A Collagen-glycosaminoglycan Scaffold Supports Adult Rat Mesenchymal Stem Cell Differentiation Along Osteogenic and Chondrogenic Routes

Addition of hyaluronic acid improves cellular infiltration and promotes early-stage chondrogenesis in a collagen-based scaffold for cartilage tissue engineering

The response of mesenchymal stem cells (MSCs) to a matrix largely depends on the composition as well as the extrinsic mechanical and morphological properties of the substrate to which they adhere to. Collagen-glycosaminoglycan (CG) scaffolds have been extensively used in a range of tissue engineering applications with great success. This is due in part to the presence of the glycosaminoglycans (GAGs) in complementing the biofunctionality of collagen. In this context, the overall goal of this study was to investigate the effect of two GAG types: chondroitin sulphate (CS) and hyaluronic acid (HyA) on the mechanical and morphological characteristics of collagen-based scaffolds and subsequently on the differentiation of rat MSCs in vitro. Morphological characterisation revealed that the incorporation of HyA resulted in a significant reduction in scaffold mean pore size (93.9 μm) relative to collagen-CS (CCS) scaffolds (136.2 μm). In addition, the collagen-HyA (CHyA) scaffolds exhibited greater levels of MSC infiltration in comparison to the CCS scaffolds. Moreover, these CHyA scaffolds showed significant acceleration of early stage gene expression of SOX-9 (approximately 60-fold higher, p<0.01) and collagen type II (approximately 35-fold higher, p<0.01) as well as cartilage matrix production (7-fold higher sGAG content) in comparison to CCS scaffolds by day 14. Combining their ability to stimulate MSC migration and chondrogenesis in vitro, these CHyA scaffolds show great potential as appropriate matrices for promoting cartilage tissue repair.

Mesenchymal stem cell fate is regulated by the composition and mechanical properties of collagen-glycosaminoglycan scaffolds

In stem cell biology, focus has recently turned to the influence of the intrinsic properties of the extracellular matrix (ECM), such as structural, composition and elasticity, on stem cell differentiation. Utilising collagen-glycosaminoglycan (CG) scaffolds as an analogue of the ECM, this study set out to determine the effect of scaffold stiffness and composition on naive mesenchymal stem cell (MSC) differentiation in the absence of differentiation supplements. Dehydrothermal (DHT) and 1-ethyl-3-3-dimethyl aminopropyl carbodiimide (EDAC) crosslinking treatments were used to produce three homogeneous CG scaffolds with the same composition but different stiffness values: 0.5, 1 and 1.5 kPa. In addition, the effect of scaffold composition on MSC differentiation was investigated by utilising two glycosaminoglycan (GAG) types: chondroitin sulphate (CS) and hyaluronic acid (HyA). Results demonstrated that scaffolds with the lowest stiffness (0.5 kPa) facilitated a significant up-regulation in SOX9 expression indicating that MSCs are directed towards a chondrogenic lineage in more compliant scaffolds. In contrast, the greatest level of RUNX2 expression was found in the stiffest scaffolds (1.5 kPa) indicating that MSCs are directed towards an osteogenic lineage in stiffer scaffolds. Furthermore, results demonstrated that the level of up-regulation of SOX9 was higher within the CHyA scaffolds in comparison to the CCS scaffolds indicating that hyaluronic acid further influences chondrogenic differentiation. In contrast, enhanced RUNX2 expression was observed in the CCS scaffolds in comparison to the CHyA scaffolds suggesting an osteogenic influence of chondroitin sulphate on MSC differentiation. In summary, this study demonstrates that, even in the absence of differentiation supplements, scaffold stiffness can direct the fate of MSCs, an effect that is further enhanced by the GAG type used within the CG scaffolds. These results have significant implications for the therapeutic uses of stem cells and enhance our understanding of the physical effects of the in vivo microenvironment on stem cell behaviour.

The development of non-viral gene-activated matrices for bone regeneration using polyethyleneimine (PEI) and collagen-based scaffolds

The healing potential of scaffolds for tissue engineering can be enhanced by combining them with genes to produce gene-activated matrices (GAMs) for tissue regeneration. We examined the potential of using polyethyleneimine (PEI) as a vector for transfection of mesenchymal stem cells (MSCs) in monolayer culture and in 3D collagen-based GAMs. PEI-pDNA polyplexes were fabricated at a range of N/P ratios and their optimal transfection parameters (N/P 7 ratio, 2μg dose) and transfection efficiencies (30±8%) determined in monolayer culture. The polyplexes were then loaded onto collagen, collagen-glycosaminoglycan and collagen-nanohydroxyapatite scaffolds where gene expression was observed up to 21 days with a polyplex dose as low as 2μg. Transient expression profiles indicated that the GAMs act as a polyplex depot system whereby infiltrating cells become transfected over time as they migrate throughout the scaffold. The collagen-nHa GAM exhibited the most prolonged and elevated levels of transgene expression. This research has thus demonstrated that PEI is a highly efficient pDNA transfection agent for both MSC monolayer cultures and in the 3D GAM environment. By combining therapeutic gene therapy with highly engineered scaffolds, it is proposed that these GAMs might have immense capability to promote tissue regeneration.

Evaluation of the ability of collagen-glycosaminoglycan scaffolds with or without mesenchymal stem cells to heal bone defects in Wistar rats

PURPOSE

The aim of this experiment was to examine the capacity of collagen-glycosaminoglycan scaffolds, with or without mesenchymal stem cells, to satisfactorily repair a 5-mm rat calvarial defect.

METHODS

Fifty-five Wistar rats were used in the study. The defects were either left empty to serve as controls (n = 7) or filled with cell-free scaffolds (n = 11), cell-seeded scaffolds that were pre-cultured in standard culture medium (n = 13), cell-seeded scaffolds that were pre-cultured in osteoinductive factor-supplemented medium (n = 12) or particulate autogenous bone (n = 12). The animals were sacrificed at 12 weeks after surgery, and specimens were prepared for histomorphometric analysis. The linear bone healing and the bone area within the defect were measured.

RESULTS

Comparable results were obtained using cell-free collagen-glycosaminoglycan scaffolds and autogenous bone both in terms of linear bone healing (P < 0.986) and area of new bone (P < 0.846). While the test groups showed significantly more bone formation compared to the empty defect control group, the linear bone healing and area of new bone within the defect were significantly lower in the cell-seeded scaffolds than in the cell-free scaffolds. The results have demonstrated that a cell-free collagen-glycosaminoglycan scaffold is capable of repairing a 5-mm rat calvarial defect as effectively as autogenous bone and that seeding the scaffold with pre-cultured mesenchymal stem cells prior to implantation offered no beneficial effect and resulted in incomplete healing of the defect.

CONCLUSIONS

The results thus suggest that the scaffold has immense potential for tissue repair showing favorable osteoconductive properties, biocompatibility and degradability.

Electrospun nanofiber-based regeneration of cartilage enhanced by mesenchymal stem cells

Application of biomaterials in combination with stem cells is a novel tissue engineering approach to regenerate cartilage. The objective of this study was to investigate the potential of poly(vinyl alcohol)/polycaprolactone (PVA/PCL) nanofiber scaffolds seeded with rabbit bone marrow-mesenchymal stem cell (BM-MSC) for cartilage tissue engineering in vitro and in vivo. We tested the biocompatibility and mechanical properties of nanofibrous scaffolds using scanning electron microscope, MTT assay, and tensile measurements. The capacity of MSC for chondrogenic differentiation on scaffolds was examined using reverse transcription-polymer chain reaction and immunostaining. For in vivo assessments, PVA/PCL nanofiber scaffolds with or without MSC were implanted into rabbit full-thickness cartilage defects. To evaluate cartilage regeneration, semi-quantitative grading and histological analysis were performed. Our results showed that PVA/PCL scaffolds supported the proliferation and chondrogenic differentiation of MSC in vitro. Moreover, the animals treated with cell-seeded PVA/PCL scaffolds showed improved healing of defects compared with untreated control and those which received cell-free scaffolds. Our findings suggest that PVA/PCL scaffolds incorporated with MSC can serve as a suitable graft for articular cartilage reconstruction.

The development of collagen-GAG scaffold-membrane composites for tendon tissue engineering

Current tissue engineering approaches for tendon defects require improved biomaterials to balance microstructural and mechanical design criteria. Collagen-glycosaminoglycan (CG) scaffolds have shown considerable success as in vivo regenerative templates and in vitro constructs to study cell behavior. While these scaffolds possess many advantageous qualities, their mechanical properties are typically orders of magnitude lower than orthopedic tissues such as tendon. Taking inspiration from mechanically efficient core-shell composites in nature such as plant stems and porcupine quills, we have created core-shell CG composites that display high bioactivity and improved mechanical integrity. These composites feature integration of a low density, anisotropic CG scaffold core with a high density, CG membrane shell. CG membranes were fabricated via an evaporative process that allowed separate tuning of membrane thickness and elastic moduli and were found to be isotropic in-plane. The membranes were then integrated with an anisotropic CG scaffold core via freeze-drying and subsequent crosslinking. Increasing the relative thickness of the CG membrane shell was shown to increase composite tensile elastic modulus by as much as a factor of 36 in a manner consistent with predictions from layered composites theory. CG scaffold-membrane composites were found to support tendon cell viability, proliferation, and metabolic activity in vitro, suggesting they maintain sufficient permeability while demonstrating improved mechanical strength. This work suggests an effective, biomimetic approach for balancing strength and bioactivity requirements of porous scaffolds for tissue engineering.

The generation of biomolecular patterns in highly porous collagen-GAG scaffolds using direct photolithography

The extracellular matrix (ECM) is a complex organization of structural proteins found within tissues and organs. Heterogeneous tissues with spatially and temporally modulated properties play an important role in organism physiology. Here we present a benzophenone (BP) based direct, photolithographic approach to spatially pattern solution phase biomolecules within collagen-GAG (CG) scaffolds and demonstrate creation of a wide range of patterns composed of multiple biomolecular species in a manner independent from scaffold fabrication steps. We demonstrate the ability to immobilize biomolecules at surface densities of up to 1000 ligands per square micron on the scaffold strut surface and to depths limited by the penetration depth of the excitation source into the scaffold structure. Importantly, while BP photopatterning does further crosslink the CG scaffold, evidenced by increased mechanical properties and collagen crystallinity, it does not affect scaffold microstructural or compositional properties or negatively influence cell adhesion, viability, or proliferation. We show that covalently photoimmobilized fibronectin within a CG scaffold significantly increases the speed of MC3T3-E1 cell attachment relative to the bare CG scaffold or non-specifically adsorbed fibronectin, suggesting that this approach can be used to improve scaffold bioactivity. Our findings, on the whole, establish the use of direct, BP photolithography as a methodology for covalently incorporating activity-improving biochemical cues within 3D collagen biomaterial scaffolds with spatial control over biomolecular deposition.

Evaluation of early healing events around mesenchymal stem cell-seeded collagen-glycosaminoglycan scaffold. An experimental study in Wistar rats

PURPOSE

Tissue engineering using cell-seeded biodegradable scaffolds offers a new bone regenerative approach that might circumvent many of the limitations of current therapeutic modalities. The aim of this experiment was to study the early healing events around mesenchymal stem cell-seeded collagen-glycosaminoglycan scaffolds.

METHODS

The 5-mm critical size defects were created in the calvarial bones of 41 Wistar rats. The defects were either left empty to serve as controls (n = 11), filled with cell-free scaffolds (n = 12), cell-seeded scaffolds that were maintained in standard culture medium (n = 9), or cell-seeded scaffolds that were maintained in osteoinductive factor-supplemented medium (n = 9). The animals were sacrificed at 7 days after surgery, and specimens were prepared for histological analysis. Early healing events such as host cell penetration, blood vessel in-growth, and scaffold integration were observed. The degree of inflammatory cell infiltrate was assessed.

RESULTS

While defects in the control group healed with a thin fibrous tissue, the collagen-glycosaminoglycan scaffold in the test groups preserved the three-dimensional form of the defects. After 7 days in vivo, the scaffold maintained its integrity and appeared populated with host cells. The cell-seeded scaffold induced more inflammatory response compared to the cell-free scaffolds. New blood vessels and areas of early bone formation were also evident in the cell-seeded scaffolds.

CONCLUSIONS

In conclusion, the findings show that mesenchymal stem cell-seeded collagen-glycosaminoglycan scaffolds have good tissue tolerance and exhibit an osteoinductive effect as indicated by early stage healing.

In-vivo generation of bone via endochondral ossification by in-vitro chondrogenic priming of adult human and rat mesenchymal stem cells

BACKGROUND

Bone grafts are required to repair large bone defects after tumour resection or large trauma. The availability of patients' own bone tissue that can be used for these procedures is limited. Thus far bone tissue engineering has not lead to an implant which could be used as alternative in bone replacement surgery. This is mainly due to problems of vascularisation of the implanted tissues leading to core necrosis and implant failure. Recently it was discovered that embryonic stem cells can form bone via the endochondral pathway, thereby turning in-vitro created cartilage into bone in-vivo. In this study we investigated the potential of human adult mesenchymal stem cells to form bone via the endochondral pathway.

METHODS

MSCs were cultured for 28 days in chondrogenic, osteogenic or control medium prior to implantation. To further optimise this process we induced mineralisation in the chondrogenic constructs before implantation by changing to osteogenic medium during the last 7 days of culture.

RESULTS

After 8 weeks of subcutaneous implantation in mice, bone and bone marrow formation was observed in 8 of 9 constructs cultured in chondrogenic medium. No bone was observed in any samples cultured in osteogenic medium. Switch to osteogenic medium for 7 days prevented formation of bone in-vivo. Addition of β-glycerophosphate to chondrogenic medium during the last 7 days in culture induced mineralisation of the matrix and still enabled formation of bone and marrow in both human and rat MSC cultures. To determine whether bone was formed by the host or by the implanted tissue we used an immunocompetent transgenic rat model. Thereby we found that osteoblasts in the bone were almost entirely of host origin but the osteocytes are of both host and donor origin.

CONCLUSIONS

The preliminary data presented in this manuscript demonstrates that chondrogenic priming of MSCs leads to bone formation in vivo using both human and rat cells. Furthermore, addition of β-glycerophosphate to the chondrogenic medium did not hamper this process. Using transgenic animals we also demonstrated that both host and donor cells played a role in bone formation. In conclusion these data indicate that in-vitro chondrogenic differentiation of human MSCs could lead to an alternative and superior approach for bone tissue engineering.

Three hours of perfusion culture prior to 28 days of static culture, enhances osteogenesis by human cells in a collagen GAG scaffold

In tissue engineering, bioreactors can be used to aid in the in vitro development of new tissue by providing biochemical and physical regulatory signals to cells and encouraging them to undergo differentiation and/or to produce extracellular matrix prior to in vivo implantation. This study examined the effect of short term flow perfusion bioreactor culture, prior to long-term static culture, on human osteoblast cell distribution and osteogenesis within a collagen glycosaminoglycan (CG) scaffold for bone tissue engineering. Human fetal osteoblasts (hFOB 1.19) were seeded onto CG scaffolds and pre-cultured for 6 days. Constructs were then placed into the bioreactor and exposed to 3 × 1 h bouts of steady flow (1 mL/min) separated by 7 h of no flow over a 24-h period. The constructs were then cultured under static osteogenic conditions for up to 28 days. Results show that the bioreactor and static culture control groups displayed similar cell numbers and metabolic activity. Histologically, however, peripheral cell-encapsulation was observed in the static controls, whereas, improved migration and homogenous cell distribution was seen in the bioreactor groups. Gene expression analysis showed that all osteogenic markers investigated displayed greater levels of expression in the bioreactor groups compared to static controls. While static groups showed increased mineral deposition; mechanical testing revealed that there was no difference in the compressive modulus between bioreactor and static groups. In conclusion, a flow perfusion bioreactor improved construct homogeneity by preventing peripheral encapsulation whilst also providing an enhanced osteogenic phenotype over static controls.

Understanding the effect of mean pore size on cell activity in collagen-glycosaminoglycan scaffolds

Mean pore size is an essential aspect of scaffolds for tissue-engineering. If pores are too small cells cannot migrate in towards the center of the construct limiting the diffusion of nutrients and removal of waste products. Conversely, if pores are too large there is a decrease in specific surface area available limiting cell attachment. However the relationship between scaffold pore size and cell activity is poorly understood and as a result there are conflicting reports within the literature on the optimal pore size required for successful tissue-engineering. Previous studies in bone tissue-engineering have indicated a range of mean pore sizes (96–150 µm) to facilitate optimal attachment. Other studies have shown a need for large pores (300–800 µm) for successful bone growth in scaffolds. These conflicting results indicate that a balance must be established between obtaining optimal cell attachment and facilitating bone growth. In this commentary we discuss our recent investigations into the effect of mean pore size in collagen-glycosaminoglycan (CG) scaffolds with pore sizes ranging from 85–325 μm and how it has provided an insight into the divergence within the literature.

Potential applications of natural origin polymer-based systems in soft tissue regeneration

Despite the many advances in tissue engineering approaches, scientists still face significant challenges in trying to repair and replace soft tissues. Nature-inspired routes involving the creation of polymer-based systems of natural origins constitute an interesting alternative route to produce novel materials. The interest in these materials comes from the possibility of constructing multi-component systems that can be manipulated by composition allowing one to mimic the tissue environment required for the cellular regeneration of soft tissues. For this purpose, factors such as the design, choice, and compatibility of the polymers are considered to be key factors for successful strategies in soft tissue regeneration. More recently, polysaccharide-protein based systems have being increasingly studied and proposed for the treatment of soft tissues. The characteristics, properties, and compatibility of the resulting materials investigated in the last 10 years, as well as commercially available matrices or those currently under investigation are the subject matter of this review.

Influence of shear stress in perfusion bioreactor cultures for the development of three-dimensional bone tissue constructs: a review

Bone tissue engineering aims to generate clinically applicable bone graft substitutes in an effort to ease the demands and reduce the potential risks associated with traditional autograft and allograft bone replacement procedures. Biomechanical stimuli play an important role under physiologically relevant conditions in the normal formation, development, and homeostasis of bone tissue--predominantly, strain (predicted levels in vivo for humans <2000 με) caused by physical deformation, and fluid shear stress (0.8-3 Pa), generated by interstitial fluid movement through lacunae caused by compression and tension under loading. Therefore, in vitro bone tissue cultivation strategies seek to incorporate biochemical stimuli in an effort to create more physiologically relevant constructs for grafting. This review is focused on collating information pertaining to the relationship between fluid shear stress, cellular deformation, and osteogenic differentiation, providing further insight into the optimal culture conditions for the creation of bone tissue substitutes.

Membrane-based cultures generate scaffold-free neocartilage in vitro: influence of growth factors

Scaffold-free cultures provide promising potential in chondrogenic differentiation of human mesenchymal stem cells (hMSCs). In this study, a novel scaffold-free membrane-based culture system, in which hMSCs were cultivated on a cellulose acetate membrane filter at medium-gas interface, was evaluated for chondrogenesis under the addition of growth factors. Chondrogenic differentiation of hMSCs has been described in scaffold-free pellet cultures with good results. In our study membrane-based cultures (1 x 10(6) hMSCs) were produced, maintained at the medium-gas interface and cultured for 21 days. Results were compared with findings from standard pellet cultures (2.5 x 10(5) hMSCs). The effects of the following growth factors were examined: human transforming growth factor-beta(3) (TGF-beta(3)) +/- insulin-like growth factor-1 or +/- human fibroblast growth factor 2. After 3 weeks of culture, chondrogenesis was assessed by Safranin-O staining, immunohistochemistry, a dimethylmethylene blue dye binding assay for glycosaminoglycans, and quantitative real-time polymerase chain reaction for cartilage-specific proteins. Membrane-based cultures containing growth factors formed hemispherical structures with a large surface area (65 mm(2)). When removed from the membrane they showed a histologically smooth cartilage-like surface. Membrane-based cultures stained positive for Safranin-O and collagen type II and contained a high content of glycosaminoglycans. Expression of cartilage-specific markers like collagen type II, aggrecan, and SOX9 was observed under the addition of TGF-beta(3), whereas combinations of growth factors let to a significant increase of collagen type II expression. A markedly reduced expression of collagen type X was found in membrane-based cultures when only TGF-beta(3) was added. Pellet cultures showed similar results besides an increased expression of collagen type X and type II that were observed. Membrane-based cultures provide a differentiation system, comparable in chondrogenesis to pellet cultures, which is able to generate scaffold-free neocartilage. The key benefit factors of membrane-based cultures are a histologically smooth cartilage-like surface and reduced expression of collagen type X, both of which are suitable features for its future application in cartilage regeneration.

The healing of bony defects by cell-free collagen-based scaffolds compared to stem cell-seeded tissue engineered constructs

One of the key challenges in tissue engineering is to understand the host response to scaffolds and engineered constructs. We present a study in which two collagen-based scaffolds developed for bone repair: a collagen-glycosaminoglycan (CG) and biomimetic collagen-calcium phosphate (CCP) scaffold, are evaluated in rat cranial defects, both cell-free and when cultured with MSCs prior to implantation. The results demonstrate that both cell-free scaffolds showed excellent healing relative to the empty defect controls and somewhat surprisingly, to the tissue engineered (MSC-seeded) constructs. Immunological analysis of the healing response showed higher M1 macrophage activity in the cell-seeded scaffolds. However, when the M2 macrophage response was analysed, both groups (MSC-seeded and non-seeded scaffolds) showed significant activity of these cells which are associated with an immunomodulatory and tissue remodelling response. Interestingly, the location of this response was confined to the construct periphery, where a capsule had formed, in the MSC-seeded groups as opposed to areas of new bone formation in the non-seeded groups. This suggests that matrix deposited by MSCs during in vitro culture may adversely affect healing by acting as a barrier to macrophage-led remodelling when implanted in vivo. This study thus improves our understanding of host response in bone tissue engineering.

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

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

Tissue differentiation in an in vivo bioreactor: in silico investigations of scaffold stiffness

Scaffold design remains a main challenge in tissue engineering due to the large number of requirements that need to be met in order to create functional tissues in vivo. Computer simulations of tissue differentiation within scaffolds could serve as a powerful tool in elucidating the design requirements for scaffolds in tissue engineering. In this study, a lattice-based model of a 3D porous scaffold construct derived from micro CT and a mechano-biological simulation of a bone chamber experiment were combined to investigate the effect of scaffold stiffness on tissue differentiation inside the chamber. The results indicate that higher scaffold stiffness, holding pore structure constant, enhances bone formation. This study demonstrates that a lattice approach is very suitable for modelling scaffolds in mechano-biological simulations, since it can accurately represent the micro-porous geometries of scaffolds in a 3D environment and reduce computational costs at the same time.

A novel collagen scaffold supports human osteogenesis--applications for bone tissue engineering

Collagen glycosaminoglycan (CG) scaffolds have been clinically approved as an application for skin regeneration. The goal of this study has been to examine whether a CG scaffold is a suitable biomaterial for generating human bone tissue. Specifically, we have asked the following questions: (1) can the scaffold support human osteoblast growth and differentiation and (2) how might recombinant human transforming growth factor-beta (TGF-beta(1)) enhance long-term in vitro bone formation? We show human osteoblast attachment, infiltration and uniform distribution throughout the construct, reaching the centre within 14 days of seeding. We have identified the fully differentiated osteoblast phenotype categorised by the temporal expression of alkaline phosphatase, collagen type 1, osteonectin, bone sialo protein, biglycan and osteocalcin. Mineralised bone formation has been identified at 35 days post-seeding by using von Kossa and Alizarin S Red staining. Both gene expression and mineral staining suggest the benefit of introducing an initial high treatment of TGF-beta(1) (10 ng/ml) followed by a low continuous treatment (0.2 ng/ml) to enhance human osteogenesis on the scaffold. Osteogenesis coincides with a reduction in scaffold size and shape (up to 70% that of original). A notable finding is core degradation at the centre of the tissue-engineered construct after 49 days of culture. This is not observed at earlier time points. Therefore, a maximum of 35 days in culture is appropriate for in vitro studies of these scaffolds. We conclude that the CG scaffold shows excellent potential as a biomaterial for human bone tissue engineering.

Tuning the elastic modulus of hydrated collagen fibrils

Systematic variation of solution conditions reveals that the elastic modulus (E) of individual collagen fibrils can be varied over a range of 2-200 MPa. Nanoindentation of reconstituted bovine Achilles tendon fibrils by atomic force microscopy (AFM) under different aqueous and ethanol environments was carried out. Titration of monovalent salts up to a concentration of 1 M at pH 7 causes E to increase from 2 to 5 MPa. This stiffening effect is more pronounced at lower pH where, at pH 5, e.g., there is an approximately 7-fold increase in modulus on addition of 1 M KCl. An even larger increase in modulus, up to approximately 200 MPa, can be achieved by using increasing concentrations of ethanol. Taken together, these results indicate that there are a number of intermolecular forces between tropocollagen monomers that govern the elastic response. These include hydration forces and hydrogen bonding, ion pairs, and possibly the hydrophobic effect. Tuning of the relative strengths of these forces allows rational tuning of the elastic modulus of the fibrils.

Screening for 3D environments that support human mesenchymal stem cell viability using hydrogel arrays

In this study we generated 3D poly(ethylene glycol) (PEG) hydrogel arrays to screen for the individual and combinatorial effects of extracellular matrix (ECM) degradability, cell adhesion ligand type, and cell adhesion ligand density on human mesenchymal stem cell (hMSC) viability. In particular, we explored the influence of two well-characterized ECM-derived cell adhesion ligands: the fibronectin-derived Arg-Gly-Asp-Ser-Pro (RGDSP) sequence, and the laminin-derived Ile-Lys-Val-Ala-Val (IKVAV) sequence. PEG network degradation, the RGDSP ligand, and the IKVAV ligand each individually increased hMSC viability in a dose-dependent manner. The RGDSP ligand also improved hMSC viability in a dose-dependent manner in degradable PEG hydrogels, while the effect of IKVAV was less pronounced in degradable hydrogels. Combinations of RGDSP and IKVAV promoted high viability of hMSCs in nondegradable PEG networks, while the combined effects of the ligands were not significant in degradable PEG hydrogels. Although hMSC spreading was not commonly observed within PEG hydrogels, we qualitatively observed hMSC spreading after 5 days only in degradable PEG hydrogels prepared with 2.5 mM of both RGDSP and IKVAV. These results suggest that the enhanced throughput approach described herein can be used to rapidly study the influence of a broad range of ECM parameters, as well as their combinations, on stem cell behavior.

Multiphasic collagen fibre-PLA composites seeded with human mesenchymal stem cells for osteochondral defect repair: an in vitro study

A promising approach for the repair of osteochondral defects is the use of a scaffold with a well-defined cartilage-bone interface. In this study, we used a multiphasic composite scaffold with an upper collagen I fibre layer for articular cartilage repair, separated by a hydrophobic interface from a lower polylactic acid (PLA) part for bone repair. Focusing initially on the engineering of cartilage, the upper layer was seeded with human mesenchymal stem cells (hMSCs) suspended in a collagen I hydrogel for homogeneous cell distribution. The constructs were cultured in a defined chondrogenic differentiation medium supplemented with 10 ng/ml transforming growth factor-beta1 (TGFbeta1) or in DMEM with 10% fetal bovine serum as a control. After 3 weeks a slight contraction of the collagen I fibre layer was seen in the TGFbeta1-treated group. Furthermore, a homogeneous cell distribution and chondrogenic differentiation was achieved in the upper third of the collagen I fibre layer. In the TGFbeta1-treated group cells showed a chondrocyte-like appearance and were surrounded by a proteoglycan and collagen type II-rich extracellular matrix. Also, a high deposition of glycosaminoglycans could be measured in this group and RT-PCR analyses confirmed the induction of chondrogenesis, with the expression of cartilage-specific marker genes, such as aggrecan and collagen types II and X. This multiphasic composite scaffold with the cartilage layer on top might be a promising construct for the repair of osteochondral defects.

Concurrent differentiation of marrow stromal cells to osteogenic and vasculogenic lineages

When rat bone marrow stromal (BMS) cells were seeded on aligned type I collagen scaffolds and cultured in osteogenic media, they underwent simultaneous maturation and differentiation into osteogenic and vascular cell lineages. In addition, these cells produced mineralized matricellular deposits. BMS cells were seeded in Petri dish or the collagen scaffold, cultured in osteogenic media for 3, 6, and 9 d and subsequently processed for immunohistochemical and cytochemical analysis. Immunolocalization of lineage-specific proteins were visualized using confocal microscopy and mRNA transcript analysis was performed by real-time quantitative polymerase chain reaction (RT-qPCR). The alkaline phosphatase activity and calcium content significantly increased over the observed period of time in an osteogenic medium. Sheets of abundant Pecam (CD31), Flk-1 (VEGFR-2), tomato lectin (TL/LEL), and alpha-smooth muscle actin (alpha-SMA) positive cells were observed in the collagen scaffolds. Nascent capillary-like vessels were also seen amidst the osteoblasts in osteogenic culture, augmenting the maturation and differentiation of BMS cells into osteoblasts. In our in vitro study, concurrent differentiation of BMS cells, a heterogeneous cell population with multilineage differentiation potential, to osteogenic and vascular lineages demonstrated that the substrates (three-dimensional (3-D), collagen type I, aligned fibrils) had a profound effect on guiding the differentiation pathway of BMS cells.

From natural bone grafts to tissue engineering therapeutics: Brainstorming on pharmaceutical formulative requirements and challenges

Tissue engineering is an emerging multidisciplinary field of investigation focused on the regeneration of diseased or injured tissues through the delivery of appropriate molecular and mechanical signals. Therefore, bone tissue engineering covers all the attempts to reestablish a normal physiology or to speed up healing of bone in all musculoskeletal disorders and injuries that are lashing modern societies. This article attempts to give a pharmaceutical perspective on the production of engineered man-made bone grafts that are described as implantable tissue engineering therapeutics, and to highlight the importance of understanding bone composition and structure, as well as osteogenesis and bone healing processes, to improve the design and development of such implants. In addition, special emphasis is given to pharmaceutical aspects that are frequently minimized, but that, instead, may be useful for formulation developments and in vitro/in vivo correlations.

Chondrogenic and osteogenic differentiations of human bone marrow-derived mesenchymal stem cells on a nanofibrous scaffold with designed pore network

The utilization of 3D scaffolds and stem cells is a promising approach to solve the problem of bone and cartilage tissue shortage and to construct osteochondral (cartilage/bone composite) tissues. In this study, 3D highly porous nanofibrous (NF) poly(L-lactic acid) (PLLA) scaffolds fabricated using a phase separation technique were seeded with multi-potent human bone marrow-derived mesenchymal stem cells (hMSCs) and the constructs were induced along osteogenic and chondrogenic development routes in vitro. Histological analysis and calcium content quantification showed that NF scaffolds supported in vitro bone differentiation. SEM observation showed an altered shape for cells cultured on an NF matrix compared with those on smooth films. Consistent with the morphological change, the gene expression of early chondrogenic commitment marker Sox-9 was enhanced on the NF matrix. NF scaffolds were then used to support long-term in vitro 3D cartilaginous development. It was found that in the presence of TGF-beta1, cartilage tissue developed on PLLA NF scaffolds, with the cartilage-specific gene expressed, glycosaminoglycan and type II collagen accumulated, and typical cartilage morphology formed. These findings suggest that NF scaffolds can support both bone and cartilage development and are excellent candidate scaffolds for osteochondral defect repair.

The effect of dehydrothermal treatment on the mechanical and structural properties of collagen-GAG scaffolds

The mechanical properties of tissue engineering scaffolds are critical for preserving the structural integrity and functionality during both in vivo implantation and long-term performance. In addition, the mechanical and structural properties of the scaffold can direct cellular activity within a tissue-engineered construct. In this context, the aim of this study was to investigate the effects of dehydrothermal (DHT) treatment on the mechanical and structural properties of collagen-glycosaminoglycan (CG) scaffolds. Temperature (105-180 degrees C) and exposure period (24-120 h) of DHT treatment were varied to determine their effect on the mechanical properties, crosslinking density, and denaturation of CG scaffolds. As expected, increasing the temperature and duration of DHT treatment resulted in an increase in the mechanical properties. Compressive properties increased up to twofold, while tensile properties increased up to 3.8-fold. Crosslink density was found to increase with DHT temperature but not exposure period. Denaturation also increased with DHT temperature and exposure period, ranging from 25% to 60% denaturation. Crosslink density was found to be correlated with compressive modulus, whilst denaturation was found to correlate with tensile modulus. Taken together, these results indicate that DHT treatment is a viable technique for altering the mechanical properties of CG scaffolds. The enhanced mechanical properties of DHT-treated CG scaffolds improve their suitability for use both in vitro and in vivo. In addition, this work facilitates the investigation of the effects of mechanical properties and denaturation on cell activity in a 3D environment.

Deformation simulation of cells seeded on a collagen-GAG scaffold in a flow perfusion bioreactor using a sequential 3D CFD-elastostatics model

Tissue-engineered bone shows promise in meeting the huge demand for bone grafts caused by up to 4 million bone replacement procedures per year, worldwide. State-of-the-art bone tissue engineering strategies use flow perfusion bioreactors to apply biophysical stimuli to cells seeded on scaffolds and to grow tissue suitable for implantation into the patient's body. The aim of this study was to quantify the deformation of cells seeded on a collagen-GAG scaffold which was perfused by culture medium inside a flow perfusion bioreactor. Using a microCT scan of an unseeded collagen-GAG scaffold, a sequential 3D CFD-deformation model was developed. The wall shear stress and the hydrostatic wall pressure acting on the cells were computed through the use of a CFD simulation and fed into a linear elastostatics model in order to calculate the deformation of the cells. The model used numerically seeded cells of two common morphologies where cells are either attached flatly on the scaffold wall or bridging two struts of the scaffold. Our study showed that the displacement of the cells is primarily determined by the cell morphology. Although cells of both attachment profiles were subjected to the same mechanical load, cells bridging two struts experienced a deformation up to 500 times higher than cells only attached to one strut. As the scaffold's pore size determines both the mechanical load and the type of attachment, the design of an optimal scaffold must take into account the interplay of these two features and requires a design process that optimizes both parameters at the same time.

Gene expression by marrow stromal cells in a porous collagen-glycosaminoglycan scaffold is affected by pore size and mechanical stimulation

Marrow stromal cell (MSC) populations, which are a potential source of undifferentiated mesenchymal cells, and culture scaffolds that mimic natural extracellular matrix are attractive options for orthopaedic tissue engineering. A type I collagen-glycosaminoglycan (CG) scaffold that has previously been used clinically for skin regeneration was recently shown to support expression of bone-associated proteins and mineralisation by MSCs cultured in the presence of osteogenic supplements. Here we follow RNA markers of osteogenic differentiation in this scaffold. We demonstrate that transcripts of the late stage markers bone sialoprotein and osteocalcin are present at higher levels in scaffold constructs than in two-dimensional culture, and that considerable gene induction can occur in this scaffold even in the absence of soluble osteogenic supplements. We also find that bone-related gene expression is affected by pore size, mechanical constraint, and uniaxial cyclic strain of the CG scaffold. The data presented here further establish the CG scaffold as a potentially valuable substrate for orthopaedic tissue engineering and for research on the mechanical interactions between cells and their environment, and suggest that a more freely-contracting scaffold with larger pore size may provide an environment more conducive to osteogenesis than constrained scaffolds with smaller pore sizes.

Mechanisms of Strain-Mediated Mesenchymal Stem Cell Apoptosis

Mechanical conditioning of mesenchymal stem cells (MSCs) has been adopted widely as a biophysical signal to aid tissue engineering applications. The replication of in vivo mechanical signaling has been used in in vitro environments to regulate cell differentiation, and extracellular matrix synthesis, so that both the chemical and mechanical properties of the tissue-engineered construct are compatible with the implant site. While research in these areas contributes to tissue engineering, the effects of mechanical strain on MSC apoptosis remain poorly defined. To evaluate the effects of uniaxial cyclic tensile strain on MSC apoptosis and to investigate mechanotransduction associated with strain-mediated cell death, MSCs seeded on a 2D silicone membrane were stimulated by a range of strain magnitudes for 3days. Mechanotransduction was investigated using the stretch-activated cation channel blocker gadolinium chloride, the L-type voltage-activated calcium channel blocker nicardipine, the c-jun NH2-terminal kinase (JNK) blocker D-JNK inhibitor 1, and the calpain inhibitor MDL 28170. Apoptosis was assessed through DNA fragmentation using the terminal deoxynucleotidyl transferase mediated-UTP-end nick labeling method. Results demonstrated that tensile strains of 7.5% or greater induce apoptosis in MSCs. L-type voltage-activated calcium channels coupled mechanical stress to activation of calpain and JNK, which lead to apoptosis through DNA fragmentation. The definition of the in vitro boundary conditions for tensile strain and MSCs along with a proposed mechanism for apoptosis induced by mechanical events positively contributes to the development of MSC biology, bioreactor design for tissue engineering, and development of computational methods for mechanobiology.

Hypoxia promotes chondrogenesis in rat mesenchymal stem cells: a role for AKT and hypoxia-inducible factor (HIF)-1alpha

Mesenchymal stem cells (MSCs) are multipotent cells capable of developing along the chondrogenic, osteogenic and adipogenic lineages. As such, they have received interest as a potential cell source for tissue engineering strategies. Cartilage is an avascular tissue and thus resides in a microenvironment with reduced oxygen tension. The aim of this study was to examine the effect of a low oxygen environment on MSC differentiation along the chondrogenic route. In MSCs exposed to chondrogenic growth factors, transforming growth factor-beta and dexamethasone, in a hypoxic environment (2% oxygen), the induction of collagen II expression and proteoglygan deposition was significantly greater than that observed when cells were exposed to the chondrogenic growth factors under normoxic (20% oxygen) conditions. The transcription factor, hypoxia-inducible factor-1alpha (HIF-1alpha), is a crucial mediator of the cellular response to hypoxia. Following exposure of MSCs to hypoxia (2% oxygen), HIF-1alpha translocated from the cytosol to the nucleus and bound to its target DNA consensus sequence. Similarly, hypoxia evoked an increase in phosphorylation of both AKT and p38 mitogen activated protein kinase, upstream of HIF-1alpha activation. Furthermore, the PI3 kinase/AKT inhibitor, LY294002, and p38 inhibitor, SB 203580, prevented the hypoxia-mediated stabilisation of HIF-1alpha. To assess the role of HIF-1alpha in the hypoxia-induced increase in chondrogenesis, we employed an siRNA knockdown approach. In cells exposed to HIF-1alpha siRNA, the hypoxia-induced enhancement of chondrogenesis, as evidenced by upregulation of collagen II, sox-9 and proteoglycan deposition, was absent. This provides evidence for HIF-1alpha being a key mediator of the beneficial effect of a low oxygen environment on chondrogenesis.

In vitro chondrogenic differentiation of human mesenchymal stem cells in collagen microspheres: influence of cell seeding density and collagen concentration

Given the inadequacies of existing repair strategies for cartilage injuries, tissue engineering approach using biomaterials and stem cells offers new hope for better treatments. Recently, we have fabricated injectable collagen-human mesenchymal stem cell (hMSC) microspheres using microencapsulation. Apart from providing a protective matrix for cell delivery, the collagen microspheres may also act as a bio-mimetic matrix facilitating the functional remodeling of hMSCs. In this study, whether the encapsulated hMSCs can be pre-differentiated into chondrogenic phenotype prior to implantation has been investigated. The effects of cell seeding density and collagen concentration on the chondrogenic differentiation potential of hMSCs have been studied. An in vivo implantation study has also been conducted. Fabrication of cartilage-like tissue micro-masses was demonstrated by positive immunohistochemical staining for cartilage-specific extracellular matrix components including type II collagen and aggrecan. The meshwork of collagen fibers was remodeled into a highly ordered microstructure, characterized by thick and parallel bundles, upon differentiation. Higher cell seeding density and higher collagen concentration favored the chondrogenic differentiation of hMSCs, yielding increased matrix production and mechanical strength of the micro-masses. These micro-masses were also demonstrated to integrate well with the host tissue in NOD/SCID mice.

Smart biomaterials for tissue engineering of cartilage

Biological regeneration using cartilage tissue engineering in which cells are grown on biomaterial scaffolds and then implanted into the cartilage defects could provide new treatment options for articular cartilage defects. This review aims to give an overview of the wide variety of biomaterials that are currently developed as scaffolds for cartilage tissue engineering. Emphasis will be placed on the current development of the materials that are able to direct cell differentiation and metabolism. These so-called "smart" biomaterials are produced by modifying the physical properties of the scaffolds using peptide sequences and most importantly by developing materials that can deliver proteins to enhance tissue regeneration. Besides providing drug delivery systems, the materials respond to environmental stimuli or release their cargo on cellular demand. However, critical issues remain, such as the transferability of basic science insights to clinical products and the applicability of certain data sets to human patients.

Effects of iron oxide incorporation for long term cell tracking on MSC differentiation in vitro and in vivo

Successful cell therapy will depend on the ability to monitor transplanted cells. With cell labeling, it is important to demonstrate efficient long term labeling without deleterious effects on cell phenotype and differentiation capacity. We demonstrate long term (7 weeks) retention of superparamagnetic iron oxide particles (SPIO) by mesenchymal stem cells (MSCs) in vivo, detectable by MRI. In vitro, multilineage differentiation (osteogenic, chondrogenic and adipogenic) was demonstrated by histological evaluation and molecular analysis in SPIO labeled and unlabeled cells. Gene expression levels were comaparable to unlabeled controls in adipogenic and chondrogenic conditions however not in the osteogenic condition. MSCs seeded into a scaffold for 21 days and implanted subcutaneously into nude mice for 4 weeks, showed profoundly altered phenotypes in SPIO labeled samples compared to implanted unlabeled control scaffolds, indicating chondrogenic differentiation. This study demonstrates long term MSC traceability using SPIO and MRI, uninhibited multilineage MSC differentiation following SPIO labeling, though with subtle but significant phenotypical alterations.

A comparison of the involvement of p38, ERK1/2 and PI3K in growth factor-induced chondrogenic differentiation of mesenchymal stem cells

Adult mesenchymal stem cells (MSCs) are under investigation as an alternative cell source for the engineering of cartilage tissue in three-dimensional (3D) scaffolds. However, little is known about the intracellular mechanisms involved in the chondrogenic differentiation of MSCs. This study investigated the signaling pathways evoked by TGF-beta1 and IGF-1 that mediated chondrogenic differentiation in adult rat bone-marrow derived MSCs in (i) monolayer on plastic and (ii) a 3D collagen-GAG scaffold. The data demonstrated involvement of the p38 pathway, but not ERK1/2 or PI3K in TGF-beta1-induced chondrogenic differentiation in monolayer. Similarly, when the MSCs were seeded onto a collagen-GAG scaffold and treated with TGF-beta1, the chondrogenic differentiation was dependent upon p38. In contrast, IGF-1-induced chondrogenic differentiation in monolayer involved p38, ERK1/2, as well as PI3K. The phosphorylation of Akt occurred downstream of PI3K and phospho-Akt was found to accumulate in the nucleus of IGF-1-treated cells. When MSCs were seeded onto the collagen-GAG scaffold and exposed to IGF-1, PI3K was required for chondrogenesis. These findings highlight the respective and differential involvement of p38, ERK1/2 and PI3K in growth factor-induced chondrogenesis of MSCs and demonstrates that intracellular signaling pathways are similar when differentiation is stimulated in a 2D or 3D environment.