theory, Zener model, was applied to correlate the time-dependent deformation of the constructs with various gel concentrations, and the creep parameters can therefore be quantitatively estimated. The value of Young's modulus was shown to increase in proportion with gel concentration. This finding is consistent with other publications. Our results also showed the great capability of using the technique to measure gels with incorporated corneal stromal cells. This study demonstrates a novel and convenient technique to measure mechanical properties of hydrogel in a non-destructive, online and real-time fashion. Thus this novel technique can become a valuable tool for soft tissue engineering.

Delivery of TGF-β1 and chondrocytes via injectable, biodegradable hydrogels for cartilage tissue engineering applications

In this work, novel hydrogel composites, based on the biodegradable polymer, oligo(poly(ethylene glycol) fumarate) (OPF) and gelatin microparticles (MPs) were utilized as injectable cell and growth factor carriers for cartilage tissue engineering applications. Specifically, bovine chondrocytes were embedded in composite hydrogels co-encapsulating gelatin MPs loaded with transforming growth factor-beta1 (TGF-beta1). Hydrogels with embedded cells co-encapsulating unloaded MPs and those with no MPs served as controls in order to assess the effects of MPs and TGF-beta1 on chondrocyte function. Samples were cultured up to 28 days in vitro. By 14 days, cell attachment to embedded gelatin MPs within the constructs was observed via light microscopy. Bioassay results showed that, over the 21 day period, there was a statistically significant increase in cellular proliferation for samples containing gelatin MPs, but no increase was exhibited in samples without MPs over the culture period. The release of TGF-beta1 further increased cell construct cellularity. Over the same time period, glycosaminoglycan content per cell remained constant for all formulations, suggesting that the dramatic increase in cell number for samples with TGF-beta1-loaded MPs was accompanied by maintenance of the cell phenotype. Overall, these data indicate the potential of OPF hydrogel composites containing embedded chondrocytes and TGF-beta1-loaded gelatin MPs as a novel strategy for cartilage tissue engineering.

Subsieve-size agarose capsules enclosing ifosfamide-activating cells: a strategy toward chemotherapeutic targeting to tumors

Localized activation of the prodrug ifosfamide in or close to tumors by implanting encapsulated ifosfamide-activating cells is an efficacious strategy for tumor therapy. The aim of this study was to evaluate the feasibility of subsieve-size agarose capsules for enclosing the cells in this application. Compared with many conventional microcapsules, subsieve-size agarose capsules are about one-tenth the size and have both higher mechanical stability and allow better molecular exchangeability than other systems. Cells that have been genetically modified to express cytochrome P450 2B1 enzyme were encapsulated in subsieve-size agarose capsules of ∼90 μm in diameter and implanted into preformed tumors in nude mice. Living cells were detected for &gt;1 month after encapsulation in vitro and showed enzymatic activity (i.e., they were able to activate ifosfamide). More significant regression of preformed tumors was observed in the recipients implanted with cell-enclosing capsules compared with those implanted with empty capsules. These results suggest that the strategy of using subsieve-size agarose capsules enclosing cytochrome P450 2B1–expressing cells is feasible for tumor therapy by chemotherapeutic targeting in combination with ifosfamide administration.

The Effect of Two Different Bioreactors on the Neocartilage Formation in Type II Collagen Modified Polyester Scaffolds Seeded With Chondrocytes

The effect of dynamic culture conditions on neocartilage formation in type II collagen modified polyester scaffolds was studied. Porcine or human articular chondrocytes were seeded in the scaffolds. The cell-scaffold constructs were cultivated statically, in a rotating-type bioreactor or in a shaker for up to 4 weeks. The cell proliferation, morphology, NO production, synthesis of proteoglycans and collagen, and mechanical properties were evaluated. The results demonstrated that the rotating-type bioreactor promoted the growth of primary porcine chondrocytes, helped to maintain their phenotype, and increased the production of extracellular matrix. The constructs also had the largest dynamic compressive modulus. In the static condition, chondrocytes occupied only the outer margin of the cell-polymer constructs. The poor mass transfer in static condition may have caused a lower pH value in the middle of the constructs and lead further to faster scaffold degradation as well as the weakest neocartilage. Constructs in the shaker produced the highest amount of NO as well as the lowest amount of cells and matrix production. Human or porcine chondrocytes of the second passage seeded in scaffolds were much less viable, with the largest amount of cells and matrix when cultured in rotating-type bioreactors. A larger seeding density was required to form neocartilage from passaged adult chondrocytes.