Addition of Laponite to gelatin methacryloyl bioinks improves the rheological properties and printability to create mechanically tailorable cell culture matrices

Extrusion-based bioprinting has gained widespread popularity in biofabrication due to its ability to assemble cells and biomaterials in precise patterns and form tissue-like constructs. To achieve this, bioinks must have rheological properties suitable for printing while maintaining cytocompatibility. However, many commonly used biomaterials do not meet the rheological requirements and therefore require modification for bioprinting applications. This study demonstrates the incorporation of Laponite-RD (LPN) into gelatin methacryloyl (GelMA) to produce highly customizable bioinks with desired rheological and mechanical properties for extrusion-based bioprinting. Bioink formulations were based on GelMA (5%-15% w/v) and LPN (0%-4% w/v), and a comprehensive rheological design was applied to evaluate key rheological properties necessary for extrusion-based bioprinting. The results showed that GelMA bioinks with LPN (1%-4% w/v) exhibited pronounced shear thinning and viscoelastic behavior, as well as improved thermal stability. Furthermore, a concentration window of 1%-2% (w/v) LPN to 5%-15% GelMA demonstrated enhanced rheological properties and printability required for extrusion-based bioprinting. Construct mechanical properties were highly tunable by varying polymer concentration and photocrosslinking parameters, with Young's moduli ranging from ∼0.2 to 75 kPa. Interestingly, at higher Laponite concentrations, GelMA cross-linking was inhibited, resulting in softer hydrogels. High viability of MCF-7 breast cancer cells was maintained in both free-swelling droplets and printed hydrogels, and metabolically active spheroids formed over 7 days of culture in all conditions. In summary, the addition of 1%-2% (w/v) LPN to gelatin-based bioinks significantly enhanced rheological properties and retained cell viability and proliferation, suggesting its suitability for extrusion-based bioprinting.

Highly compliant biomimetic scaffolds for small diameter tissue-engineered vascular grafts (TEVGs) produced via melt electrowriting (MEW)

Biofabrication approaches toward the development of tissue-engineered vascular grafts (TEVGs) have been widely investigated. However, successful translation has been limited to large diameter applications, with small diameter grafts frequently failing due to poor mechanical performance, in particular mismatched radial compliance. Herein, melt electrowriting (MEW) of poly(ϵ-caprolactone) has enabled the manufacture of highly porous, biocompatible microfibre scaffolds with physiological anisotropic mechanical properties, as substrates for the biofabrication of small diameter TEVGs. Highly reproducible scaffolds with internal diameter of 4.0 mm were designed with 500 and 250µm pore sizes, demonstrating minimal deviation of less than 4% from the intended architecture, with consistent fibre diameter of 15 ± 2µm across groups. Scaffolds were designed with straight or sinusoidal circumferential microfibre architecture respectively, to investigate the influence of biomimetic fibre straightening on radial compliance. The results demonstrate that scaffolds with wave-like circumferential microfibre laydown patterns mimicking the architectural arrangement of collagen fibres in arteries, exhibit physiological compliance (12.9 ± 0.6% per 100 mmHg), while equivalent control geometries with straight fibres exhibit significantly reduced compliance (5.5 ± 0.1% per 100 mmHg). Further mechanical characterisation revealed the sinusoidal scaffolds designed with 250µm pores exhibited physiologically relevant burst pressures of 1078 ± 236 mmHg, compared to 631 ± 105 mmHg for corresponding 500µm controls. Similar trends were observed for strength and failure, indicating enhanced mechanical performance of scaffolds with reduced pore spacing. Preliminaryin vitroculture of human mesenchymal stem cells validated the MEW scaffolds as suitable substrates for cellular growth and proliferation, with high cell viability (>90%) and coverage (>85%), with subsequent seeding of vascular endothelial cells indicating successful attachment and preliminary endothelialisation of tissue-cultured constructs. These findings support further investigation into long-term tissue culture methodologies for enhanced production of vascular extracellular matrix components, toward the development of the next generation of small diameter TEVGs.

A tumour-spheroid manufacturing and cryopreservation process that yields a highly reproducible product ready for direct use in drug screening assays

If it were possible to purchase tumour-spheroids as a standardised product, ready for direct use in assays, this may contribute to greater research reproducibility, potentially reducing costs and accelerating outcomes. Herein, we describe a workflow where uniformly sized cancer tumour-spheroids are mass-produced using microwell culture, cryopreserved with high viability, and then cultured in neutral buoyancy media for drug testing. C4-2B prostate cancer or MCF-7 breast cancer cells amalgamated into uniform tumour-spheroids after 48 h of culture. Tumour-spheroids formed from 100 cells each tolerated the cryopreservation process marginally better than tumour-spheroids formed from 200 or 400 cells. Post-thaw, tumour-spheroid metabolic activity was significantly reduced, suggesting mitochondrial damage. Metabolic function was rescued by thawing the tumour-spheroids into medium supplemented with 10 µM N-Acetyl-l-cysteine (NAC). Following thaw, the neutral buoyancy media, Happy Cell ASM, was used to maintain tumour-spheroids as discrete tissues during drug testing. Fresh and cryopreserved C4-2B or MCF-7 tumour-spheroids responded similarly to titrations of Docetaxel. This protocol will contribute to a future where tumour-spheroids may be available for purchase as reliable and reproducible products, allowing laboratories to efficiently replicate and build on published research, in many cases, making tumour-spheroids simply another cell culture reagent.

Spray nebulization enables polycaprolactone nanofiber production in a manner suitable for generation of scaffolds or direct deposition of nanofibers onto cells

Spray nebulization is an elegant, but relatively unstudied, technique for scaffold production. Herein we fabricated mesh scaffolds of polycaprolactone (PCL) nanofibers via spray nebulization of 8% PCL in dichloromethane (DCM) using a 55.2 kPa compressed air stream and 17 ml h-1polymer solution flow rate. Using a refined protocol, we tested the hypothesis that spray nebulization would simultaneously generate nanofibers and eliminate solvent, yielding a benign environment at the point of fiber deposition that enabled the direct deposition of nanofibers onto cell monolayers. Nanofibers were collected onto a rotating plate 20 cm from the spray nozzle, but could be collected onto any static or moving surface. Scaffolds exhibited a mean nanofiber diameter of 910 ± 190 nm, ultimate tensile strength of 2.1 ± 0.3 MPa, elastic modulus of 3.3 ± 0.4 MPa, and failure strain of 62 ± 6%.In vitro, scaffolds supported growth of human keratinocyte cell epithelial-like layers, consistent with potential utility as a dermal scaffold. Fourier-transform infrared spectroscopy demonstrated that DCM had vaporized and was undetectable in scaffolds immediately following production. Exploiting the rapid elimination of DCM during fiber production, we demonstrated that nanofibers could be directly deposited on to cell monolayers, without compromising cell viability. This is the first description of spray nebulization generating nanofibers using PCL in DCM. Using this method, it is possible to rapidly produce nanofiber scaffolds, without need for high temperatures or voltages, yielding a method that could potentially be used to deposit nanofibers onto cell cultures or wound sites.

Method for manufacture and cryopreservation of cartilage microtissues

The financial viability of a cell and tissue-engineered therapy may depend on the compatibility of the therapy with mass production and cryopreservation. Herein, we developed a method for the mass production and cryopreservation of 3D cartilage microtissues. Cartilage microtissues were assembled from either 5000 human bone marrow-derived stromal cells (BMSC) or 5000 human articular chondrocytes (ACh) each using a customized microwell platform (the Microwell-mesh). Microtissues rapidly accumulate homogenous cartilage-like extracellular matrix (ECM), making them potentially useful building blocks for cartilage defect repair. Cartilage microtissues were cultured for 5 or 10 days and then cryopreserved in 90% serum plus 10% dimethylsulfoxide (DMSO) or commercial serum-free cryopreservation media. Cell viability was maximized during thawing by incremental dilution of serum to reduce oncotic shock, followed by washing and further culture in serum-free medium. When assessed with live/dead viability dyes, thawed microtissues demonstrated high viability but reduced immediate metabolic activity relative to unfrozen control microtissues. To further assess the functionality of the freeze-thawed microtissues, their capacity to amalgamate into a continuous tissue was assess over a 14 day culture. The amalgamation of microtissues cultured for 5 days was superior to those that had been cultured for 10 days. Critically, the capacity of cryopreserved microtissues to amalgamate into a continuous tissue in a subsequent 14-day culture was not compromised, suggesting that cryopreserved microtissues could amalgamate within a cartilage defect site. The quality ECM was superior when amalgamation was performed in a 2% O2 atmosphere than a 20% O2 atmosphere, suggesting that this process may benefit from the limited oxygen microenvironment within a joint. In summary, cryopreservation of cartilage microtissues is a viable option, and this manipulation can be performed without compromising tissue function.

Personalized Volumetric Tissue Generation by Enhancing Multiscale Mass Transport through 3D Printed Scaffolds in Perfused Bioreactors

Engineered tissues provide an alternative to graft material, circumventing the use of donor tissue such as autografts or allografts and non-physiological synthetic implants. However, their lack of vasculature limits the growth of volumetric tissue more than several millimeters thick which limits their success post-implantation. Perfused bioreactors enhance nutrient mass transport inside lab-grown tissue but remain poorly customizable to support the culture of personalized implants. Here, a multiscale framework of computational fluid dynamics (CFD), additive manufacturing, and a perfusion bioreactor system are presented to engineer personalized volumetric tissue in the laboratory. First, microscale 3D printed scaffold pore geometries are designed and 3D printed to characterize media perfusion through CFD and experimental fluid testing rigs. Then, perfusion bioreactors are custom-designed to combine 3D printed scaffolds with flow-focusing inserts in patient-specific shapes as simulated using macroscale CFD. Finally, these computationally optimized bioreactor-scaffold assemblies are additively manufactured and cultured with pre-osteoblast cells for 7, 20, and 24 days to achieve tissue growth in the shape of human calcaneus bones of 13 mL volume and 1 cm thickness. This framework enables an intelligent model-based design of 3D printed scaffolds and perfusion bioreactors which enhances nutrient transport for long-term volumetric tissue growth in personalized implant shapes. The novel methods described here are readily applicable for use with different cell types, biomaterials, and scaffold microstructures to research therapeutic solutions for a wide range of tissues.

Biofabrication of small diameter tissue-engineered vascular grafts

Current clinical treatment strategies for the bypassing of small diameter (<6 mm) blood vessels in the management of cardiovascular disease frequently fail due to a lack of suitable autologous grafts, as well as infection, thrombosis, and intimal hyperplasia associated with synthetic grafts. The rapid advancement of 3D printing and regenerative medicine technologies enabling the manufacture of biological, tissue-engineered vascular grafts (TEVGs) with the ability to integrate, remodel, and repair in vivo, promises a paradigm shift in cardiovascular disease management. This review comprehensively covers current state-of-the-art biofabrication technologies for the development of biomimetic TEVGs. Various scaffold based additive manufacturing methods used in vascular tissue engineering, including 3D printing, bioprinting, electrospinning and melt electrowriting, are discussed and assessed against the biomechanical and functional requirements of human vasculature, while the efficacy of decellularization protocols currently applied to engineered and native vessels are evaluated. Further, we provide interdisciplinary insight into the outlook of regenerative medicine for the development of vascular grafts, exploring key considerations and perspectives for the successful clinical integration of evolving technologies. It is expected that continued advancements in microscale additive manufacturing, biofabrication, tissue engineering and decellularization will culminate in the development of clinically viable, off-the-shelf TEVGs for small diameter applications in the near future. STATEMENT OF SIGNIFICANCE: Current clinical strategies for the management of cardiovascular disease using small diameter vessel bypassing procedures are inadequate, with up to 75% of synthetic grafts failing within 3 years of implantation. It is this critically important clinical problem that researchers in the field of vascular tissue engineering and regenerative medicine aim to alleviate using biofabrication methods combining additive manufacturing, biomaterials science and advanced cellular biology. While many approaches facilitate the development of bioengineered constructs which mimic the structure and function of native blood vessels, several challenges must still be overcome for clinical translation of the next generation of tissue-engineered vascular grafts.

Characterisation of ovine bone marrow-derived stromal cells (oBMSC) and evaluation of chondrogenically induced micro-pellets for cartilage tissue repair in vivo

Abstract

Bone marrow stromal cells (BMSC) show promise in cartilage repair, and sheep are the most common large animal pre-clinical model.

Objective

The objective of this study was to characterise ovine BMSC (oBMSC) in vitro, and to evaluate the capacity of chondrogenic micro-pellets manufactured from oBMSC or ovine articular chondrocytes (oACh) to repair osteochondral defects in sheep.

Design

oBMSC were characterised for surface marker expression using flow cytometry and evaluated for tri-lineage differentiation capacity. oBMSC micro-pellets were manufactured in a microwell platform, and chondrogenesis was compared at 2%, 5%, and 20% O₂. The capacity of cartilage micro-pellets manufactured from oBMSC or oACh to repair osteochondral defects in adult sheep was evaluated in an 8-week pilot study.

Results

Expanded oBMSC were positive for CD44 and CD146 and negative for CD45. The common adipogenic induction ingredient, 3-Isobutyl-1-methylxanthine (IBMX), was toxic to oBMSC, but adipogenesis could be restored by excluding IBMX from the medium. BMSC chondrogenesis was optimal in a 2% O₂ atmosphere. Micro-pellets formed from oBMSC or oACh appeared morphologically similar, but hypertrophic genes were elevated in oBMSC micro-pellets. While oACh micro-pellets formed cartilage-like repair tissue in sheep, oBMSC micro-pellets did not.

Conclusion

The sensitivity of oBMSC, compared to human BMSC, to IBMX in standard adipogenic assays highlights species-associated differences. Micro-pellets manufactured from oACh were more effective than micro-pellets manufactured from oBMSC in the repair of osteochondral defects in sheep. While oBMSC can be driven to form cartilage-like tissue in vitro, the effective use of these cells in cartilage repair will depend on the successful mitigation of hypertrophy and tissue integration.

Supplementary information

The online version contains supplementary material available at 10.1186/s13287-020-02045-3.

A single day of TGF-β1 exposure activates chondrogenic and hypertrophic differentiation pathways in bone marrow-derived stromal cells

Virtually all bone marrow-derived stromal cell (BMSC) chondrogenic induction cultures include greater than 2 weeks exposure to transforming growth factor-β (TGF-β), but fail to generate cartilage-like tissue suitable for joint repair. Herein we used a micro-pellet model (5 × 103 BMSC each) to determine the duration of TGF-β1 exposure required to initiate differentiation machinery, and to characterize the role of intrinsic programming. We found that a single day of TGF-β1 exposure was sufficient to trigger BMSC chondrogenic differentiation and tissue formation, similar to 21 days of TGF-β1 exposure. Despite cessation of TGF-β1 exposure following 24 hours, intrinsic programming mediated further chondrogenic and hypertrophic BMSC differentiation. These important behaviors are obfuscated by diffusion gradients and heterogeneity in commonly used macro-pellet models (2 × 105 BMSC each). Use of more homogenous micro-pellet models will enable identification of the critical differentiation cues required, likely in the first 24-hours, to generate high quality cartilage-like tissue from BMSC.

Intermittent parathyroid hormone (1-34) supplementation of bone marrow stromal cell cultures may inhibit hypertrophy, but at the expense of chondrogenesis

Background

Bone marrow stromal cells (BMSC) have promise in cartilage tissue engineering, but for their potential to be fully realised, the propensity to undergo hypertrophy must be mitigated. The literature contains diverging reports on the effect of parathyroid hormone (PTH) on BMSC differentiation. Cartilage tissue models can be heterogeneous, confounding efforts to improve media formulations.

Methods

Herein, we use a novel microwell platform (the Microwell-mesh) to manufacture hundreds of small-diameter homogeneous micro-pellets and use this high-resolution assay to quantify the influence of constant or intermittent PTH(1–34) medium supplementation on BMSC chondrogenesis and hypertrophy. Micro-pellets were manufactured from 5000 BMSC each and cultured in standard chondrogenic media supplemented with (1) no PTH, (2) intermittent PTH, or (3) constant PTH.

Results

Relative to control chondrogenic cultures, BMSC micro-pellets exposed to intermittent PTH had reduced hypertrophic gene expression following 1 week of culture, but this was accompanied by a loss in chondrogenesis by the second week of culture. Constant PTH treatment was detrimental to chondrogenic culture.

Conclusions

This study provides further clarity on the role of PTH on chondrogenic differentiation in vitro and suggests that while PTH may mitigate BMSC hypertrophy, it does so at the expense of chondrogenesis.

Integration of an ultra-strong poly(lactic-co-glycolic acid) (PLGA) knitted mesh into a thermally induced phase separation (TIPS) PLGA porous structure to yield a thin biphasic scaffold suitable for dermal tissue engineering

We aimed to capture the outstanding mechanical properties of meshes, manufactured using textile technologies, in thin biodegradable biphasic tissue-engineered scaffolds through encapsulation of meshes into porous structures formed from the same polymer. Our novel manufacturing process used thermally induced phase separation (TIPS), with ethylene carbonate (EC) as the solvent, to encapsulate a poly(lactic-co-glycolic acid) (PLGA) mesh into a porous PLGA network. Biphasic scaffolds (1 cm × 4 cm × 300 μm) were manufactured by immersing strips of PLGA mesh in 40 °C solutions containing 5% PLGA in EC, supercooling at 4 °C for 4 min, triggering TIPS by manually agitating the supercooled solution, and lastly eluting EC into 4 °C Milli-Q water. EC processing was rapid and did not compromise mesh tensile properties. Biphasic scaffolds exhibited a tensile strength of 40.7 ± 2.2 MPa, porosity of 94%, pore size of 16.85 ± 3.78 μm, supported HaCaT cell proliferation, and degraded in vitro linearly over the first ∼3 weeks followed by rapid degradation over the following three weeks. The successful integration of textile-type meshes yielded scaffolds with exceptional mechanical properties. This thin, porous, high-strength scaffold is potentially suitable for use in dermal wound repair or repair of tubular organs.

Immunogold FIB-SEM: Combining Volumetric Ultrastructure Visualization with 3D Biomolecular Analysis to Dissect Cell-Environment Interactions

Volumetric imaging techniques capable of correlating structural and functional information with nanoscale resolution are necessary to broaden the insight into cellular processes within complex biological systems. The recent emergence of focused ion beam scanning electron microscopy (FIB-SEM) has provided unparalleled insight through the volumetric investigation of ultrastructure; however, it does not provide biomolecular information at equivalent resolution. Here, immunogold FIB-SEM, which combines antigen labeling with in situ FIB-SEM imaging, is developed in order to spatially map ultrastructural and biomolecular information simultaneously. This method is applied to investigate two different cell-material systems: the localization of histone epigenetic modifications in neural stem cells cultured on microstructured substrates and the distribution of nuclear pore complexes in myoblasts differentiated on a soft hydrogel surface. Immunogold FIB-SEM offers the potential for broad applicability to correlate structure and function with nanoscale resolution when addressing questions across cell biology, biomaterials, and regenerative medicine.

Effect of gelatin source and photoinitiator type on chondrocyte redifferentiation in gelatin methacryloyl-based tissue-engineered cartilage constructs

This work investigates neocartilage formation in bovine and porcine gelatin methacryloyl-based hydrogels photocrosslinked using ultraviolet or visible light photoinitiator systems.

Engineering Anisotropic Muscle Tissue using Acoustic Cell Patterning

Tissue engineering has offered unique opportunities for disease modeling and regenerative medicine; however, the success of these strategies is dependent on faithful reproduction of native cellular organization. Here, it is reported that ultrasound standing waves can be used to organize myoblast populations in material systems for the engineering of aligned muscle tissue constructs. Patterned muscle engineered using type I collagen hydrogels exhibits significant anisotropy in tensile strength, and under mechanical constraint, produced microscale alignment on a cell and fiber level. Moreover, acoustic patterning of myoblasts in gelatin methacryloyl hydrogels significantly enhances myofibrillogenesis and promotes the formation of muscle fibers containing aligned bundles of myotubes, with a width of 120-150 µm and a spacing of 180-220 µm. The ability to remotely pattern fibers of aligned myotubes without any material cues or complex fabrication procedures represents a significant advance in the field of muscle tissue engineering. In general, these results are the first instance of engineered cell fibers formed from the differentiation of acoustically patterned cells. It is anticipated that this versatile methodology can be applied to many complex tissue morphologies, with broader relevance for spatially organized cell cultures, organoid development, and bioelectronics.

Three-Dimensional Bioprinting and Its Potential in the Field of Articular Cartilage Regeneration

Three-dimensional (3D) bioprinting techniques can be used for the fabrication of personalized, regenerative constructs for tissue repair. The current article provides insight into the potential and opportunities of 3D bioprinting for the fabrication of cartilage regenerative constructs. Although 3D printing is already used in the orthopedic clinic, the shift toward 3D bioprinting has not yet occurred. We believe that this shift will provide an important step forward in the field of cartilage regeneration. Three-dimensional bioprinting techniques allow incorporation of cells and biological cues during the manufacturing process, to generate biologically active implants. The outer shape of the construct can be personalized based on clinical images of the patient's defect. Additionally, by printing with multiple bio-inks, osteochondral or zonally organized constructs can be generated. Relevant mechanical properties can be obtained by hybrid printing with thermoplastic polymers and hydrogels, as well as by the incorporation of electrospun meshes in hydrogels. Finally, bioprinting techniques contribute to the automation of the implant production process, reducing the infection risk. To prompt the shift from nonliving implants toward living 3D bioprinted cartilage constructs in the clinic, some challenges need to be addressed. The bio-inks and required cartilage construct architecture need to be further optimized. The bio-ink and printing process need to meet the sterility requirements for implantation. Finally, standards are essential to ensure a reproducible quality of the 3D printed constructs. Once these challenges are addressed, 3D bioprinted living articular cartilage implants may find their way into daily clinical practice.

Structural analysis of photocrosslinkable methacryloyl-modified protein derivatives

Biochemically modified proteins have attracted significant attention due to their widespread applications as biomaterials. For instance, chemically modified gelatin derivatives have been widely explored to develop hydrogels for tissue engineering and regenerative medicine applications. Among the reported methods, modification of gelatin with methacrylic anhydride (MA) stands out as a convenient and efficient strategy to introduce functional groups and form hydrogels via photopolymerization. Combining light-activation of modified gelatin with soft lithography has enabled the materialization of microfabricated hydrogels. So far, this gelatin derivative has been referred to in the literature as gelatin methacrylate, gelatin methacrylamide, or gelatin methacryloyl, with the same abbreviation of GelMA. Considering the complex composition of gelatin and the presence of different functional groups on the amino acid residues, both hydroxyl groups and amine groups can possibly react with methacrylic anhydride during functionalization of the protein. This can also apply to the modification of other proteins, such as recombinant human tropoelastin to form MA-modified tropoelastin (MeTro). Here, we employed analytical methods to quantitatively determine the amounts of methacrylate and methacrylamide groups in MA-modified gelatin and tropoelastin to better understand the reaction mechanism. By combining two chemical assays with instrumental techniques, such as proton nuclear magnetic resonance (1H NMR) and liquid chromatography tandem-mass spectrometry (LC-MS/MS), our results indicated that while amine groups had higher reactivity than hydroxyl groups and resulted in a majority of methacrylamide groups, modification of proteins by MA could lead to the formation of both methacrylamide and methacrylate groups. It is therefore suggested that the standard terms for GelMA and MeTro should be defined as gelatin methacryloyl and methacryloyl-substituted tropoelastin, respectively, to remain consistent with the widespread abbreviations used in literature.

Tailoring hydrogel surface properties to modulate cellular response to shear loading

Biological tissues at articulating surfaces, such as articular cartilage, typically have remarkable low-friction properties that limit tissue shear during movement. However, these frictional properties change with trauma, aging, and disease, resulting in an altered mechanical state within the tissues. Yet, it remains unclear how these surface changes affect the behaviour of embedded cells when the tissue is mechanically loaded. Here, we developed a cytocompatible, bilayered hydrogel system that permits control of surface frictional properties without affecting other bulk physicochemical characteristics such as compressive modulus, mass swelling ratio, and water content. This hydrogel system was applied to investigate the effect of variations in surface friction on the biological response of human articular chondrocytes to shear loading. Shear strain in these hydrogels during dynamic shear loading was significantly higher in high-friction hydrogels than in low-friction hydrogels. Chondrogenesis was promoted following dynamic shear stimulation in chondrocyte-encapsulated low-friction hydrogel constructs, whereas matrix synthesis was impaired in high-friction constructs, which instead exhibited increased catabolism. Our findings demonstrate that the surface friction of tissue-engineered cartilage may act as a potent regulator of cellular homeostasis by governing the magnitude of shear deformation during mechanical loading, suggesting a similar relationship may also exist for native articular cartilage.

STATEMENT OF SIGNIFICANCE

Excessive mechanical loading is believed to be a major risk factor inducing pathogenesis of articular cartilage and other load-bearing tissues. Yet, the mechanisms leading to increased transmission of mechanical stimuli to cells embedded in the tissue remain largely unexplored. Here, we demonstrate that the tribological properties of loadbearing tissues regulate cellular behaviour by governing the magnitude of mechanical deformation arising from physiological tissue function. Based on these findings, we propose that changes to articular surface friction as they occur with trauma, aging, or disease, may initiate tissue pathology by increasing the magnitude of mechanical stress on embedded cells beyond a physiological level.

Challenges in engineering large customized bone constructs

The ability to treat large tissue defects with customized, patient-specific scaffolds is one of the most exciting applications in the tissue engineering field. While an increasing number of modestly sized tissue engineering solutions are making the transition to clinical use, successfully scaling up to large scaffolds with customized geometry is proving to be a considerable challenge. Managing often conflicting requirements of cell placement, structural integrity, and a hydrodynamic environment supportive of cell culture throughout the entire thickness of the scaffold has driven the continued development of many techniques used in the production, culturing, and characterization of these scaffolds. This review explores a range of technologies and methods relevant to the design and manufacture of large, anatomically accurate tissue-engineered scaffolds with a focus on the interaction of manufactured scaffolds with the dynamic tissue culture fluid environment. Biotechnol. Bioeng. 2017;114: 1129-1139. © 2016 Wiley Periodicals, Inc.

A Hydrogel Model Incorporating 3D-Plotted Hydroxyapatite for Osteochondral Tissue Engineering

The concept of biphasic or multi-layered compound scaffolds has been explored within numerous studies in the context of cartilage and osteochondral regeneration. To date, no system has been identified that stands out in terms of superior chondrogenesis, osteogenesis or the formation of a zone of calcified cartilage (ZCC). Herein we present a 3D plotted scaffold, comprising an alginate and hydroxyapatite paste, cast within a photocrosslinkable hydrogel made of gelatin methacrylamide (GelMA), or GelMA with hyaluronic acid methacrylate (HAMA). We hypothesized that this combination of 3D plotting and hydrogel crosslinking would form a high fidelity, cell supporting structure that would allow localization of hydroxyapatite to the deepest regions of the structure whilst taking advantage of hydrogel photocrosslinking. We assessed this preliminary design in terms of chondrogenesis in culture with human articular chondrocytes, and verified whether the inclusion of hydroxyapatite in the form presented had any influence on the formation of the ZCC. Whilst the inclusion of HAMA resulted in a better chondrogenic outcome, the effect of HAP was limited. We overall demonstrated that formation of such compound structures is possible, providing a foundation for future work. The development of cohesive biphasic systems is highly relevant for current and future cartilage tissue engineering.

O-Phenanthroline as modulator of the hypoxic and catabolic response in cartilage tissue-engineering models

Hypoxia has been shown to be important for maintaining cartilage homeostasis as well as for inducing chondrogenic differentiation. Ensuring low oxygen levels during in vitro culture is difficult, therefore we assessed the chondro-inductive capabilities of the hypoxia-mimicking agent O-phenanthroline, which is also known as a non-specific matrix metalloproteinase (MMP) inhibitor. We found that O-phenanthroline reduced the expression of MMP3 and MMP13 mRNA levels during chondrogenic differentiation of human chondrocytes (hChs), as well as after TNFα/IL-1β exposure in an explant model. Interestingly, O-phenanthroline significantly inhibited matrix degradation in a TNFα/IL-1β-dependent model of cartilage degeneration when compared to control and natural hypoxia (2.5% O₂ ). O-Phenanthroline had limited ability to improve the chondrogenic differentiation or matrix deposition in the chondrogenic pellet model. Additionally, O-phenanthroline alleviated MMP-induced cartilage degradation without affecting chondrogenesis in the explant culture. The data presented in this study indicate that the inhibitory effect of O-phenanthroline on MMP expression is dominant over the hypoxia-mimicking effect. Copyright © 2014 John Wiley & Sons, Ltd.

Multiphasic construct studied in an ectopic osteochondral defect model

In vivo osteochondral defect models predominantly consist of small animals, such as rabbits. Although they have an advantage of low cost and manageability, their joints are smaller and more easily healed compared with larger animals or humans. We hypothesized that osteochondral cores from large animals can be implanted subcutaneously in rats to create an ectopic osteochondral defect model for routine and high-throughput screening of multiphasic scaffold designs and/or tissue-engineered constructs (TECs). Bovine osteochondral plugs with 4 mm diameter osteochondral defect were fitted with novel multiphasic osteochondral grafts composed of chondrocyte-seeded alginate gels and osteoblast-seeded polycaprolactone scaffolds, prior to being implanted in rats subcutaneously with bone morphogenic protein-7. After 12 weeks of in vivo implantation, histological and micro-computed tomography analyses demonstrated that TECs are susceptible to mineralization. Additionally, there was limited bone formation in the scaffold. These results suggest that the current model requires optimization to facilitate robust bone regeneration and vascular infiltration into the defect site. Taken together, this study provides a proof-of-concept for a high-throughput osteochondral defect model. With further optimization, the presented hybrid in vivo model may address the growing need for a cost-effective way to screen osteochondral repair strategies before moving to large animal preclinical trials.

A biomimetic extracellular matrix for cartilage tissue engineering centered on photocurable gelatin, hyaluronic acid and chondroitin sulfate

The development of hydrogels tailored for cartilage tissue engineering has been a research and clinical goal for over a decade. Directing cells towards a chondrogenic phenotype and promoting new matrix formation are significant challenges that must be overcome for the successful application of hydrogels in cartilage tissue therapies. Gelatin-methacrylamide (Gel-MA) hydrogels have shown promise for the repair of some tissues, but have not been extensively investigated for cartilage tissue engineering. We encapsulated human chondrocytes in Gel-MA-based hydrogels, and show that with the incorporation of small quantities of photocrosslinkable hyaluronic acid methacrylate (HA-MA), and to a lesser extent chondroitin sulfate methacrylate (CS-MA), chondrogenesis and mechanical properties can be enhanced. The addition of HA-MA to Gel-MA constructs resulted in more rounded cell morphologies, enhanced chondrogenesis as assessed by gene expression and immunofluorescence, and increased quantity and distribution of the newly synthesized extracellular matrix (ECM) throughout the construct. Consequently, while the compressive moduli of control Gel-MA constructs increased by 26 kPa after 8 weeks culture, constructs with HA-MA and CS-MA increased by 114 kPa. The enhanced chondrogenic differentiation, distribution of ECM, and improved mechanical properties make these materials potential candidates for cartilage tissue engineering applications.

Effect of preculture and loading on expression of matrix molecules, matrix metalloproteinases, and cytokines by expanded osteoarthritic chondrocytes

OBJECTIVE

One of the pathologic changes that occurs during osteoarthritis (OA) is the degeneration of the pericellular matrix (PCM). Since the PCM is likely to be involved in mechanotransduction, this study was undertaken to investigate the effects of PCM-like matrix accumulation in zonal OA chondrocytes and their influence on chondrocyte response to compression.

METHODS

Superficial and middle/deep zone chondrocytes from macroscopically normal cartilage of OA knees were expanded and encapsulated in alginate gels. The effects of compression (short-term or long-term) and preculture on chondrocyte expression of various matrix molecules, cytokines, and matrix metalloproteinases (MMPs) were assessed. Additionally, nonexpanded chondrocytes were encapsulated in alginate and cultured in the presence or absence of transforming growth factor β1 (TGFβ1) and dexamethasone and analyzed following short-term compression experiments.

RESULTS

Expanded OA chondrocytes (superficial and middle/deep zone) that were precultured for 2 weeks under free-swelling conditions prior to dynamic compression responded more sensitively to loading and had increased matrix accumulation, increased interleukin-1β (IL-1β) and IL-4 levels, and decreased levels of MMP-2 (in the middle/deep zone) compared to the nonloaded controls. Compression also decreased MMP-3 and MMP-13 levels even without preculture. Nonexpanded chondrocytes did not respond to compression, but differences in gene expression were found depending on the zone of harvest, time in culture, and medium composition.

CONCLUSION

Our findings demonstrate that with predeposited PCM-like matrix, compressive stimulation can enhance matrix protein accumulation in expanded OA chondrocytes. Investigations into how PCM or other matrix components differentially affect this balance under mechanical loading may provide invaluable insight into OA pathogenesis and the use of expanded cells in tissue engineering and regenerative medicine-based applications.

Stage-specific embryonic antigen-4 is not a marker for chondrogenic and osteogenic potential in cultured chondrocytes and mesenchymal progenitor cells

One important challenge for regenerative medicine is to produce a clinically relevant number of cells with consistent tissue-forming potential. Isolation and expansion of cells from skeletal tissues results in a heterogeneous population of cells with variable regenerative potential. A more consistent tissue formation could be achieved by identification and selection of potent progenitors based on cell surface molecules. In this study, we assessed the expression of stage-specific embryonic antigen-4 (SSEA-4), a classic marker of undifferentiated stem cells, and other surface markers in human articular chondrocytes (hACs), osteoblasts, and bone marrow-derived mesenchymal stromal cells (bmMSCs) and characterized their differentiation potential. Further, we sorted SSEA-4-expressing hACs and followed their potential to proliferate and to form cartilage in vitro. Cells isolated from cartilage and bone exhibited remarkably heterogeneous SSEA-4 expression profiles in expansion cultures. SSEA-4 expression levels increased up to ∼5 population doublings, but decreased following further expansion and differentiation cultures; levels were not related to the proliferation state of the cells. Although SSEA-4-sorted chondrocytes showed a slightly better chondrogenic potential than their SSEA-4-negative counterparts, differences were insufficient to establish a link between SSEA-4 expression and chondrogenic potential. SSEA-4 levels in bmMSCs also did not correlate to the cells' chondrogenic and osteogenic potential in vitro. SSEA-4 is clearly expressed by subpopulations of proliferating somatic cells with a MSC-like phenotype. However, the predictive value of SSEA-4 as a specific marker of superior differentiation capacity in progenitor cell populations from adult human tissue and even its usefulness as a stem cell marker appears questionable.

Non-invasive identification of proteoglycans and chondrocyte differentiation state by Raman microspectroscopy

Proteoglycans (PGs) are crucial extracellular matrix (ECM) components that are present in all tissues and organs. Pathological remodeling of these macromolecules can lead to severe diseases such as osteoarthritis or rheumatoid arthritis. To date, PG-associated ECM alterations are routinely diagnosed by invasive analytical methods. Here, we employed Raman microspectroscopy, a laser-based, marker-free and non-destructive technique that allows the generation of spectra with peaks originating from molecular vibrations within a sample, to identify specific Raman bands that can be assigned to PGs within human and porcine cartilage samples and chondrocytes. Based on the non-invasively acquired Raman spectra, we further revealed that a prolonged in vitro culture leads to phenotypic alterations of chondrocytes, resulting in a decreased PG synthesis rate and loss of lipid contents. Our results are the first to demonstrate the applicability of Raman microspectroscopy as an analytical and potential diagnostic tool for non-invasive cell and tissue state monitoring of cartilage in biomedical research.

Cartilage regeneration using zonal chondrocyte subpopulations: a promising approach or an overcomplicated strategy?

Cartilage defects heal imperfectly and osteoarthritic changes develop frequently as a result. Although the existence of specific behaviours of chondrocytes derived from various depth-related zones in vitro has been known for over 20 years, only a relatively small body of in vitro studies has been performed with zonal chondrocytes and current clinical treatment strategies do not reflect these native depth-dependent (zonal) differences. This is surprising since mimicking the zonal organization of articular cartilage in neo-tissue by the use of zonal chondrocyte subpopulations could enhance the functionality of the graft. Although some research groups including our own have made considerable progress in tailoring culture conditions using specific growth factors and biomechanical loading protocols, we conclude that an optimal regime has not yet been determined. Other unmet challenges include the lack of specific zonal cell sorting protocols and limited amounts of cells harvested per zone. As a result, the engineering of functional tissue has not yet been realized and no long-term in vivo studies using zonal chondrocytes have been described. This paper critically reviews the research performed to date and outlines our view of the potential future significance of zonal chondrocyte populations in regenerative approaches for the treatment of cartilage defects. Secondly, we briefly discuss the capabilities of additive manufacturing technologies that can not only create patient-specific grafts directly from medical imaging data sets but could also more accurately reproduce the complex 3D zonal extracellular matrix architecture using techniques such as hydrogel-based cell printing.

Comparative study of depth-dependent characteristics of equine and human osteochondral tissue from the medial and lateral femoral condyles

Articular cartilage defects are common after joint injuries. When left untreated, the biomechanical protective function of cartilage is gradually lost, making the joint more susceptible to further damage, causing progressive loss of joint function and eventually osteoarthritis (OA). In the process of translating promising tissue-engineering cartilage repair approaches from bench to bedside, pre-clinical animal models including mice, rabbits, goats, and horses, are widely used. The equine species is becoming an increasingly popular model for the in vivo evaluation of regenerative orthopaedic approaches. As there is also an increasing body of evidence suggesting that successful lasting tissue reconstruction requires an implant that mimics natural tissue organization, it is imperative that depth-dependent characteristics of equine osteochondral tissue are known, to assess to what extent they resemble those in humans. Therefore, osteochondral cores (4-8 mm) were obtained from the medial and lateral femoral condyles of equine and human donors. Cores were processed for histology and for biochemical quantification of DNA, glycosaminoglycan (GAG) and collagen content. Equine and human osteochondral tissues possess similar geometrical (thickness) and organizational (GAG, collagen and DNA distribution with depth) features. These comparable trends further underscore the validity of the equine model for the evaluation of regenerative approaches for articular cartilage.

Dynamic compression improves biosynthesis of human zonal chondrocytes from osteoarthritis patients

OBJECTIVE

We hypothesize that chondrocytes from distinct zones of articular cartilage respond differently to compressive loading, and that zonal chondrocytes from osteoarthritis (OA) patients can benefit from optimized compressive stimulation. Therefore, we aimed to determine the transcriptional response of superficial (S) and middle/deep (MD) zone chondrocytes to varying dynamic compressive strain and loading duration. To confirm effects of compressive stimulation on overall matrix production, we subjected zonal chondrocytes to compression for 2 weeks.

DESIGN

Human S and MD chondrocytes from osteoarthritic joints were encapsulated in 2% alginate, pre-cultured, and subjected to compression with varying dynamic strain (5, 15, 50% at 1 Hz) and loading duration (1, 3, 12 h). Temporal changes in cartilage-specific, zonal, and dedifferentiation genes following compression were evaluated using quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR). The benefits of long-term compression (50% strain, 3 h/day, for 2 weeks) were assessed by measuring construct glycosaminoglycan (GAG) content and compressive moduli, as well as immunostaining.

RESULTS

Compressive stimulation significantly induced aggrecan (ACAN), COL2A1, COL1A1, proteoglycan 4 (PRG4), and COL10A1 gene expression after 2 h of unloading, in a zone-dependent manner (P < 0.05). ACAN and PRG4 mRNA levels depended on strain and load duration, with 50% and 3 h loading resulting in highest levels (P < 0.05). Long-term compression increased collagen type II and ACAN immunostaining and total GAG (P < 0.05), but only S constructs showed more PRG4 stain, retained more GAG (P < 0.01), and developed higher compressive moduli than non-loaded controls.

CONCLUSIONS

The biosynthetic activity of zonal chondrocytes from osteoarthritis joints can be enhanced with selected compression regimes, indicating the potential for cartilage tissue engineering applications.

Tissue engineering of articular cartilage with biomimetic zones

Articular cartilage damage is a persistent and increasing problem with the aging population, and treatments to achieve biological repair or restoration remain a challenge. Cartilage tissue engineering approaches have been investigated for over 20 years, but have yet to achieve the consistency and effectiveness for widespread clinical use. One of the potential reasons for this is that the engineered tissues do not have or establish the normal zonal organization of cells and extracellular matrix that appears critical for normal tissue function. A number of approaches are being taken currently to engineer tissue that more closely mimics the organization of native articular cartilage. This review focuses on the zonal organization of native articular cartilage, strategies being used to develop such organization, the reorganization that occurs after culture or implantation, and future prospects for the tissue engineering of articular cartilage with biomimetic zones.

Prioritizing land and sea conservation investments to protect coral reefs

Background

Coral reefs have exceptional biodiversity, support the livelihoods of millions of people, and are threatened by multiple human activities on land (e.g. farming) and in the sea (e.g. overfishing). Most conservation efforts occur at local scales and, when effective, can increase the resilience of coral reefs to global threats such as climate change (e.g. warming water and ocean acidification). Limited resources for conservation require that we efficiently prioritize where and how to best sustain coral reef ecosystems.

Methodology/Principal Findings

Here we develop the first prioritization approach that can guide regional-scale conservation investments in land- and sea-based conservation actions that cost-effectively mitigate threats to coral reefs, and apply it to the Coral Triangle, an area of significant global attention and funding. Using information on threats to marine ecosystems, effectiveness of management actions at abating threats, and the management and opportunity costs of actions, we calculate the rate of return on investment in two conservation actions in sixteen ecoregions. We discover that marine conservation almost always trumps terrestrial conservation within any ecoregion, but terrestrial conservation in one ecoregion can be a better investment than marine conservation in another. We show how these results could be used to allocate a limited budget for conservation and compare them to priorities based on individual criteria.

Conclusions/Significance

Previous prioritization approaches do not consider both land and sea-based threats or the socioeconomic costs of conserving coral reefs. A simple and transparent approach like ours is essential to support effective coral reef conservation decisions in a large and diverse region like the Coral Triangle, but can be applied at any scale and to other marine ecosystems.

Strategies for zonal cartilage repair using hydrogels

Articular cartilage is a highly hydrated tissue with depth-dependent cellular and matrix properties that provide low-friction load bearing in joints. However, the structure and function are frequently lost and there is insufficient repair response to regenerate high-quality cartilage. Several hydrogel-based tissue-engineering strategies have recently been developed to form constructs with biomimetic zonal variations to improve cartilage repair. Modular hydrogel systems allow for systematic control over hydrogel properties, and advanced fabrication techniques allow for control over construct organization. These technologies have great potential to address many unanswered questions involved in prescribing zonal properties to tissue-engineered constructs for cartilage repair.

Zonal chondrocyte subpopulations reacquire zone-specific characteristics during in vitro redifferentiation

BACKGROUND

If chondrocytes from the superficial, middle, and deep zones of articular cartilage could maintain or regain their characteristic properties during in vitro culture, it would be feasible to create constructs comprising these distinctive zones.

HYPOTHESIS

Zone-specific characteristics of zonal cell populations will disappear during 2-dimensional expansion but will reappear after 3-dimensional redifferentiation, independent of the culture technique used (alginate beads versus pellet culture).

STUDY DESIGN

Controlled laboratory study.

METHODS

Equine articular chondrocytes from the 3 zones were expanded in monolayer culture (8 donors) and subsequently redifferentiated in pellet and alginate bead cultures for up to 4 weeks. Glycosaminoglycans and DNA were quantified, along with immunohistochemical assessment of the expression of various zonal markers, including cartilage oligomeric protein (marking cells from the deeper zones) and clusterin (specifically expressed by superficial chondrocytes).

RESULTS

Cell yield varied between zones, but proliferation rates did not show significant differences. Expression of all evaluated zonal markers was lost during expansion. Compared to the alginate bead cultures, pellet cultures showed a higher amount of glycosaminoglycans produced per DNA after redifferentiation. In contrast to cells in pellet cultures, cells in alginate beads regained zonal differences, as evidenced by zone-specific reappearance of cartilage oligomeric protein and clusterin, as well as significantly higher glycosaminoglycans production by cells from the deep zone compared to the superficial zone.

CONCLUSION

Chondrocytes isolated from the 3 zones of equine cartilage can restore their zone-specific matrix expression when cultured in alginate after in vitro expansion.

CLINICAL RELEVANCE

Appreciation of the zonal differences can lead to important advances in cartilage tissue engineering. Findings support the use of hydrogels such as alginate for engineering zonal cartilage constructs.

The Roles of Hypoxia in theIn VitroEngineering of Tissues

Oxygen is a potent modulator of cell function and wound repair in vivo. The lack of oxygen (hypoxia) can create a potentially lethal environment and limit cellular respiration and growth or, alternatively, enhance the production of the specific extracellular matrix components and increase angiogenesis through the hypoxia-inducible factor-1 pathway. For the in vitro generation of clinically relevant tissue-engineered grafts, these divergent actions of hypoxia should be addressed. Diffusion through culture medium and tissue typically limits oxygen transport in vitro, leading to hypoxic regions and limiting the viable tissue thickness. Approaches to overcoming the transport limitations include culture with bioreactors, scaffolds with artificial microvasculature, oxygen carriers, and hyperbaric oxygen chambers. As an alternate approach, angiogenesis after implantation may be enhanced by incorporating endothelial cells, genetically modified cells, or specific factors (including vascular endothelial growth factor) into the scaffold or exposing the graft to a hypoxic environment just before implantation. Better understanding of the roles of hypoxia will help prevent common problems and exploit potential benefits of hypoxia in engineered tissues.

Short-Term Retention of Labeled Chondrocyte Subpopulations in Stratified Tissue-Engineered Cartilaginous Constructs Implanted In Vivo in Mini-Pigs

It is likely that effective application of cell-laden implants for cartilage defects depends on retention of implanted cells and interaction between implanted and host cells. The objectives of this study were to characterize stratified cartilaginous constructs seeded sequentially with superficial (S) and middle (M) chondrocyte subpopulations labeled with fluorescent cell tracking dye PKH26 (*) and determine the degree to which these stratified cartilaginous constructs maintain their architecture in vivo after implantation in mini-pigs for 1 week. Alginate-recovered cells were seeded sequentially to form stratified S*/M (only S cells labeled) and S*/M* (both S and M cells labeled) constructs. Full-thickness defects (4 mm diameter) were created in the patellofemoral groove of adult Yucatan mini-pigs and filled with portions of constructs or left empty. Constructs were characterized biochemically, histologically, and biomechanically, and stratification visualized and quantified, before and after implant. After 1 week, animals were sacrificed and implants retrieved. After 1 week in vivo, glycosaminoglycan and collagen content of constructs remained similar to that at implant, whereas DNA content increased. Histological analyses revealed features of an early repair response, with defects filled with tissues containing little matrix and abundant cells. Some implanted (PKH26-labeled) cells persisted in the defects, although constructs did not maintain a stratified organization. Of the labeled cells, 126 +/- 38% and 32 +/- 8% in S*/M and S*/M* constructs, respectively, were recovered. Distribution of labeled cells indicated interactions between implanted and host cells. Longer-term in vivo studies will be useful in determining whether implanted cells are sufficient to have a positive effect in repair.

Continuous passive motion applied to whole joints stimulates chondrocyte biosynthesis of PRG4

Continuous passive motion (CPM) is currently a part of patient rehabilitation regimens after a variety of orthopedic surgical procedures. While CPM can enhance the joint healing process, the direct effects of CPM on cartilage metabolism remain unknown. Recent in vivo and in vitro observations suggest that mechanical stimuli can regulate articular cartilage metabolism of proteoglycan 4 (PRG4), a putative lubricating and chondroprotective molecule found in synovial fluid and at the articular cartilage surface.

OBJECTIVES

(1) Determine the topographical variation in intrinsic cartilage PRG4 secretion. (2) Apply a CPM device to whole joints in bioreactors and assess effects of CPM on PRG4 biosynthesis.

METHODS

A bioreactor was developed to apply CPM to bovine stifle joints in vitro. Effects of 24h of CPM on PRG4 biosynthesis were determined.

RESULTS

PRG4 secretion rate varied markedly over the joint surface. Rehabilitative joint motion applied in the form of CPM regulated PRG4 biosynthesis, in a manner dependent on the duty cycle of cartilage sliding against opposing tissues. Specifically, in certain regions of the femoral condyle that were continuously or intermittently sliding against meniscus and tibial cartilage during CPM, chondrocyte PRG4 synthesis was higher with CPM than without.

CONCLUSIONS

Rehabilitative joint motion, applied in the form of CPM, stimulates chondrocyte PRG4 metabolism. The stimulation of PRG4 synthesis is one mechanism by which CPM may benefit cartilage and joint health in post-operative rehabilitation.

Microenvironment regulation of PRG4 phenotype of chondrocytes

Articular cartilage is a heterogeneous tissue with superficial (S), middle (M), and deep (D) zones. Chondrocytes in the S zone secrete the lubricating PRG4 protein, while chondrocytes from the M and D zones are more specialized in producing large amounts of the glycosaminoglycan (GAG) component of the extracellular matrix. Soluble and insoluble chemicals and mechanical stimuli regulate cartilage development, growth, and homeostasis; however, the mechanisms of regulation responsible for the distinct PRG4-positive and negative phenotypes of chondrocytes are unknown. The objective of this study was to determine if interaction between S and M chondrocytes regulates chondrocyte phenotype, as determined by coculture in monolayer at different ratios of S:M (100:0, 75:25, 50:50, 25:75, 0:100) and at different densities (240,000, 120,000, 60,000, and 30,000 cells/cm(2)), and by measurement of PRG4 secretion and expression, and GAG accumulation. Coculture of S and M cells resulted in significant up-regulation in PRG4 secretion and the percentage of cells expressing PRG4, with simultaneous down-regulation of GAG accumulation. Tracking M cells with PKH67 dye in coculture revealed that they maintained a PRG4-negative phenotype, and proliferated less than S cells. Taken together, these results indicate that the up-regulated PRG4 expression in coculture is a result of preferential proliferation of PRG4-expressing S cells. This finding may have practical implications for generating a large number of phenotypically normal S cells, which can be limited in source, for tissue engineering applications.

Depth-dependent biomechanical and biochemical properties of fetal, newborn, and tissue-engineered articular cartilage

Adult articular cartilage has depth-dependent mechanical and biochemical properties which contribute to zone-specific functions. The compressive moduli of immature cartilage and tissue-engineered cartilage are known to be lower than those of adult cartilage. The objective of this study was to determine if such tissues exhibit depth-dependent compressive properties, and how these depth-varying properties were correlated with cell and matrix composition of the tissue. The compressive moduli of fetal and newborn bovine articular cartilage increased with depth (p<0.05) by a factor of 4-5 from the top 0.1 mm (28+/-13 kPa, 141+/-10 kPa, respectively) to 1 mm deep into the tissue. Likewise, the glycosaminoglycan and collagen content increased with depth (both p<0.001), and correlated with the modulus (both p<0.01). In contrast, tissue-engineered cartilage formed by either layering or mixing cells from the superficial and middle zone of articular cartilage exhibited similarly soft regions at both construct surfaces, as exemplified by large equilibrium strains. The properties of immature cartilage may provide a template for developing tissue-engineered cartilage which aims to repair cartilage defects by recapitulating the natural development and growth processes. These results suggest that while depth-dependent properties may be important to engineer into cartilage constructs, issues other than cell heterogeneity must be addressed to generate such tissues.

Tailoring secretion of proteoglycan 4 (PRG4) in tissue-engineered cartilage

Articular cartilage provides a low-friction surface for joint articulation, with boundary lubrication facilitated by proteoglycan 4 (PRG4), which is secreted by chondrocytes of the superficial zone. Chondrocytes from different zones are phenotypically distinct, and their phenotypes in vitro are influenced by the system in which they are cultured. We hypothesized that culturing cells from the superficial (S) zone in two-dimensional monolayer or three-dimensional alginate would affect their synthesis of PRG4, and that subsequently seeding them atop alginate-recovered cells from the middle/ deep (M) zone in various proportions would result in tissue-engineered constructs with varying levels of PRG4 secretion and matrix accumulation. During monolayer culture, S cells retained their PRG4-secreting phenotype, whereas in alginate culture the percentage of cells secreting PRG4 decreased with time. Constructs formed with increasing percentages of S cells decreased in thickness and matrix accumulation, depending on both the culture conditions before construct formation and the S-cell density. PRG4-secreting cells were localized to the S-cell seeded construct surface, with secretion rates of 0.1-4 pg/cell/day or 0.1-1 pg/cell/day for constructs formed with monolayer-recovered or alginate-recovered S cells, respectively. Tailoring secretion of PRG4 in cartilage constructs may be useful for enhancing low-friction properties at the articular surface, while maintaining other surfaces free of PRG4 for enhancing integration with surrounding tissues.

Tracking chondrocytes and assessing their proliferation with PKH26: effects on secretion of proteoglycan 4 (PRG4)

Distinguishing between implanted and host-derived cells, as well as between distinct cell phenotypes, would be useful in assessing the mechanisms of cell-based repair of cartilage. The fluorescent tracker dye, PKH26, was previously applied to several cell types to assess proliferation in vitro and to track cells in vivo. The objectives of this study were to assess the utility of PKH26 for tracking chondrocytes from superficial and middle zones and their proliferation, and determine the effects of PKH26 on chondrocyte functions, in particular, proliferation and secretion of Proteoglycan 4 (PRG4). PKH26-labeled and unlabeled superficial and middle zone chondrocytes were plated in either low- or high-density monolayer culture and analyzed for retention of PKH26 by flow cytometry and fluorescence microscopy at days 0 and 7. Cell suspensions and conditioned media were analyzed for DNA and secretion of PRG4, respectively. Flow cytometric histograms were deconvolved so that the number of cells in each doubling generation contributing to the final cell population could be estimated. Chondrocytes were consistently and intensely labeled with PKH26 through 7 cycles of division. At day 7 of culture, >97% of superficial zone cells seeded at low or high density could be distinguished as fluorescent, as could middle zone cells seeded at high density. Retention of cell fluorescence after PKH26 labeling and lack of adverse effects on cell proliferation and synthesis of PRG4 suggest that PKH26 can be useful in determining the fate and function of implanted chondrocytes in vivo, as well as monitoring proliferation in vitro.

Modulation of depth-dependent properties in tissue-engineered cartilage with a semi-permeable membrane and perfusion: a continuum model of matrix metabolism and transport

The functional properties of cartilaginous tissues are determined predominantly by the content, distribution, and organization of proteoglycan and collagen in the extracellular matrix. Extracellular matrix accumulates in tissue-engineered cartilage constructs by metabolism and transport of matrix molecules, processes that are modulated by physical and chemical factors. Constructs incubated under free-swelling conditions with freely permeable or highly permeable membranes exhibit symmetric surface regions of soft tissue. The variation in tissue properties with depth from the surfaces suggests the hypothesis that the transport processes mediated by the boundary conditions govern the distribution of proteoglycan in such constructs. A continuum model (DiMicco and Sah in Transport Porus Med 50:57-73, 2003) was extended to test the effects of membrane permeability and perfusion on proteoglycan accumulation in tissue- engineered cartilage. The concentrations of soluble, bound, and degraded proteoglycan were analyzed as functions of time, space, and non-dimensional parameters for several experimental configurations. The results of the model suggest that the boundary condition at the membrane surface and the rate of perfusion, described by non-dimensional parameters, are important determinants of the pattern of proteoglycan accumulation. With perfusion, the proteoglycan profile is skewed, and decreases or increases in magnitude depending on the level of flow-based stimulation. Utilization of a semi-permeable membrane with or without unidirectional flow may lead to tissues with depth-increasing proteoglycan content, resembling native articular cartilage.