Influence of medium composition, static and stirred conditions on the proliferation of and matrix protein expression of bovine articular chondrocytes cultured in a 3-D collagen scaffold

A Novel Bioreactor System Capable of Simulating the In Vivo Conditions of Synovial Joints

Any significant in vitro evaluation of cartilage tissue engineering and cartilage repair strategies has to be performed under the harsh conditions encountered in vivo within synovial joints. To this end, we have developed a novel automated physiological robot reactor system (PRRS) that is capable of recapitulating complex physiological motions and load patterns within an environment similar to that found in the human knee. The PRRS consists of a mechanical stimulation unit (MSU) and an automatic sample changer (ASC) within an environment control box in which the humidity, temperature, and gas composition are tightly regulated. The MSU has three linear (orthogonal) axes and one rotational degree of freedom (around the z-axis). The ASC provides space for up to 24 samples, which can be allocated to individual stimulation patterns. Cell-seeded scaffolds and ex vivo tissue culture systems were established to demonstrate the applicability of the PRRS to the investigation of the effect of load and environmental conditions on engineering and maintenance of articular cartilage in vitro. The bioreactor is a flexible system that has the potential to be applied for culturing connective tissues other than cartilage, such as bone and intervertebral disc tissue, even though the mechanical and environmental parameters are very different.

A Hybrid Model of Cartilage Regeneration Capturing the Interactions Between Cellular Dynamics and Porosity

To accelerate the development of strategies for cartilage tissue engineering, models are necessary to investigate the interactions between cellular dynamics and the local microenvironment. We use a discrete framework to capture the individual behavior of cells, modeling experiments where cells are seeded in a porous scaffold or hydrogel and over the time course of a month, the scaffold slowly degrades while cells divide and synthesize extracellular matrix constituents. The movement of cells and the ability to proliferate is a function of the local porosity, defined as the volume fraction of fluid in the surrounding region. A phenomenological approach is used to capture a continuous profile for the degrading scaffold and accumulating matrix, which will then change the local porosity throughout the construct. We parameterize the model by first matching total cell counts in the construct to chondrocytes seeded in a polyglycolic acid scaffold (Freed et al. in Biotechnol Bioeng 43:597-604, 1994). We investigate the influence of initial scaffold porosity on the total cell count and spatial profiles of cell and ECM in the construct. Cell counts were higher at day 30 in scaffolds of lower initial porosity, and similar cell counts were obtained using different models of scaffold degradation and matrix accumulation (either uniform or cell-specific). Using this modeling framework, we study the interplay between a phenomenological representation of scaffold architecture and porosity as well as the potential continuous application of growth factors. We determine parameter regimes where large cellular aggregates occur, which can hinder matrix accumulation and cellular proliferation. The developed modeling framework can easily be extended and can be used to identify optimal scaffolds and culture conditions that lead to a desired distribution of extracellular matrix and cell counts throughout the construct.

A Guide for Using Mechanical Stimulation to Enhance Tissue-Engineered Articular Cartilage Properties

The use of tissue-engineered articular cartilage (TEAC) constructs has the potential to become a powerful treatment option for cartilage lesions resulting from trauma or early stages of pathology. Although fundamental tissue-engineering strategies based on the use of scaffolds, cells, and signals have been developed, techniques that lead to biomimetic AC constructs that can be translated to in vivo use are yet to be fully confirmed. Mechanical stimulation during tissue culture can be an effective strategy to enhance the mechanical, structural, and cellular properties of tissue-engineered constructs toward mimicking those of native AC. This review focuses on the use of mechanical stimulation to attain and enhance the properties of AC constructs needed to translate these implants to the clinic. In vivo, mechanical loading at maximal and supramaximal physiological levels has been shown to be detrimental to AC through the development of degenerative changes. In contrast, multiple studies have revealed that during culture, mechanical stimulation within narrow ranges of magnitude and duration can produce anisotropic, mechanically robust AC constructs with high cellular viability. Significant progress has been made in evaluating a variety of mechanical stimulation techniques on TEAC, either alone or in combination with other stimuli. These advancements include determining and optimizing efficacious loading parameters (e.g., duration and frequency) to yield improvements in construct design criteria, such as collagen II content, compressive stiffness, cell viability, and fiber organization. With the advancement of mechanical stimulation as a potent strategy in AC tissue engineering, a compendium detailing the results achievable by various stimulus regimens would be of great use for researchers in academia and industry. The objective is to list the qualitative and quantitative effects that can be attained when direct compression, hydrostatic pressure, shear, and tensile loading are used to tissue-engineer AC. Our goal is to provide a practical guide to their use and optimization of loading parameters. For each loading condition, we will also present and discuss benefits and limitations of bioreactor configurations that have been used. The intent is for this review to serve as a reference for including mechanical stimulation strategies as part of AC construct culture regimens.

Digital light procession three-dimensional printing acrylate/collagen composite airway stent for tracheomalacia

Recently, more and more researchers have focused on airway stent applied in tracheomalacia. The airway stents for clinical application were usually manufactured in accordance with a fixed pattern, which were difficult to perfect match with children, especially infants. Digital light procession of light curing acrylate resin implantation showed higher accuracy and printing speed over traditional three-dimensional printing techniques. In this article, a novel personalized airway stent was developed by digital light procession three-dimensional printing and was modified by collagen I extracted from the fish scales. The morphology of the collagen-modified airway stent was examined by scanning electron microscopy, and the chemical structures were examined by attenuated total internal reflectance Fourier transform infrared spectroscopy. The biocompatibility of this synthetic acrylate/collagen composite airway stent was characterized by water contact angle test and cell culture. The results confirmed that the composite airway stent was hydrophilic and non-cytotoxic toward a cultured human bronchial epithelial cell line with good cell viability and show excellent physicochemical and biological properties. In conclusion, this study presented the three-dimensional printing composite acrylate and collagen airway stent may have potential in customized treatment for tracheomalacia.

Jellyfish collagen matrices conserve the chondrogenic phenotype in two- and three-dimensional collagen matrices

Cartilage is a tissue with a very low capability of self-repair and the search for suitable materials supporting the chondrogenic phenotype and thus avoiding fibrotic dedifferentiation for matrix-associated chondrocyte transplantation (MACI) is ongoing. Jellyfish collagen was thought to be a suitable material mainly because of its good availability and easy handling. Collagen was extracted from jellyfish Rhopilema esculentum and the spreading of porcine chondrocytes on two (2D) and three dimensional (3D) collagen matrices examined in comparison with vertebrate collagens, placenta collagen and a commercially available matrix from porcine collagen type I (Optimaix®). In 2D, most chondrocytes kept their round shape on jellyfish collagen and vertebrate collagen type II compared with vertebrate collagen type I. This was also confirmed in 3D experiments, where chondrocytes preserved their phenotype on jellyfish collagen, as indicated by high collagen II/(II + I) ratios (≥54 % and ~92 % collagen type II in mRNA and protein, respectively) and no proliferation during 28 days of cultivation. These observations were discussed with a view to potential structural differences of jellyfish collagen, which might influence the integrin-mediated adhesion mechanisms of vertebrate cells on jellyfish collagen. This probably results from a lack of integrin-binding sites and the existence of an alternative binding mechanism such that cells kept their round shape on jellyfish collagen, preventing chondrocytes from dedifferentiation. Thus, collagen from R. esculentum is a very suitable and promising material for cartilage tissue engineering. Copyright © 2015 John Wiley & Sons, Ltd.

Mesenchymal stem cell therapy in the sports knee: where are we in 2011?

Background:

The relationship between biological tissue healing following knee injury or surgery and long-term clinical outcome has come to the forefront of sports medicine practice. This has led many knee surgeons to incorporate biologically mediated healing factors into the management of knee injuries. In particular, the clinical use of mesenchymal stem cells has opened new horizons.

Evidence Acquisition:

Relevant studies were identified through a search of PubMed from January 2000 to April 2011, combining the term mesenchymal stem cells with articular cartilage, anterior cruciate ligament, and meniscus. Relevant citations from the reference lists of selected studies were also reviewed.

Results:

Knee injury treatment with mesenchymal stem cells shows potential. Most reports represent animal model studies; few advances have been translated to human clinical applications.

Conclusion:

Mesenchymal stem cell use to promote healing following knee injury is likely to increase. There are scientific methodological concerns and ethical and legal issues regarding mesenchymal stem cell use for treating knee injuries.

Fibrin-chitosan composite substrate for in vitro culture of chondrocytes

The aim of this study was to develop a biocompatible monolayer substrate based on fibrin and chitosan for in vitro culture of chondrocytes. Fibrin-chitosan composite substrates combined the proved cell adhesion properties of fibrin with the hydrophilicity and poor adhesion capacity of chitosan. Chitosan microspheres were produced by coacervation method, agglomerated within a fibrin network and subsequently crosslinked with genipin. The composite substrate was stable for 28 days of culture due to the high crosslinking density. Human chondrocytes cultured on the composite substrate were viable during the culture period. At the end of culture time (28 days) the composite substrate showed low cellular proliferation, 41% more collagen type II and 13% more production of sulfated glycosaminoglycans with respect to the amounts found at 14 days. The study revealed that dedifferentiated chondrocytes cultured in monolayer on the composite substrate can re-acquire characteristics of differentiated cells without using three-dimensional substrates or chondrogenic media.

Enhanced chondrogenesis in co-cultures with articular chondrocytes and mesenchymal stem cells

In this work, articular chondrocytes (ACs) and mesenchymal stem cells (MSCs) with 1:1 and 1:3 cell ratios were co-cultured in order to evaluate if a majority of primary ACs can be replaced with MSCs without detrimental effects on in vitro chondrogenesis. We further used a xenogeneic culture model to study if such co-cultures can result in redifferentiation of passaged ACs. Cells were cultured in porous scaffolds for four weeks and their cellularity, cartilage-like matrix formation and chondrogenic gene expression levels (collagen I and II, aggrecan) were measured. Constructs with primary bovine ACs had ~1.6 and 5.5 times higher final DNA and glycosaminoglycan contents, respectively, in comparison to those with culture expanded chondrocytes or MSCs harvested from the same animals. Equally robust chondrogenesis was also observed in co-cultures, even when up to 75% of primary ACs were initially replaced with MSCs. Furthermore, species-specific RT-PCR analysis indicated a gradual loss of MSCs in bovine-rabbit co-cultures. Finally, co-cultures using primary and culture expanded ACs resulted in similar outcomes. We conclude that the most promising cell source for cartilage engineering was the co-cultures, as the trophic effect of MSCs may highly increase the chondrogenic potential of ACs thus diminishing the problems with primary chondrocyte harvest and expansion.

Clinical relevance of scaffolds for cartilage engineering

The repair of articular cartilage defects in patients' knees presents a particular challenge to the orthopedic surgeon because cartilage lacks the ability to repair or regenerate itself. Various cartilage repair techniques have not produced a superior or uniform outcome, which has led to a new generation of cartilage repair based on tissue-engineering strategies and the use of biological scaffolds. Clinical advances have been made regarding the regeneration of articular cartilage, and continue to be made toward the achievement of a suitable treatment method for resurfacing osteochondral defects, through cartilage tissue engineering and the use of pluripotent cells seeded on bio-scaffolds.

Physiological cartilage tissue engineering effect of oxygen and biomechanics

In vitro engineering of cartilaginous tissues has been studied for many years, and tissue-engineered constructs are sought to be used clinically for treating articular cartilage defects. Even though there is a plethora of studies and data available, no breakthroughs have been achieved yet that allow for implanting in vivo cultured articular cartilaginous tissues in patients. A review of contributions to cartilage tissue engineering over the past decades emphasizes that most of the studies were performed under environmental conditions neglecting the physiological situation. This is specifically pronounced in the use of bioreactor systems which neither allow for application of near physiomechanical stimulations nor for controlling a hypoxic environment as it is experienced in synovial joints. It is suspected that the negligence of these important parameters has slowed down progress and prevented major breakthroughs in the field. This review focuses on the main aspects of cartilage tissue engineering with emphasis on the relation and understanding of employing physiological conditions.

Strategies for enhancing the accumulation and retention of extracellular matrix in tissue-engineered cartilage cultured in bioreactors

Production of tissue-engineered cartilage involves the synthesis and accumulation of key constituents such as glycosaminoglycan (GAG) and collagen type II to form insoluble extracellular matrix (ECM). During cartilage culture, macromolecular components are released from nascent tissues into the medium, representing a significant waste of biosynthetic resources. This work was aimed at developing strategies for improving ECM retention in cartilage constructs and thus the quality of engineered tissues produced in bioreactors. Human chondrocytes seeded into polyglycolic acid (PGA) scaffolds were cultured in perfusion bioreactors for up to 5 weeks. Analysis of the size and integrity of proteoglycans in the constructs and medium showed that full-sized aggrecan was being stripped from the tissues without proteolytic degradation. Application of low (0.075 mL min(-1)) and gradually increasing (0.075-0.2 mL min(-1)) medium flow rates in the bioreactor resulted in the generation of larger constructs, a 4.0-4.4-fold increase in the percentage of GAG retained in the ECM, and a 4.8-5.2-fold increase in GAG concentration in the tissues compared with operation at 0.2 mL min(-1). GAG retention was also improved by pre-culturing seeded scaffolds in flasks for 5 days prior to bioreactor culture. In contrast, GAG retention in PGA scaffolds infused with alginate hydrogel did not vary significantly with medium flow rate or pre-culture treatment. This work demonstrates that substantial improvements in cartilage quality can be achieved using scaffold and bioreactor culture strategies that specifically target and improve ECM retention.

Application of high throughput perfusion micro 3-D cell culture platform for the precise study of cellular responses to extracellular conditions -effect of serum concentrations on the physiology of articular chondrocytes

Mammalian cells are sensitive to extracellular microenvironments. In order to faithfully explore the physiological responses of cells to extracellular conditions, a steady, homogenous, and three-dimensional (3-D) culture environment is required because it can provide a more quantifiable and biologically-relevant culture condition. To achieve this, this study reports a perfusion micro cell culture platform encompassing 22 microbioreactor units for high throughput 3-D cell culture. The cell culture platform structurally consisting of a plug and a microbioreactor chamber module was simply fabricated by replica molding of polydimethylsiloxane (PDMS) polymer. The platform features in the proposed plug module with multiple molds incorporated, facilitating the preparation of cell encapsulated 3-D hydrogel constructs in a precise and efficient manner. This trait is found particularly useful for high-precision and high-throughput micro 3-D cell culture-based assay. In this study, the real value of the proposed platform to maintain a stable and homogenous culture condition was discussed. Besides, the application of the presented platform for precisely investigating the effect of serum concentration on the metabolic activities and biosynthetic abilities of articular chondrocytes was also demonstrated. As a whole, the proposed device has paved an alternative route to carry out high throughput micro-scale 3-D perfusion cell culture in a simple, cost-effective and precise manner. The promising applications include 3-D cell culture-based high throughput drug or toxicity testing/screening, or other investigations on the cell biology, where the precise quantification of the links between the cellular responses and extracellular conditions is required.

Fabrication and repair of cartilage defects with a novel acellular cartilage matrix scaffold

The objectives of this study were to develop a three-dimensional acellular cartilage matrix (ACM) and investigate its possibility for use as a scaffold in cartilage tissue engineering. Bovine articular cartilage was decellularized sequentially with trypsin, nuclease solution, hypotonic buffer, and Triton x 100 solution; molded with freeze-drying process; and cross-linked by ultraviolet irradiation. Histological and biochemical analysis showed that the ACM was devoid of cells and still maintained the collagen and glycosaminoglycan components of cartilage. Scanning electronic microscopy and mercury intrusion porosimetry showed that the ACM had a sponge-like structure of high porosity. The ACM scaffold had good biocompatibility with cultured rabbit bone marrow mesenchymal stem cells with no indication of cytotoxicity both in contact and in extraction assays. The cartilage defects repair in rabbit knees with the mesenchymal stem cell-ACM constructs had a significant improvement of histological scores when compared to the control groups at 6 and 12 weeks. In summary, the ACM possessed the characteristics that afford it as a potential scaffold for cartilage tissue engineering.

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.

Cartilage tissue engineering using electrospun PCL nanofiber meshes and MSCs

Mesenchymal stem cells (MSCs) have been recognized for their ability to differentiate into cells of different tissues such as bone, cartilage, or adipose tissue, and therefore are of great interest for potential therapeutic strategies. Adherent, colony-forming, fibroblastic cells were isolated from human bone marrow aspirates, from patients undergoing knee arthroplasties, and the MSCs phenotype characterized by flow cytometry. Afterward, cells were seeded onto electrospun polycaprolactone nanofiber meshes and cultured in a multichamber flow perfusion bioreactor to determine their ability to produce cartilagineous extracellular matrix. Results indicate that the flow perfusion bioreactor increased the chondrogenic differentiation of hBM-MSCs, as confirmed either by morphological and RT-PCR analysis. Cartilage-related genes such as aggrecan, collagen type II, and Sox9 were expressed. ECM deposition was also detected by histological procedures. Collagen type II was present in the samples, as well as collagen type I. Despite no statistically significant values being obtained for gene expression, the other results support the choice of the bioreactor for this type of culture.

Strategies for articular cartilage lesion repair and functional restoration

Injury of articular cartilage due to trauma or pathological conditions is the major cause of disability worldwide, especially in North America. The increasing number of patients suffering from joint-related conditions leads to a concomitant increase in the economic burden. In this review article, we focus on strategies to repair and replace knee joint cartilage, since knee-associated disabilities are more prevalent than any other joint. Because of inadequacies associated with widely used approaches, the orthopedic community has an increasing tendency to develop biological strategies, which include transplantation of autologous (i.e., mosaicplasty) or allogeneic osteochondral grafts, autologous chondrocytes (autologous chondrocyte transplantation), or tissue-engineered cartilage substitutes. Tissue-engineered cartilage constructs represent a highly promising treatment option for knee injury as they mimic the biomechanical environment of the native cartilage and have superior integration capabilities. Currently, a wide range of tissue-engineering-based strategies are established and investigated clinically as an alternative to the routinely used techniques (i.e., knee replacement and autologous chondrocyte transplantation). Tissue-engineering-based strategies include implantation of autologous chondrocytes in combination with collagen I, collagen I/III (matrix-induced autologous chondrocyte implantation), HYAFF 11 (Hyalograft C), and fibrin glue (Tissucol) or implantation of minced cartilage in combination with copolymers of polyglycolic acid along with polycaprolactone (cartilage autograft implantation system), and fibrin glue (DeNovo NT graft). Tissue-engineered cartilage replacements show better clinical outcomes in the short term, and with advances that have been made in orthopedics they can be introduced arthroscopically in a minimally invasive fashion. Thus, the future is bright for this innovative approach to restore function.

Chitosan/polyester-based scaffolds for cartilage tissue engineering: assessment of extracellular matrix formation

Naturally derived polymers have been extensively used in scaffold production for cartilage tissue engineering. The present work aims to evaluate and characterize extracellular matrix (ECM) formation in two types of chitosan-based scaffolds, using bovine articular chondrocytes (BACs). The influence of these scaffolds' porosity, as well as pore size and geometry, on the formation of cartilagineous tissue was studied. The effect of stirred conditions on ECM formation was also assessed. Chitosan-poly(butylene succinate) (CPBS) scaffolds were produced by compression moulding and salt leaching, using a blend of 50% of each material. Different porosities and pore size structures were obtained. BACs were seeded onto CPBS scaffolds using spinner flasks. Constructs were then transferred to the incubator, where half were cultured under stirred conditions, and the other half under static conditions for 4 weeks. Constructs were characterized by scanning electron microscopy, histology procedures, immunolocalization of collagen type I and collagen type II, and dimethylmethylene blue assay for glycosaminoglycan (GAG) quantification. Both materials showed good affinity for cell attachment. Cells colonized the entire scaffolds and were able to produce ECM. Large pores with random geometry improved proteoglycans and collagen type II production. However, that structure has the opposite effect on GAG production. Stirred culture conditions indicate enhancement of GAG production in both types of scaffold.

Evaluation of histological scoring systems for tissue-engineered, repaired and osteoarthritic cartilage

OBJECTIVE

Regeneration of hyaline cartilage has been the focus of an increasing number of research groups around the world. One of the most important outcome measures in evaluation of its success is the histological quality of cartilaginous tissue. Currently, a variety of histological scoring systems is used to describe the quality of osteoarthritic, in vivo repaired or in vitro engineered tissue. This review aims to provide an overview of past and currently used histological scoring systems, in an effort to aid cartilage researchers in choosing adequate and validated cartilage histological scoring systems.

METHODS

Histological scoring systems for analysis of osteoarthritic, tissue engineered and in vivo repaired cartilage were reviewed. The chronological development as well as the validity and practical applicability of the scoring systems is evaluated.

RESULTS

The Histological-Histochemical Grading System (HHGS) or a HHGS-related score is most often used for evaluation of osteoarthritic cartilage, however the Osteoarthritis Research Society International (OARSI) Osteoarthritis Cartilage Histopathology Assessment System seems a valid alternative. The O'Driscoll score and the International Cartilage Repair Society (ICRS) II score may be used for in vivo repaired cartilage. The 'Bern score' seems most adequate for evaluation of in vitro engineered cartilage.

CONCLUSION

A great variety of histological scoring systems exists for analysis of osteoarthritic or normal, in vivo repaired or tissue-engineered cartilage, but only few have been validated. Use of these validated scores may considerably improve exchange of information necessary for advances in the field of cartilage regeneration.

Development of serum-free, chemically defined conditions for human embryonic stem cell-derived fibrochondrogenesis

This study established serum-free, chemically defined conditions to generate fibrocartilage with human embryonic stem cells (hESCs). Three sequential experimental phases were performed to eliminate serum because of its variability and antigenic potential and characterize the performance of hESCs in serum-free and serum-based conditions. Each phase used a two-stage modular experiment: chondrogenic differentiation followed by scaffold-less tissue engineering, called self-assembly. Phase I studied serum effects, and showed that a 1% serum chondrogenic medium (CM) during differentiation resulted in uniform constructs, whereas a 20% serum CM did not. Furthermore, a no-serum CM during self-assembly led to a collagen content 50% to 200% greater than a 1% serum CM. Thus, a "serum standard" of 1% serum during differentiation and no serum during self-assembly was carried forward. Phase II compared this with serum-free formulations, using 5% knock-out serum replacer or 1-ng/mL transforming growth factor beta 1 (TGF-beta1). The TGF-beta1 group was chosen as a "serum-free standard" because it performed similarly to the serum standard in terms of morphological, biochemical, and biomechanical properties. In Phase III, the serum-free standard had significantly more collagen (100%) and greater tensile ( approximately 150%) and compressive properties ( approximately 80%) than the serum standard with TGF-beta1 treatment during self-assembly. These advances are important to the understanding of mechanisms of chondrogenesis and creating clinically relevant stem cell therapies.

Comparative phenotypic analysis of articular chondrocytes cultured within type I or type II collagen scaffolds

Among the existing repair strategies for cartilage injury, tissue engineering approach using biomaterials and chondrocytes offers hope for treatments. In this context, collagen-based biomaterials are good candidates as scaffolds for chondrocytes in cell transplantation procedures. These scaffolds are provided under different forms (gel or crosslinked sponge) made with either type I collagen or type I or type II atelocollagen molecules. The present study was undertaken to investigate how bovine articular chondrocytes sense and respond to differences in the structure and organization of these collagen scaffolds, over a 12-day culture period. When chondrocytes were seeded in the collagen scaffolds maintained in free-floating conditions, cells contracted gels to 40-60% and sponges to 15% of their original diameter. Real-time polymerase chain reaction analysis indicated that the chondrocyte phenotype, assessed notably by the ratio of COL2A1/COL1A2 mRNA and alpha10/alpha11 integrin subunit mRNA, was comparatively better sustained in type I collagen sponges when seeded at high cell density, also in type I atelocollagen gels. Besides, proteoglycan accumulation in the different scaffolds, as assessed by measuring the sulfated glycosaminoglycan content, was found be highest in type I collagen sponges seeded at high cell density. In addition, gene expression of matrix metalloproteinase-13 increased dramatically (up to 90-fold) in chondrocytes cultured in the different gels, whereas it remained stable in the sponges. Our data taken together reveal that type I collagen sponges seeded at high cell density represent a suitable material for tissue engineering of cartilage.

Cross-linking density alters early metabolic activities in chondrocytes encapsulated in poly(ethylene glycol) hydrogels and cultured in the rotating wall vessel

In designing a tissue engineering strategy for cartilage repair, selection of both the bioreactor, and scaffold is important to the development of a mechanically functional tissue. The hydrodynamic environment associated with many bioreactors enhances nutrient transport, but also introduces fluid shear stress, which may influence cellular response. This study examined the combined effects of hydrogel cross-linking and the hydrodynamic environment on early chondrocyte response. Specifically, chondrocytes were encapsulated in poly(ethylene glycol) (PEG) hydrogels having two different cross-linked structures, corresponding to a low and high cross-linking density. Both cross-linked gels yielded high water contents (92% and 79%, respectively) and mesh sizes of 150 and 60 A respectively. Cell-laden PEG hydrogels were cultured in rotating wall vessels (RWV) or under static cultures for up to 5 days. Rotating cultures yielded low fluid shear stresses (< or = 0.11 Pa) at the hydrogel periphery indicating a laminar hydrodynamic environment. Chondrocyte response was measured through total DNA content, total nitric oxide (NO) production, and matrix deposition for glycosaminoglycans (GAG). In static cultures, gel cross-linking had no effect on DNA content, NO production, or GAG production; although GAG production increased with culture time for both cross-linked gels. In rotating cultures, DNA content increased, NO production decreased, and overall GAG production decreased when compared to static controls for the low cross-linked gels. For the high cross-linked gels, the hydrodynamic environment had no effect on DNA content, but exhibited similar results to the low cross-linked gel for NO production, and matrix production. Our findings demonstrated that at early culture times, when there is limited matrix production, the hydrodynamic environment dramatically influences cell response in a manner dependent on the gel cross-linking, which may impact long-term tissue development.

Autospecies and post-myocardial infarction sera enhance the viability, proliferation, and maturation of 3D cardiac cell culture

The limited ability of cardiac muscle to regenerate after an extensive myocardial infarction (MI) and the scarcity of cardiac donors have fueled the field of cardiac tissue engineering as a potential therapeutic approach to enhance cardiac function in post-MI patients. We are exploring the ex vivo bioengineering of cardiac muscle tissue by seeding isolated cardiac cells within alginate scaffolds and supplementing the culture with "smart" media. The hypothesis investigated herein is that sera derived from autospecies and from post-MI animals contain agents that might induce cell proliferation, survival, and maturation in vitro. The results of the metabolic activity of the neonatal cardiac cell constructs (6.4-51x10(6) cells/cm(3)), as measured by MTT viability assay, indicated a significant advantage (p < 0.05) to the constructs supplemented with serum from normal and post-MI adult rats compared to fetal calf serum (FCS) supplementation. H&E staining and alpha-sarcomeric actin immunofluorescence staining revealed thick viable cardiac cell clusters (150-300 microm), with abundant 3D architecture in the cardiac cell constructs supplemented with post-MI and normal adult rat serum. The number of cells positively immunostained with Ki-67, a cell proliferation marker, was significantly higher in post-MI adult rat serum-supplemented cultures compared to negative results in the FCS-supplemented culture. The results presented in this study indicate that media supplemented with post-MI adult rat serum and normal adult rat serum compared to FCS have a significant advantage in the regeneration of injured cardiac tissue.

Monitoring of metabolite gradients in tissue-engineered constructs

At present, the assessment of developing tissue-engineered constructs is almost always carried out destructively using biochemical or histological methods to determine cell number, viability and tissue growth throughout the construct. Since many of these experiments are long, taking weeks or even months to complete, simple and readily applicable non-destructive methods of monitoring changes in cell metabolism, viability and tissue deposition within the construct would be invaluable; such methods could point out adverse responses during the early stages of culture. Here, we describe the use of microdialysis for detecting local changes in cellular metabolism within a tissue-engineered construct. Three-dimensional constructs consisting of bovine articular chondrocytes entrapped in an alginate gel were cultured in a bioreactor for two weeks. Glucose and lactate were monitored by microdialysis, as the major nutrient and metabolite, respectively. Concentration gradients within the construct were evident, with the highest lactate concentrations in the construct centre. The local lactate concentration was a measure of cellular metabolic activity, decreasing as cellular activity fell and increasing as cellular activity was stimulated. Nutrient starvation and cell death in the construct centre could be readily detected in constructs deliberately cultured under adverse conditions. The results show that probe measurements can give an early warning of inappropriate local metabolic changes. Such information during the growth of tissue-engineered constructs would allow either corrective action or else an early end to an unsuccessful test.

Confocal laser scanning microscopy using dialkylcarbocyanine dyes for cell tracing in hard and soft biomaterials

The aim of this work was to study, in vitro, cell colonization of two biomaterials currently used for bone and cartilage repair, this step being important to understand the function of engineered tissues. Current methods that use histological approaches are not always suited to tissue-engineering analysis. We, therefore, set up a protocol to assess cell distribution, utilizing noninvasive confocal microscopy and fluorescent labels with a far red emission wavelength to optimize scaffold transparency and minimize light scattering. Hard (ceramic substitute) and soft (collagen sponge) biomaterials were seeded respectively, on one side of the scaffold, with human fibroblasts and bovine chondrocytes labelled with carbocyanine dyes (DiD and DiR). The mean penetration depth for DiR labelled fibroblasts and chondrocytes in the two scaffolds, around 270 m, was greater than for DiD (136-218 microm) labelled cells. These depths were independent of cell origin but were influenced by the nature of the scaffolds. Collagen sponge is transparent in contrast to ceramic substitutes where measurements could only be made in opened macropores. Besides the limits of the equipment, the limits of the supports were diffusion for collagen sponges and transmission for ceramic substitutes. Confocal microscopy techniques could thus be used to address the question of cell colonization of porous biomaterials in a noninvasive manner.

Effect of seeding and bioreactor culture conditions on the development of human tissue-engineered cartilage

Human cartilage was produced using fetal chondrocytes seeded into polyglycolic acid (PGA) mesh scaffolds and cultured in recirculation bioreactors. The effect of scaffold thickness, seeding cell density, and bioreactor operating conditions on the quality of the engineered cartilage was investigated. Thin (2.15-mm-thick) PGA scaffolds lost their structural integrity during bioreactor culture and the resulting constructs were small and misshapen compared with tissues generated using 4.75-mm-thick scaffolds. Increasing the seeding cell number from 1.2 x 10(7) to 2.2 x 10(7 )per 4.75-mm-thick scaffold resulted in a doubling of the construct wet weight, a 4.4-fold increase in glycosaminoglycan (GAG) concentration, and a 2.9-fold increase in total collagen concentration in the tissues. Levels of GAG and total collagen were also improved significantly when 100 mL or 50% v/v of the culture medium was replaced periodically during operation of the bioreactors compared with 50, 25, or 5 mL. The proportion of GAG lost from the tissues into the medium was reduced by increasing the seeding cell number and replaced medium volume. This work demonstrates that the quality of tissue-engineered cartilage can be manipulated substantially depending on the cell seeding and bioreactor culture conditions employed.

A Novel Simulation Model for Engineered Cartilage Growth in Static Systems

A novel mathematical model to simulate the growth of engineered cartilage in static systems is proposed. This model is based on material balances for the involved species (glycosaminoglycan and collagen, both pertaining to extracellular matrix), as well as mass-structured population balance for simulating cell growth and its proliferation within the scaffold. This model may simulate tissue growth on static culture taking place in Petri dishes, static flasks, and well plates for different types of scaffolds (i.e., poly(glycolic acid) [PGA], PGA/poly(l-lactic acid), and collagen sponge). This work aimed to demonstrate that the model approach proposed in previous works, regarding engineered cartilage growth on PGA scaffolds performed in rotating bioreactors, may also be applied to different scaffolds and system configurations. In particular, the balance equation for simulating collagen production is introduced, as well as the use of spatial averaging over the spatial region to compare experimental data with the model. Experimental data from the literature in terms of cells, glycosaminoglycans, and collagen content have been successfully compared with model results, thus demonstrating the validity of the proposed model, as well as its predictive capability.

Activation by IL-1 of bovine articular chondrocytes in culture within a 3D collagen-based scaffold. An in vitro model to address the effect of compounds with therapeutic potential in osteoarthritis

OBJECTIVE

To determine the best protocol for the preparation of a tissue-engineered cartilage to investigate the potential anti-arthritic and/or anti-osteoarthritic effects of drugs.

METHODS

Calf articular chondrocytes, seeded in collagen sponges were grown in culture for up to 1 month. At day 14 cultures received interleukin (IL)-1beta (ranging from 0.1 to 20 ng/ml) for 1 to 3 days. Analyses of gene expression for extracellular matrix proteins, collagen-binding integrins, matrix metalloproteinases (MMPs), aggrecanases, TIMPs, IL-1Ra and Ikappa-Balpha were carried out using real-time polymerase chain reaction (PCR). Metalloproteinase activities were analysed in the culture medium using both zymography and fluorogenic peptide substrates.

RESULTS

We selected a culture for 15 or 17 days with collagen sponges seeded with 10(7) chondrocytes showing a minimal cell proliferation, a maximal sulphated glycosaminoglycan (sGAG) deposition and a high expression of COL2A1, aggrecan and the alpha10 integrin sub-unit and low expression of COL1A2 and the alpha11 integrin sub-unit. In the presence of 1 ng/ml IL-1beta, we observed at day 15 up-regulations of 450-fold for MMP-1, 60-fold for MMP-13, 54-fold for ADAMTS-4 and MMP-3 and 10-fold for ADAMTS-5 and IL-1Ra. Down-regulations of 2.5-fold for COL2A1 and aggrecan were observed only at day 17. At the protein level a dose-dependent increase of total MMP-1 and MMP-13 was noted with less than 15% in the active form.

CONCLUSIONS

This in vitro model of chondrocyte culture in three dimensional (3D) seems well adapted to investigate the responses of these cells to inflammatory cytokines and to evaluate the potential anti-inflammatory effects of drugs.

Bovine chondrocyte behaviour in three-dimensional type I collagen gel in terms of gel contraction, proliferation and gene expression

This study evaluated the in vitro behaviour of bovine chondrocytes seeded in collagen gels, promising recently reported scaffolds for the treatment of full-thickness cartilage defects. To determine how chondrocytes respond to a collagen gel environment, 2 x 10(6) chondrocytes isolated from fetal, calf and adult bovine cartilage were seeded within type I collagen gels and grown for 12 days in both attached and floating (detached from the culture dish after polymerisation) conditions. Monolayer cultures were performed in parallel. All chondrocytes contracted floating gels to 55% of the initial size, by day 12. Contraction was dependent on initial cell density and inhibited by the presence of dihydrocytochalasin B as previously observed with fibroblasts. Gene expression was determined using conventional and real-time PCR. The chondrocyte phenotype was better maintained in floating gels compared to attached gels and monolayers. This was demonstrated by comparing the ratio of COL2A1/ COL1A2 mRNA and also of alpha10/alpha11 integrin mRNA. A strong up-regulation of MMP13 expression was measured at day 12 in floating gels. The composition of cartilage-like tissue obtained by growing chondrocytes in a collagen gel varied depending on the floating or attached conditions and initial cell density. It is thus important to consider these parameters when using this culture system in order to prepare a well-defined implant for cartilage repair.

Directing stem cell differentiation into the chondrogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in cartilage tissue engineering and reconstructive surgery. This requires well-defined and efficient protocols for directing the differentiation of stem cells into the chondrogenic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying chondrogenesis and cartilaginous tissue biology. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for cartilage-related biomaterials and drugs could also utilize protocols developed for the chondrogenic differentiation of stem cells. Hence, this review critically examines the various strategies that could be used to direct the differentiation of stem cells into the chondrogenic lineage in vitro.

Optimization of Dynamic Culture Conditions: Effects on Biosynthetic Activities of Chondrocytes Grown in Collagen Sponges

Application of mechanical stimulation, using dynamic bioreactors, is considered an effective strategy to enhance cellular behavior in load-bearing tissues. In this study, two types of perfusion mode (direct and free flow) are investigated in terms of the biosynthetic activities of chondrocytes grown in collagen sponges by assessment of cell proliferation rate, matrix production, and tissue morphology. Effects of the duration of preculture and dynamic conditioning are further determined. Our results have demonstrated that both bovine and human-derived chondrocytes demonstrate a dose-dependent response to flow rate (0-1 mL/min) in terms of cell number and glycosaminoglycan (GAG) content. This may reflect the weak adhesion of cells to the sponge scaffolds and the immature state of the constructs even after 3 weeks of proliferative culture. Our studies define an optimal flow rate between 0.1 and 0.3 mL/min for direct perfusion and free flow bioreactors. Using fresh bovine chondrocytes and a lower flow rate of 0.1 mL/min, a comparison was made between free flow system and direct perfusion system. In the free flow bioreactor, no cell loss was observed and higher GAG production was measured compared with static cultured controls. However, as with direct perfusion, the enhancement effect of free flow perfusion was strongly dependent on the maturation and organization of the constructs before the stimulation. To address the maturation of the matrix, preculture periods were varied before mechanical conditioning. An increase in culture duration of 18 days before mechanical conditioning resulted in enhanced GAG production compared with controls. Interestingly, additional enhancement was found in specimens that were further subjected to a prolonged duration of perfusion (63% increase after an additional 4 days of perfusion) after prematuration. The free flow system has an advantage over the direct perfusion system, especially when using sponge scaffolds, which have lower mechanical properties; however, mass transfer of nutrients is still more optimal throughout the scaffolds in a direct perfusion system as demonstrated by histological analysis.

Tissue engineering of human cartilage in bioreactors using single and composite cell-seeded scaffolds

Chondrocytes isolated from human fetal epiphyseal cartilage were seeded under mixed conditions into 15-mm-diameter polyglycolic acid (PGA) scaffolds and cultured in recirculation column bioreactors to generate cartilage constructs. After seeding, the cell distributions in thick (4.75 mm) and thin (2.15 mm) PGA disks were nonuniform, with higher cell densities accumulating near the top surfaces. Composite scaffolds were developed by suturing together two thin PGA disks after seeding to manipulate the initial cell distribution before bioreactor culture. The effect of medium flow direction in the bioreactors, including periodic reversal of medium flow, was also investigated. The quality of the tissue-engineered cartilage was assessed after 5 weeks of culture in terms of the tissue wet weight, glycosaminoglycan (GAG), total collagen and collagen type II contents, histological analysis of cell, GAG and collagen distributions, and immunohistochemical analysis of collagen types I and II. Significant enhancement in construct quality was achieved using composite scaffolds compared with single PGA disks. Operation of the bioreactors with periodic medium flow reversal instead of unidirectional flow yielded further improvements in tissue weight and GAG and collagen contents with the composite scaffolds. At harvest, the constructs contained GAG concentrations similar to those measured in ex vivo human adult articular cartilage; however, total collagen and collagen type II levels were substantially lower than those in adult tissue. This study demonstrates that the location of regions of high cell density in the scaffold coupled with application of dynamic bioreactor operating conditions has a significant influence on the quality of tissue-engineered cartilage.

Molecular diffusion in tissue-engineered cartilage constructs: Effects of scaffold material, time, and culture conditions

Diffusion is likely to be the primary mechanism for macromolecular transport in tissue-engineered cartilage, and providing an adequate nutrient supply via diffusion may be necessary for cell proliferation and extracellular matrix production. The goal of this study was to measure the diffusivity of tissue-engineered cartilage constructs as a function of scaffold material, culture conditions, and time in culture. Diffusion coefficients of four different-sized fluorescent dextrans were measured by fluorescence recovery after photobleaching in tissue-engineered cartilage constructs seeded with human adipose-derived stem cells or acellular constructs on scaffolds of alginate, agarose, gelatin, or fibrin that were cultured for 1 or 28 days in either chondrogenic or control conditions. Diffusivities in the constructs were much greater than those of native cartilage. The diffusivity of acellular constructs increased 62% from Day 1 to Day 28, whereas diffusivity of cellular constructs decreased 42% and 27% in chondrogenic and control cultures, respectively. The decrease in diffusivity in cellular constructs is likely due to new matrix synthesis, which may be enhanced with chondrogenic media, and matrix contraction by the cells in the fibrin and gelatin scaffolds. The increase in diffusivity in the acellular constructs is probably due to scaffold degradation and swelling.