Vascularized Tissue-Engineered Model for Studying Drug Resistance in Neuroblastoma

Neuroblastoma is a vascularized pediatric tumor derived from neural crest stem cells that displays vasculogenic mimicry and can express a number of stemness markers, such as SOX2 and NANOG. Tumor relapse is the major cause of succumbing to this disease, and properties attributed to cancer stem-like cells (CSLC), such as drug-resistance and cell plasticity, seem to be the key mechanisms. However, the lack of controllable models that recapitulate the features of human neuroblastoma limits our understanding of the process and impedes the development of new therapies. In response to these limitations, we engineered a perfusable, vascularized in vitro model of three-dimensional human neuroblastoma to study the effects of retinoid therapy on tumor vasculature and drug-resistance.

METHODS

The in vitro model of neuroblastoma was generated using cell-sheet engineering and cultured in a perfusion bioreactor. Firstly, we stacked three cell sheets containing SKNBE(2) neuroblastoma cells and HUVEC. Then, a vascular bed made of fibrin, collagen I and HUVEC cells was placed onto a collagen-gel base with 8 microchannels. After gelling, the stacked cell sheets were placed on the vascular bed and cultured in the perfusion bioreactor (perfusion rate: 0.5 mL/min) for 4 days. Neuroblastoma models were treated with 10μM isotretionin in single daily doses for 5 days.

RESULTS

The bioengineered model recapitulated vasculogenic mimicry (vessel-like structure formation and tumor-derived endothelial cells-TECs), and contained CSLC expressing SOX2 and NANOG. Treatment with Isotretinoin destabilized vascular networks but failed to target vasculogenic mimicry and augmented populations of CSLCs expressing high levels of SOX2. Our results suggest that CSLCs can transdifferentiate into drug resistant CD31⁺-TECs, and reveal the presence of an intermediate state STEC (stem tumor-derived endothelial cell) expressing both SOX2 and CD31.

CONCLUSION

Our results reveal some roles of SOX2 in drug resistance and tumor relapse, and suggest that SOX2 could be a therapeutic target in neuroblastoma.

Mimicking biophysical stimuli within bone tumor microenvironment

In vivo, cells reside in a complex environment regulating their fate and function. Most of this complexity is lacking in standard in vitro models, leading to readouts falling short of predicting the actual in vivo situation. The use of engineering tools, combined with deep biological knowledge, leads to the development and use of bioreactors providing biologically sound niches. Such bioreactors offer new tools for biological research, and are now also entering the field of cancer research. Here we present the development and validation of a modular bioreactor system providing: (i) high throughput analyses, (ii) a range of biological conditions, (iii) high degree of control, and (iv) application of physiological stimuli to the cultured samples. The bioreactor was used to engineer a three-dimensional (3D) tissue model of cancer, where the effects of mechanical stimulation on the tumor phenotype were evaluated. Mechanical stimuli applied to the engineered tumor model activated the mechanotransduction machinery and resulted in measurable changes of mRNA levels towards a more aggressive tumor phenotype.

Design of electrical stimulation bioreactors for cardiac tissue engineering

Electrical stimulation has been shown to improve functional assembly of cardiomyocytes in vitro for cardiac tissue engineering. Carbon electrodes were found in past studies to have the best current injection characteristics. The goal of this study was to develop rational experimental design principles for the electrodes and stimulation regime, in particular electrode configuration, electrode ageing, and stimulation amplitude. Carbon rod electrodes were compared via electrochemical impedance spectroscopy (EIS) and we identified a safety range of 0 to 8 V/cm by comparing excitation thresholds and maximum capture rates for neonatal rat cardiomyocytes cultured with electrical stimulation. We conclude with recommendations for studies involving carbon electrodes for cardiac tissue engineering.

Engineering of functional contractile cardiac tissues cultured in a perfusion system

Overcoming the limitations of diffusional transport in conventional culture systems remains an open issue for successfully generating thick, compact and functional cardiac tissues. Previously, it was shown that perfusion systems enhance the yield and uniformity of cell seeding and cell survival in thick cardiac constructs. The aim of our study was to form highly functional cardiac constructs starting from spatially uniform, high density cell seeded constructs. Disk-shaped elastomeric poly(glycerol sebacate) scaffolds were seeded with neonatal rat cardiomyocytes and cultured for eight days with direct perfusion of culture medium or statically in a six-well plate. In the perfusion experimental group, the integrity of some disks was well maintained, whereas in others a central hole was formed, resulting in ring-shaped constructs. This allowed us to also study the effects of construct geometry and of interstitial flow versus channel perfusion. The ring-shaped constructs appeared to have a denser and more uniform deposition of extracellular matrix. In response to electrical stimulation, the fractional area change of the ring-shaped constructs was 7.3 and 2.7 times higher than for disk-shaped tissues cultured in perfusion or statically, respectively. These findings suggest that a combination of many factors, including scaffold elasticity and geometry and the type of perfusion system applied, need to be considered in order to engineer a cardiac construct with high contractile activity.

Biomimetic approach to cardiac tissue engineering

Here, we review an approach to tissue engineering of functional myocardium that is biomimetic in nature, as it involves the use of culture systems designed to recapitulate some aspects of the actual in vivo environment. To mimic the capillary network, subpopulations of neonatal rat heart cells were cultured on a highly porous elastomer scaffold with a parallel array of channels perfused with culture medium. To mimic oxygen supply by haemoglobin, the culture medium was supplemented with a perfluorocarbon (PFC) emulsion. Constructs cultivated in the presence of PFC contained higher amounts of DNA and cardiac markers and had significantly better contractile properties than control constructs cultured without PFC. To induce synchronous contractions of cultured constructs, electrical signals mimicking those in native heart were applied. Over only 8 days of cultivation, electrical stimulation induced cell alignment and coupling, markedly increased the amplitude of synchronous construct contractions and resulted in a remarkable level of ultrastructural organization. The biomimetic approach is discussed in the overall context of cardiac tissue engineering, and the possibility to engineer functional human cardiac grafts based on human stem cells.

Porous silk scaffolds can be used for tissue engineering annulus fibrosus

There is no optimal treatment for symptomatic degenerative disc disease which affects millions of people worldwide. One novel approach would be to form a patch or tissue replacement to repair the annulus fibrosus (AF) through which the NP herniates. As the optimal scaffold for this has not been defined the purpose of this study was to determine if porous silk scaffolds would support AF cell attachment and extracellular matrix accumulation and whether chemically decorating the scaffold with RGD peptide, which has been shown to enhance attachment for other cell types, would further improve AF cell attachment and tissue formation. Annulus fibrosus cells were isolated from bovine caudal discs and seeded into porous silk scaffolds. The percent cell attachment was quantified and the cell morphology and distribution within the scaffold was evaluated using scanning electron microscopy. The cell-seeded scaffolds were grown for up to 8 weeks and evaluated for gene expression, histological appearance and matrix accumulation. AF cells attach to porous silk scaffolds, proliferate and synthesize and accumulate extracellular matrix as demonstrated biochemically and histologically. Coupling the silk scaffold with RGD-peptides did not enhance cell attachment nor tissue formation but did affect cell morphology. As well, the cells had higher levels of type II collagen and aggrecan gene expression when compared to cells grown on the non-modified scaffold, a feature more in keeping with cells of the inner annulus. Porous silk is an appropriate scaffold on which to grow AF cells. Coupling RGD peptide to the scaffold appears to influence AF cell phenotype suggesting that it may be possible to select an appropriate scaffold that favours inner annulus versus outer annulus differentiation which will be important for tissue engineering an intervertebral disc.

Silk fibroin as an organic polymer for controlled drug delivery

The pharmaceutical utility of silk fibroin (SF) materials for drug delivery was investigated. SF films were prepared from aqueous solutions of the fibroin protein polymer and crystallinity was induced and controlled by methanol treatment. Dextrans of different molecular weights, as well as proteins, were physically entrapped into the drug delivery device during processing into films. Drug release kinetics were evaluated as a function of dextran molecular weight, and film crystallinity. Treatment with methanol resulted in an increase in beta-sheet structure, an increase in crystallinity and an increase in film surface hydrophobicity determined by FTIR, X-ray and contact angle techniques, respectively. The increase in crystallinity resulted in the sustained release of dextrans of molecular weights ranging from 4 to 40 kDa, whereas for less crystalline films sustained release was confined to the 40 kDa dextran. Protein release from the films was studied with horseradish peroxidase (HRP) and lysozyme (Lys) as model compounds. Enzyme release from the less crystalline films resulted in a biphasic release pattern, characterized by an initial release within the first 36 h, followed by a lag phase and continuous release between days 3 and 11. No initial burst was observed for films with higher crystallinity and subsequent release patterns followed linear kinetics for HRP, or no substantial release for Lys. In conclusion, SF is an interesting polymer for drug delivery of polysaccharides and bioactive proteins due to the controllable level of crystallinity and the ability to process the biomaterial in biocompatible fashion under ambient conditions to avoid damage to labile compounds to be delivered.

Towards tissue engineering of meniscus substitutes: selection of cell source and culture environment

With the ultimate goal to engineer a meniscus substitute based on autologous cells, we aimed this work at identifying (i) a human cell source capable of generating fibrocartilaginous tissues and (ii) a culture environment promoting the development of bi-zonal constructs, resembling the complex structure and function of a meniscus. The post-expansion differentiation capacity of different chondrogenic cells readily available by knee arthroscopy, namely inner meniscus, fat pad, synovial membrane cells and articular chondrocytes (AC), was assessed within hyaluronan based non-woven meshes. Under our experimental conditions, only expanded AC generated tissues containing relevant amounts of glycosaminoglycans (GAG) and with cell phenotypes compatible with those of the inner and outer meniscus regions. Physical conditioning of constructs generated by expanded AC was applied using mixed flasks. The hydrodynamic environment of mixed flasks was instrumental to promote the formation of bi-zonal tissues, with an inner region rich in GAG and stiffer in compression and an outer rim rich in collagen and stiffer in tension. Therefore, the use of AC cultured within porous scaffolds in mixed flasks allowed engineering of constructs resembling some aspects of the phenotype and function of meniscus tissue.

Bioreactor cultivation of osteochondral grafts

The clinical utility of tissue engineering depends upon our ability to direct cells to form tissues with characteristic structural and mechanical properties across different hierarchical scales. Ideally, an engineered graft should be tailored to (re)establish the structure and function of the native tissue being replaced. Engineered grafts of such high fidelity would also foster fundamental research by serving as physiologically relevant models for quantitative in vitro studies. The approach discussed here involves the use of human mesenchymal stem cells (hMSC) cultured on custom-designed scaffolds (providing a structural and logistic template for tissue development) in bioreactors (providing environmental control, biochemical and mechanical cues). Cartilage, bone and ligaments have been engineered by using hMSC, highly porous protein scaffolds (collagen; silk) and bioreactors (perfused cartridges with or without mechanical loading). In each case, the scaffold and bioreactor were designed to recapitulate some aspects of the environment present in native tissues. Medium flow facilitated mass transport to the cells and thereby enhanced the formation of all three tissues. In the case of cartilage, dynamic laminar flow patterns were advantageous as compared to either turbulent steady flow or static (no flow) cultures. In the case of bone, medium flow affected the geometry, distribution and orientation of the forming bone-like trabeculae. In the case of ligament, applied mechanical loading (a combination of dynamic stretch and torsion) markedly enhanced cell differentiation, alignment and functional assembly. Taken together, these studies provide a basis for the ongoing work on engineering osreochondral grafts for a variety of potential applications, including those in the craniofacial complex.

Adjacent tissues (cartilage, bone) affect the functional integration of engineered calf cartilage in vitro

OBJECTIVE

An in vitro model was used to test the hypothesis that culture time and adjacent tissue structure and composition affected chondrogenesis and integrative repair in engineered cartilage.

METHOD

Engineered constructs made of bovine calf chondrocytes and hyaluronan benzyl ester non-woven mesh were press-fitted into adjacent tissue rings made of articular cartilage (AC), devitalized bone (DB), or vital bone (VB) and cultured in rotating bioreactors for up to 8 weeks. Structure (light and electron microscopy), biomechanical properties (interfacial adhesive strength, construct compressive modulus), biochemical composition (construct glycosaminoglycans (GAG), collagen, and cells), and adjacent tissue diffusivity were assessed.

RESULTS

Engineered constructs were comprised predominately of hyaline cartilage, and appeared either closely apposed to adjacent cartilage or functionally interdigitated with adjacent bone due to interfacial deposition of extracellular matrix. An increase in culture time significantly improved construct adhesive strength (P<0.001), modulus (P=0.02), GAG (P=0.04) and cellularity (P<0.001). The type of adjacent tissue significantly affected construct adhesion (P<0.001), modulus (P<0.001), GAG (P<0.001) and collagen (P<0.001). For constructs cultured in rings of cartilage, negative correlations were observed between ring GAG content (log transformed) and construct adhesion (R2=0.66, P<0.005), modulus (R2=0.49, P<0.05) and GAG (R2=0.44, P<0.05). Integrative repair was better for constructs cultured adjacent to bone than cartilage, in association with its solid architectural structure and high GAG content, and best for constructs cultured adjacent to DB, in association with its high diffusivity.

CONCLUSIONS

Chondrogenesis and integrative repair in engineered cartilage improved with time and depended on adjacent tissue architecture, composition, and transport properties.

Tissue engineering of ligaments

Tissue engineering is emerging as a significant clinical option to address tissue and organ failure by implanting biological substitutes for the compromised tissues. As compared to the transplantation of cells alone, engineered tissues offer the potential advantage of immediate functionality. Engineered tissues can also serve as physiologically relevant models for controlled studies of cells and tissues designed to distinguish the effects of specific signals from the complex milieu of factors present in vivo. A high number of ligament failures and the lack of adequate options to fully restore joint functions have prompted the need to develop new tissue engineering strategies. We discuss the requirements for ligament reconstruction, the available treatment options and their limitations, and then focus on the tissue engineering of ligaments. One representative tissue engineering system involving the integrated use of adult human stem cells, custom-designed scaffolds, and advanced bioreactors with dynamic loading is described.

Mechanical stimulation promotes osteogenic differentiation of human bone marrow stromal cells on 3-D partially demineralized bone scaffolds in vitro

Bone is a dynamic tissue that is able to sense and adapt to mechanical stimuli by modulating its mass, geometry, and structure. Bone marrow stromal cells (BMSCs) are known to play an integral part in bone formation by providing an osteoprogenitor cell source capable of differentiating into mature osteoblasts in response to mechanical stresses. Characteristics of the in vivo bone environment including the three dimensional (3-D) lacunocanalicular structure and extracellular matrix composition have previously been shown to play major roles in influencing mechanotransduction processes within bone cells. To more accurately model this phenomenon in vitro, we cultured human BMSCs on 3-D, partially demineralized bone scaffolds in the presence of four-point bending loads within a novel bioreactor. The effect of mechanical loading and dexamethasone concentration on BMSC osteogenic differentiation and mineralized matrix production was studied for 8 and 16 days of culture. Mechanical stimulation after 16 days with 10 nM dexamethasone promoted osteogenic differentiation of BMSCs by significantly elevating alkaline phosphatase activity as well as alkaline phosphatase and osteopontin transcript levels over static controls. Mineralized matrix production also increased under these culture conditions. Dexamethasone concentration had a dramatic effect on the ability of mechanical stimulation to modulate these phenotypic and genotypic responses. These results provide increased insight into the role of mechanical stimulation on osteogenic differentiation of human BMSCs in vitro and may lead to improved strategies in bone tissue engineering.

[Engineering and characterization of functional osteochondral replacement tissue]

Extensive osteochondral lesions require repair of the cartilage and underlying bone. We generated osteochondral repair tissue by tissue engineering. Standardized defects, 7 x 5 x 5 mm, were created in femoropatellar grooves of adult rabbits. Engineered cartilage, generated in vitro starting from chondrocytes and a biodegradable scaffold, was implanted using Collagraft as subchondral support. Cell-free implants, defects left empty, and unoperated knee joints served as controls. Explants were characterized morphologically and mechanically. Engineered cartilage implants were superior to cell-free implants and to natural healing of empty defects with respect to the histologic score and Young's modulus of the 6-month repair tissue. These data suggest that engineered cartilage can provide primary stability for the treatment of critical osteochondral defects.

Long-term culture of tissue engineered cartilage in a perfused chamber with mechanical stimulation

One approach to functional tissue engineering of cartilage is to utilize bioreactors to provide environmental conditions that stimulate chondrogenesis in cells cultured on biomaterial scaffolds. We report the combined use of a three-dimensional in vitro model and a novel bioreactor with perfusion of culture medium and mechanical stimulation in long-term studies of cartilage development and function. To engineer cartilage, scaffolds made of a non-woven mesh of polyglycolic acid (PGA) were seeded with bovine calf articular chondrocytes, cultured for an initial 30-day period under free swelling conditions, and cultured for an additional 37 day period in one of the three groups: (1) free-swelling, (2) static compression (on 24 h/day, strain control, static offset 10%), and (3) dynamic compression (on 1 h/day; off 23 h/day; strain control, static offset 2%, dynamic strain amplitude 5%; frequency 0.3 Hz). Constructs were sampled at timed intervals and assessed with respect to structure, biochemical composition, and mechanical function. Mechanical simulation had little effect on the compositions, morphologies and on mechanical properties of construct interiors discs, but it resulted in distincly different properties of the peripheral rings and face sides. Contructs cultured with mechanical loading maintained their cylindrical shape with flat and parallel top and bottom surfaces, and retained larger amounts of GAG. The modular bioreactor system with medium perfusion and mechanical loading can be utilized to define the conditions of cultivation for functional tissue engineering of cartilage.

Bioreactor studies of native and tissue engineered cartilage

Functional tissue engineering of cartilage involves the use of bioreactors designed to provide a controlled in vitro environment that embodies some of the biochemical and physical signals known to regulate chondrogenesis. Hydrodynamic conditions can affect in vitro tissue formation in at least two ways: by direct effects of hydrodynamic forces on cell morphology and function, and by indirect flow-induced changes in mass transfer of nutrients and metabolites. In the present work, we discuss the effects of three different in vitro environments: static flasks (tissues fixed in place, static medium), mixed flasks (tissues fixed in place, unidirectional turbulent flow) and rotating bioreactors (tissues dynamically suspended in laminar flow) on engineered cartilage constructs and native cartilage explants. As compared to static and mixed flasks, dynamic laminar flow in rotating bioreactors resulted in the most rapid tissue growth and the highest final fractions of glycosaminoglycans and total collagen in both tissues. Mechanical properties (equilibrium modulus, dynamic stiffness, hydraulic permeability) of engineered constructs and explanted cartilage correlated with the wet weight fractions of glycosaminoglycans and collagen. Current research needs in the area of cartilage tissue engineering include the utilization of additional physiologically relevant regulatory signals, and the development of predictive mathematical models that enable optimization of the conditions and duration of tissue culture.

Bone morphogenetic proteins-2, -12, and -13 modulate in vitro development of engineered cartilage

Bovine calf articular chondrocytes were seeded onto biodegradable polyglycolic acid (PGA) scaffolds and cultured in either control medium or medium supplemented with 1, 10, or 100 ng/mL of bone morphogenetic proteins (BMPs) BMP-2, BMP-12, or BMP-13. Under all conditions investigated, cell-polymer constructs cultivated for 4 weeks in vitro macroscopically and histologically resembled native cartilage. Addition of 100 ng/mL of BMP-2, BMP-12, or BMP-13 increased the total mass of the constructs relative to the controls by 121%, 80%, and 62%, respectively, which was accompanied by increases in the absolute amounts of collagen, glycosaminoglycans (GAG), and cells. The addition of 100 ng/mL of BMP-2, BMP-12, or BMP-13 increased the weight percentage of GAG in the constructs by 27%, 18%, and 15%, and decreased the weight percent of total collagen to 63%, 89%, and 83% of controls, respectively. BMP-2, but not BMP-12 or BMP-13 promoted chondrocyte hypertrophy. Taken together, these data suggest that BMP-2, BMP-12, and BMP-13 increase growth rate and modulate the composition of engineered cartilage and that 100 ng/mL of BMP-2 has the greatest effect. In addition, in vitro engineered cartilage provides a system for studying the effects of BMPs on chondrogenesis in a well-defined environment.

Growth factors for sequential cellular de- and re-differentiation in tissue engineering

A model system for the in vitro generation of cartilaginous constructs was used to study a tissue engineering paradigm whereby sequentially applied growth factors promoted chondrocytes to first de-differentiate into a proliferative state and then re-differentiate and undergo chondrogenesis. Early cultivation in medium with supplemental TGF-beta1/FGF-2 doubled cell fractions in 2-week constructs compared to unsupplemented controls. Subsequent culture with supplemental IGF-I yielded large 4-week constructs with high fractions of cartilaginous extracellular matrix (ECM) and high compressive moduli, whereas prolonged culture with supplemental FGF-2 yielded small 4-week constructs with low ECM fractions and moduli. Sequential supplementation with TGF-beta1/FGF-2 and then IGF-I yielded 4-week constructs with type-specific mRNA expression and protein levels that were high for type II and negligible for type I collagen, in contrast to other growth factor regimens studied. The data demonstrate that structural, functional, and molecular properties of engineered cartilage can be modulated by sequential application of growth factors.

Integration of engineered cartilage

The structure and function of cartilaginous constructs, engineered in vitro using bovine articular chondrocytes, biodegradable scaffolds and bioreactors, can be modulated by the conditions and duration of tissue cultivation. We hypothesized that the integrative properties of engineered cartilage depend on developmental stage of the construct and the extracellular matrix content of adjacent cartilage, and that some aspects of integration can be studied under controlled in vitro conditions. Disc-shaped constructs (cultured for 5+/-1 days or 5+/-1 weeks) or explants (untreated or trypsin treated cartilage) were sutured into ring-shaped explants (untreated or trypsin treated cartilage) to form composites that were cultured for an additional 1-8 weeks in bioreactors and evaluated biochemically, histologically and mechanically (compressive stiffness of the central disk, adhesive strength of the integration interface). Immature constructs had poorer mechanical properties but integrated better than either more mature constructs or cartilage explants. Integration of immature constructs involved cell proliferation and the progressive formation of cartilaginous tissue, in contrast to the integration of more mature constructs or native cartilage which involved only the secretion of extracellular matrix components. Integration patterns correlated with the adhesive strength of the disc-ring interface, which was markedly higher for immature constructs than for either more mature constructs or cartilage explants. Trypsin treatment of the adjacent cartilage further enhanced the integration of immature constructs.

IGF-I and mechanical environment interact to modulate engineered cartilage development

Bovine calf articular chondrocytes were seeded onto biodegradable polyglycolic acid scaffolds and cultured for four weeks using in vitro systems providing different mechanical environments (static and mixed Petri dishes, static and mixed flasks, and rotating vessels) and different biochemical environments (medium with and without supplemental insulin-like growth factor I, IGF-I). Under all conditions, the resulting engineered tissue histologically resembled cartilage and contained its major constituents: glycosaminoglycans, collagen, and cells. The mechanical environment and supplemental IGF-I (a) independently modulated tissue morphology, growth, biochemical composition, and mechanical properties (equilibrium modulus) of engineered cartilage as previously reported; (b) interacted additively or in some cases nonadditively producing results not suggested by the independent responses, and (c) in combination produced tissue superior to that obtained by modifying these factors individually.

Enhanced cartilage tissue engineering by sequential exposure of chondrocytes to FGF-2 during 2D expansion and BMP-2 during 3D cultivation

Bovine calf articular chondrocytes, either primary or expanded in monolayers (2D) with or without 5 ng/ml fibroblast growth factor-2 (FGF-2), were cultured on three-dimensional (3D) biodegradable polyglycolic acid (PGA) scaffolds with or without 10 ng/ml bone morphogenetic protein-2 (BMP-2). Chondrocytes expanded without FGF-2 exhibited high intensity immunostaining for smooth muscle alpha-actin (SMA) and collagen type I and induced shrinkage of the PGA scaffold, thus resembling contractile fibroblasts. Chondrocytes expanded in the presence of FGF-2 and cultured 6 weeks on PGA scaffolds yielded engineered cartilage with 3.7-fold higher cell number, 4.2-fold higher wet weight, and 2.8-fold higher wet weight glycosaminoglycan (GAG) fraction than chondrocytes expanded without FGF-2. Chondrocytes expanded with FGF-2 and cultured on PGA scaffolds in the presence of BMP-2 for 6 weeks yielded engineered cartilage with similar cellularity and size, 1.5-fold higher wet weight GAG fraction, and more homogenous GAG distribution than the corresponding engineered cartilage cultured without BMP-2. The presence of BMP-2 during 3D culture had no apparent effect on primary chondrocytes or those expanded without FGF-2. In summary, the presence of FGF-2 during 2D expansion reduced chondrocyte expression of fibroblastic molecules and induced responsiveness to BMP-2 during 3D cultivation on PGA scaffolds.

Selective differentiation of mammalian bone marrow stromal cells cultured on three-dimensional polymer foams

Bone marrow stromal cells (BMSC) are pluripotent progenitor cells that can regenerate different skeletal tissues in response to environmental signals. In this study, we used highly porous, structurally stable three-dimensional polymer foams in conjunction with specific regulatory molecules to selectively differentiate mammalian BMSC into either cartilaginous or bone-like tissues. Bovine BMSC were expanded in monolayers and cultured on 5-mm-diameter, 2-mm-thick foams made of poly(lactic-co-glycolic acid) and poly(ethylene glycol). Constructs maintained their original size and shape for up to 4 weeks of culture and supported BMSC growth and production of extracellular matrix (ECM). By proper use of chondrogenic (dexamethasone, insulin, transforming growth factor-beta1) or osteogenic (dexamethasone, beta-glycerophosphate) medium supplements, we could control whether the generated ECM was cartilaginous (containing collagen type II and sulfated glycosaminoglycans) or bone-like (containing osteocalcin, osteonectin, and mineralized foci). After 4 weeks of cultivation, cartilaginous and bone-like ECM were uniformly distributed throughout the construct volume and respectively represented 34.2 +/- 9.3% and 12.6 +/- 3.2% of the total available area. BMSC culture on poly(lactic-co-glycolic acid)/poly(ethylene glycol) foams provides a three-dimensional model system to study the development of mesenchymal tissues in vitro and has potential applications in engineering autologous grafts for skeletal tissue repair.

Bioreactor studies of natural and tissue engineered cartilage

Bioreactors provide controlled environments for tissue cultivation and evaluation of the effect of specific biochemical and physical parameters on in vitro chondrogenesis. The hydrodynamic environment is expected to modulate the in vitro tissue formation in at least two ways: directly via hydrodynamic effects on cell morphology and function, and indirectly via flow-induced mass transfer of nutrients and metabolites. We investigated and compared the effects of three different hydrodynamic environments: static flasks (tissues fixed in place, static medium), mixed flasks (tissues fixed in place, unidirectional turbulent flow) and rotating bioreactors (tissues dynamically suspended in laminar flow) on the morphology and composition and metabolic function of engineered and natural cartilage over an in vitro culture period of 6 weeks. In general, engineered and natural cartilage responded in a similar manner. Static conditions were associated with increased rates of GAG release into tissue culture medium and the formation of an outer fibrous capsule in both engineered and natural cartilage. In contrast, dynamic laminar flow in rotating bioreactors provided efficient oxygen supply and resulted in the retention of newly synthesized macromolecules, the maintenance of cartilaginous tissue morphology, and the best overall tissue structure and composition for both engineered and natural cartilage.

Effects of mixing intensity on tissue-engineered cartilage

Mechanical forces regulate the structure and function of many tissues in vivo; recent results indicate that the mechanical environment can decisively influence the development of engineered tissues cultured in vitro. To investigate the effects of the hydrodynamic environment on tissue-engineered cartilage, primary bovine calf chondrocytes were seeded on fibrous polyglycolic acid meshes and cultured in spinner flasks either statically or at one of nine different turbulent mixing intensities. In medium from unmixed flasks, CO(2) accumulated and O(2) was depleted, whereas in medium from mixed flasks the concentrations of both gases approached their equilibrium values. Relative to constructs exposed to nonmixed conditions, constructs exposed to mixing contained higher fractions of collagen, synthesized and released more GAG, but contained lower fractions of GAG. Across the wide range of mixing intensities investigated, the presence or absence of mixing, but not the intensity of the mixing, was the primary determinant of the GAG and collagen content in the constructs. The all-or-none nature of these responses may provide insight into the mechanism(s) by which engineered cartilage perceives changes in its hydrodynamic environment and responds by modifying extracellular matrix production and release. 2001 John Wiley & Sons, Inc.

Tissue engineering of functional cardiac muscle: molecular, structural, and electrophysiological studies

The primary aim of this study was to relate molecular and structural properties of in vitro reconstructed cardiac muscle with its electrophysiological function using an in vitro model system based on neonatal rat cardiac myocytes, three-dimensional polymeric scaffolds, and bioreactors. After 1 wk of cultivation, we found that engineered cardiac muscle contained a 120- to 160-microm-thick peripheral region with cardiac myocytes that were electrically connected through gap junctions and sustained macroscopically continuous impulse propagation over a distance of 5 mm. Molecular, structural, and electrophysiological properties were found to be interrelated and depended on specific model system parameters such as the tissue culture substrate, bioreactor, and culture medium. Native tissue and the best experimental group (engineered cardiac muscle cultivated using laminin-coated scaffolds, rotating bioreactors, and low-serum medium) were comparable with respect to the conduction velocity of propagated electrical impulses and spatial distribution of connexin43. Furthermore, the structural and electrophysiological properties of the engineered cardiac muscle, such as cellularity, conduction velocity, maximum signal amplitude, capture rate, and excitation threshold, were significantly improved compared with our previous studies.

In vitro generation of osteochondral composites

Osteochondral repair involves the regeneration of articular cartilage and underlying bone, and the development of a well-defined tissue-to-tissue interface. We investigated tissue engineering of three-dimensional cartilage/bone composites based on biodegradable polymer scaffolds, chondrogenic and osteogenic cells. Cartilage constructs were created by cultivating primary bovine calf articular chondrocytes on polyglycolic acid meshes; bone-like constructs were created by cultivating expanded bovine calf periosteal cells on foams made of a blend of poly-lactic-co-glycolic acid and polyethylene glycol. Pairs of constructs were sutured together after 1 or 4 weeks of isolated culture, and the resulting composites were cultured for an additional 4 weeks. All composites were structurally stable and consisted of well-defined cartilaginous and bone-like tissues. The fraction of glycosaminoglycan in the cartilaginous regions increased with time, both in isolated and composite cultures. In contrast, the mineralization in bone-like regions increased during isolated culture, but remained approximately constant during the subsequent composite culture. The integration at the cartilage/bone interface was generally better for composites consisting of immature (1-week) than mature (4-week) constructs. This study demonstrates that osteochondral tissue composites for potential use in osteochondral repair can be engineered in vitro by culturing mammalian chondrocytes and periosteal cells on appropriate polymer scaffolds.

Modulation of the mechanical properties of tissue engineered cartilage

Cartilaginous constructs have been grown in vitro using chondrocytes, biodegradable polymer scaffolds, and tissue culture bioreactors. In the present work, we studied how the composition and mechanical properties of engineered cartilage can be modulated by the conditions and duration of in vitro cultivation, using three different environments: static flasks, mixed flasks, and rotating vessels. After 4-6 weeks, static culture yielded small and fragile constructs, while turbulent flow in mixed flasks induced the formation of an outer fibrous capsule; both environments resulted in constructs with poor mechanical properties. The constructs that were cultured freely suspended in a dynamic laminar flow field in rotating vessels had the highest fractions of glycosaminoglycans and collagen (respectively 75% and 39% of levels measured in native cartilage), and the best mechanical properties (equilibrium modulus, hydraulic permeability, dynamic stiffness, and streaming potential were all about 20% of values measured in native cartilage). Chondrocytes in cartilaginous constructs remained metabolically active and phenotypically stable over prolonged cultivation in rotating bioreactors. The wet weight fraction of glycosaminoglycans and equilibrium modulus of 7 month constructs reached or exceeded the corresponding values measured from freshly explanted native cartilage. Taken together, these findings suggest that functional equivalents of native cartilage can be engineered by optimizing the hydrodynamic conditions in tissue culture bioreactors and the duration of tissue cultivation.

Mammalian chondrocytes expanded in the presence of fibroblast growth factor 2 maintain the ability to differentiate and regenerate three-dimensional cartilaginous tissue

The differentiated phenotype of chondrocytes from hyaline cartilage is gradually lost during expansion in monolayers. Chondrocytes can reexpress their differentiated phenotype by transfer into an environment that prevents cell flattening, but serially passaged cells never completely recover their chondrogenic potential. We report that chondrocytes expanded (up to 2000-fold) in the presence of fibroblast growth factor 2 (FGF-2) dedifferentiated, but fully maintained their potential for redifferentiation in response to environmental changes. After seeding onto three-dimensional polymer scaffolds, chondrocytes expanded in the presence of FGF-2 formed cartilaginous tissue that was histologically and biochemically comparable to that obtained using primary chondrocytes, in contrast to chondrocytes expanded to the same degree but in the absence of FGF-2. The presence of FGF-2 inhibited the formation of thick F-actin structures, which otherwise formed during monolayer expansion, were maintained during tissue cultivation, and were associated with reduced ability of chondrocytes to reexpress their differentiated phenotype. This study provides evidence that FGF-2 maintains the chondrogenic potential during chondrocyte expansion in monolayers, possibly due to changes in the architecture of F-actin elements and allows more efficient utilization of harvested tissue for cartilage tissue engineering.

Frontiers in tissue engineering. In vitro modulation of chondrogenesis

Cartilage tissue engineering can provide functional cartilaginous constructs that can be used for controlled in vitro studies of chondrogenesis and potentially for in vivo articular cartilage repair. Ideally, engineered cartilage should be indistinguishable from native articular cartilage with respect to zonal organization, biochemical composition, and mechanical properties. In the model system presented here, chondrogenic cells are expanded in vitro as required, seeded onto three-dimensional polymeric scaffolds, and cultured in bioreactor vessels. During the course of in vitro cultivation, construct cellularity plateaus at a physiologic level, fractions of glycosaminoglycan and Type II collagen increase progressively, and the scaffold biodegrades. Construct structure (composition, morphology) and function (biosynthetic activity, mechanical properties) depend on cultivation conditions. This paper reviews recent studies of in vitro modulation of chondrogenesis by: (1) cell seeding density and source; (2) the tissue regeneration template; (3) biochemical regulatory signals; (4) mixing, mass transport and hydrodynamic forces; and (5) cultivation time. Key requirements and some of the critical research needs for successful cartilage tissue engineering are discussed.

Cardiac tissue engineering: cell seeding, cultivation parameters, and tissue construct characterization

Cardiac tissue engineering has been motivated by the need to create functional tissue equivalents for scientific studies and cardiac tissue repair. We previously demonstrated that contractile cardiac cell-polymer constructs can be cultivated using isolated cells, 3-dimensional scaffolds, and bioreactors. In the present work, we examined the effects of (1) cell source (neonatal rat or embryonic chick), (2) initial cell seeding density, (3) cell seeding vessel, and (4) tissue culture vessel on the structure and composition of engineered cardiac muscle. Constructs seeded under well-mixed conditions with rat heart cells at a high initial density ((6-8) x 10(6) cells/polymer scaffold) maintained structural integrity and contained macroscopic contractile areas (approximately 20 mm(2)). Seeding in rotating vessels (laminar flow) rather than mixed flasks (turbulent flow) resulted in 23% higher seeding efficiency and 20% less cell damage as assessed by medium lactate dehydrogenase levels (p < 0.05). Advantages of culturing constructs under mixed rather than static conditions included the maintenance of metabolic parameters in physiological ranges, 2-4 times higher construct cellularity (p &le 0.0001), more aerobic cell metabolism, and a more physiological, elongated cell shape. Cultivations in rotating bioreactors, in which flow patterns are laminar and dynamic, yielded constructs with a more active, aerobic metabolism as compared to constructs cultured in mixed or static flasks. After 1-2 weeks of cultivation, tissue constructs expressed cardiac specific proteins and ultrastructural features and had approximately 2-6 times lower cellularity (p < 0.05) but similar metabolic activity per unit cell when compared to native cardiac tissue.

Method for quantitative analysis of glycosaminoglycan distribution in cultured natural and engineered cartilage

Cartilage tissue engineering can provide a valuable tool for controlled studies of tissue development. As an example, analysis of the spatial distribution of glycosaminoglycans (GAG) in sections of cartilaginous tissues engineered under different culture conditions could be used to correlate the effects of environmental factors with the structure of the regenerated tissue. In this paper we describe a computer-based technique for quantitative analysis of safranin-O stained histological sections, using low magnification light microscopy images. We identified a parameter to quantify the intensity of red color in the sections, which in turn was proportional to the biochemically determined wet weight fraction of GAG in corresponding tissue samples, and to describe the spatial distribution of GAG as a function of depth from the section edge. A broken line regression model was then used to determine the thickness of an external region, with lower GAG fractions, and the spatial rate of change in GAG content. The method was applied to the quantitation of GAG distribution in samples of natural and engineered cartilage, cultured for 6 weeks in three different vessels: static flasks, mixed flasks, and rotating bioreactors.

Cardiac muscle tissue engineering: toward an in vitro model for electrophysiological studies

The objective of this study was to establish a three-dimensional (3-D) in vitro model system of cardiac muscle for electrophysiological studies. Primary neonatal rat ventricular cells containing lower or higher fractions of cardiac myocytes were cultured on polymeric scaffolds in bioreactors to form regular or enriched cardiac muscle constructs, respectively. After 1 wk, all constructs contained a peripheral tissue-like region (50-70 micrometer thick) in which differentiated cardiac myocytes were organized in multiple layers in a 3-D configuration. Indexes of cell size (protein/DNA) and metabolic activity (tetrazolium conversion/DNA) were similar for constructs and neonatal rat ventricles. Electrophysiological studies conducted using a linear array of extracellular electrodes showed that the peripheral region of constructs exhibited relatively homogeneous electrical properties and sustained macroscopically continuous impulse propagation on a centimeter-size scale. Electrophysiological properties of enriched constructs were superior to those of regular constructs but inferior to those of native ventricles. These results demonstrate that 3-D cardiac muscle constructs can be engineered with cardiac-specific structural and electrophysiological properties and used for in vitro impulse propagation studies.

Microgravity cultivation of cells and tissues

In vitro studies of cells and tissues in microgravity, either simulated by cultivation conditions on earth or actual, during spaceflight, are expected to help identify mechanisms underlying gravity sensing and transduction in biological organisms. In this paper, we review rotating bioreactor studies of engineered skeletal and cardiovascular tissues carried out in unit gravity, a four month long cartilage tissue engineering study carried out aboard the Mir Space Station, and the ongoing laboratory development and testing of a system for cell and tissue cultivation aboard the International Space Station.

Gas exchange is essential for bioreactor cultivation of tissue engineered cartilage

Tissue engineered cartilage can be grown in vitro if the necessary physical and biochemical factors are present in the tissue culture environment. Cell metabolism and tissue composition were studied for engineered cartilage cultured for 5 weeks using bovine articular chondrocytes, polymer scaffolds (5 mm diameter x 2 mm thick fibrous discs), and rotating bioreactors. Medium pH and concentrations of oxygen, carbon dioxide, glucose, lactate, ammonia, and glycosoaminoglycan (GAG) were varied by altering the exchange rates of gas and medium in the bioreactors. Cell-polymer constructs were assessed with respect to histomorphology, biochemical composition and metabolic activity. Low oxygen tension ( approximately 40 mmHg) and low pH ( approximately 6.7) were associated with anaerobic cell metabolism (yield of lactate on glucose, YL/G, of 2.2 mol/mol) while higher oxygen tension ( approximately 80 mmHg) and higher pH ( approximately 7.0) were associated with more aerobic cell metabolism (YL/G of 1.65-1.79 mol/mol). Under conditions of infrequent medium replacement (50% once per week), cells utilized more economical pathways such that glucose consumption and lactate production both decreased, cell metabolism remained relatively aerobic (YL/G of 1.67 mol/mol) and the resulting constructs were cartilaginous. More aerobic conditions generally resulted in larger constructs containing higher amounts of cartilaginous tissue components, while anaerobic conditions suppressed chondrogenesis in 3D tissue constructs.

Bioreactor cultivation conditions modulate the composition and mechanical properties of tissue-engineered cartilage

Cartilaginous constructs have been grown in vitro with use of isolated cells, biodegradable polymer scaffolds, and bioreactors. In the present work, the relationships between the composition and mechanical properties of engineered cartilage constructs were studied by culturing bovine calf articular chondrocytes on fibrous polyglycolic acid scaffolds (5 mm in diameter, 2-mm thick, and 97% porous) in three different environments: static flasks, mixed flasks, and rotating vessels. After 6 weeks of cultivation, the composition, morphology, and mechanical function of the constructs in radially confined static and dynamic compression all depended on the conditions of in vitro cultivation. Static culture yielded small and fragile constructs, while turbulent flow in mixed flasks yielded constructs with fibrous outer capsules; both environments resulted in constructs with poor mechanical properties. The constructs that were cultured freely suspended in a dynamic laminar flow field in rotating vessels were the largest, contained continuous cartilage-like extracellular matrices with the highest fractions of glycosaminoglycan and collagen, and had the best mechanical properties. The equilibrium modulus, hydraulic permeability, dynamic stiffness, and streaming potential correlated with the wet-weight fractions of glycosaminoglycan, collagen, and water. These findings suggest that the hydrodynamic conditions in tissue-culture bioreactors can modulate the composition, morphology, mechanical properties, and electromechanical function of engineered cartilage.

Collagen in tissue-engineered cartilage: types, structure, and crosslinks

The function of articular cartilage as a weight-bearing tissue depends on the specific arrangement of collagen types II and IX into a three-dimensional organized collagen network that can balance the swelling pressure of the proteoglycan/water gel. To determine whether cartilage engineered in vitro contains a functional collagen network, chondrocyte-polymer constructs were cultured for up to 6 weeks and analyzed with respect to the composition and ultrastructure of collagen by using biochemical and immunochemical methods and scanning electron microscopy. Total collagen content and the concentration of pyridinium crosslinks were significantly (57% and 70%, respectively) lower in tissue-engineered cartilage that in bovine calf articular cartilage. However, the fractions of collagen types II, IX, and X and the collagen network organization, density, and fibril diameter in engineered cartilage were not significantly different from those in natural articular cartilage. The implications of these findings for the field of tissue engineering are that differentiated chondrocytes are capable of forming a complex structure of collagen matrix in vitro, producing a tissue similar to natural articular cartilage on an ultrastructural scale.

Culture of organized cell communities

Cells cultured in vitro will tend to retain their differentiated phenotype under conditions that resemble their natural in vivo environment, for example, when cultured on polymer scaffolds in tissue culture bioreactors. In this chapter, we define organized cell communities as three-dimensional in vitro grown cell-polymer constructs that display important structural and functional features of the natural tissue. We review representative studies in which the research goal was to culture organized cell communities resembling cartilage, bone, skeletal muscle or cardiac-like tissue. These constructs can potentially serve as tissue equivalents for in vivo transplantation or as a model system for the in vitro testing of cell and tissue-level responses to molecular, mechanical or genetic manipulations.

Chondrogenesis in a cell-polymer-bioreactor system

Chondrogenesis was studied under controlled in vitro conditions using a cell-polymer-bioreactor system. Bovine calf articular chondrocytes were seeded onto biodegradable polymer scaffolds and cultured in rotating bioreactor vessels. Concomitant increases in the amounts of glycosaminoglycan (GAG) and type II collagen resulted in cell-polymer constructs with continuous cartilaginous matrix over their entire cross sections (6.7 mm diameter x 5 mm thick) after 40 days of cultivation. As compared to natural calf cartilage, constructs had comparable cellularities, 68% as much GAG and 33% as much type II collagen per gram wet weight. The progression of chondrogenesis in chondrocyte-polymer constructs was similar to that suggested previously for precursor cells in vitro and developing limbs in vivo. In particular, the polymer scaffold provided a three-dimensional structure that could be seeded with chondrocytes at high cell densities in order to establish cell-to-cell contacts and initiate cartilage tissue development, whereas the bioreactor vessel provided a permissive microenvironment for chondrogenesis. This work demonstrates the promise of using tissue engineered constructs for in vitro studies of cell interactions and differentiation.

Dynamic cell seeding of polymer scaffolds for cartilage tissue engineering

Cell seeding of three-dimensional polymer scaffolds is the first step of the cultivation of engineered tissues in bioreactors. Seeding requirements of large scaffolds to make implants for potential clinical use include: (a) high yield, to maximize the utilization of donor cells, (b) high kinetic rate, to minimize the time in suspension for anchorage-dependent and shear-sensitive cells, and (c) high and spatially uniform distribution of attached cells, for rapid and uniform tissue regeneration. Highly porous, fibrous polyglycolic acid scaffolds, 5-10 mm in diameter and 2-5 mm thick, were seeded with bovine articular chondrocytes in well-mixed spinner flasks. Essentially, all cells attached throughout the scaffold volume within 1 day. Mixing promoted the formation of 20-32-micron diameter cell aggregates that enhanced the kinetics of cell attachment without compromising the uniformity of cell distribution. The kinetics and possible mechanisms of cell seeding were related to the formation of cell aggregates by a simple mathematical model that can be used to optimize seeding conditions for cartilage tissue engineering.

In vitro differentiation of chick embryo bone marrow stromal cells into cartilaginous and bone-like tissues

Bone marrow stromal cells, progenitor cells involved in repair of bone and cartilage, can potentially provide a source for autologous skeletal tissue engineering. We investigated which factors were required to induce in vitro differentiation of avian bone marrow stromal cells into three-dimensional cartilaginous and bone-like tissues. Bone marrow stromal cells from embryonic chicks were expanded in monolayers, seeded onto biodegradable polyglycolic acid scaffolds, and cultured for 4 weeks in orbitally mixed Petri dishes. Cell-polymer constructs developed an organized extracellular matrix containing glycosaminoglycans and collagen, whereas control bone marrow stromal cell pellet cultures were smaller and consisted predominantly of fibrous tissue. Bone marrow stromal cells expanded with fibroblast growth factor-2 and seeded onto polymer scaffolds formed highly homogeneous three-dimensional tissues that contained cartilage-specific molecular markers and had biochemical compositions comparable with avian epiphyseal cartilage. When cell-polymer constructs were cultured in the presence of beta-glycerophosphate and dexamethasone, the extracellular matrix mineralized and bone-specific proteins were expressed. Our work shows that cell expansion in the presence of fibroblast growth factor-2 and cultivation on a three-dimensional polymer scaffold allows differentiation of chick bone marrow stromal cells into three-dimensional cartilaginous tissues. In the in vitro system studied, the same population could be selectively induced to regenerate either cartilaginous or bone-like tissue.

Tissue engineering of cartilage in space

Tissue engineering of cartilage, i.e., the in vitro cultivation of cartilage cells on synthetic polymer scaffolds, was studied on the Mir Space Station and on Earth. Specifically, three-dimensional cell-polymer constructs consisting of bovine articular chondrocytes and polyglycolic acid scaffolds were grown in rotating bioreactors, first for 3 months on Earth and then for an additional 4 months on either Mir (10(-4)-10(-6) g) or Earth (1 g). This mission provided a unique opportunity to study the feasibility of long-term cell culture flight experiments and to assess the effects of spaceflight on the growth and function of a model musculoskeletal tissue. Both environments yielded cartilaginous constructs, each weighing between 0.3 and 0.4 g and consisting of viable, differentiated cells that synthesized proteoglycan and type II collagen. Compared with the Earth group, Mir-grown constructs were more spherical, smaller, and mechanically inferior. The same bioreactor system can be used for a variety of controlled microgravity studies of cartilage and other tissues. These results may have implications for human spaceflight, e.g., a Mars mission, and clinical medicine, e.g., improved understanding of the effects of pseudo-weightlessness in prolonged immobilization, hydrotherapy, and intrauterine development.