The microwell-mesh: A novel device and protocol for the high throughput manufacturing of cartilage microtissues

Cellular Patterning Alone Using Bioprinting Regenerates Articular Cartilage Through Native-Like Cartilagenesis

Few studies have proved that bioprinting itself helps recapitulate native tissue functions mainly because the bioprinted macro shape can rarely, if ever, influence cell function. This can be more problematic in bioprinting cartilage, generally considered more challenging to engineer. Here a new method is shown to micro-pattern chondrocytes within bioprinted sub-millimeter micro tissues, denoted as patterned micro-articular-cartilages tissues (PA-MCTs). Under the sole influence of bioprinted cellular patterns. A pattern scoring system is developed after over 600 bioprinted cellular patterns are analyzed. The top-scored pattern mimics that of the isogenous group in native articular cartilage. Under the sole influence of this pattern during PA-MCTs bio-assembling into macro-cartilage and repairing cartilage defects, chondrogenic cell phenotype is preserved, and cartilagenesis is initiated and maintained. Neocartilage tissues from individual and assembled PA-MCTs are comparable to native articular cartilage and superior to cartilage bioprinted with homogeneously distributed cells in morphology, biochemical components, cartilage-specific protein and gene expression, mechanical properties, integration with host tissues, zonation forming and stem cell chondrogenesis. PA-MCTs can also be used as osteoarthritic and healthy cartilage models for therapeutic drug screening and cartilage development studies. This cellular patterning technique can pave a new way for bioprinting to recapitulate native tissue functions via tissue genesis.

Temporal Enzymatic Treatment to Enhance the Remodeling of Multiple Cartilage Microtissues into a Structurally Organized Tissue

Scaffold-free tissue engineering aims to recapitulate key aspects of normal developmental processes to generate biomimetic grafts. Although functional cartilaginous tissues are engineered using such approaches, considerable challenges remain. Herein, the benefits of engineering cartilage via the fusion of multiple cartilage microtissues compared to using (millions of) individual cells to generate a cartilaginous graft are demonstrated. Key advantages include the generation of a richer extracellular matrix, more hyaline-like cartilage phenotype, and superior shape fidelity. A major drawback of aggregate engineering is that individual microtissues do not completely (re)model and remnants of their initial architectures remain throughout the macrotissue. To address this, a temporal enzymatic (chondroitinase-ABC) treatment is implemented to accelerate structural (re)modeling and shown to support robust fusion between adjacent microtissues, enhance microtissue (re)modeling, and enable the development of a more biomimetic tissue with a zonally organized collagen network. Additionally, enzymatic treatment is shown to modulate matrix composition, tissue phenotype, and to a lesser extent, tissue mechanics. This work demonstrates that microtissue self-organization is an effective method for engineering scaled-up cartilage grafts with a predefined geometry and near-native levels of matrix accumulation. Importantly, key limitations associated with using biological building blocks can be alleviated by temporal enzymatic treatment during graft development.

Microtissue Culture Provides Clarity on the Relative Chondrogenic and Hypertrophic Response of Bone-Marrow-Derived Stromal Cells to TGF-β1, BMP-2, and GDF-5

Chondrogenic induction of bone-marrow-derived stromal cells (BMSCs) is typically accomplished with medium supplemented with growth factors (GF) from the transforming growth factor-beta (TGF-β)/bone morphogenetic factor (BMP) superfamily. In a previous study, we demonstrated that brief (1-3 days) stimulation with TGF-β1 was sufficient to drive chondrogenesis and hypertrophy using small-diameter microtissues generated from 5000 BMSC each. This biology is obfuscated in typical large-diameter pellet cultures, which suffer radial heterogeneity. Here, we investigated if brief stimulation (2 days) of BMSC microtissues with BMP-2 (100 ng/mL) or growth/differentiation factor (GDF-5, 100 ng/mL) was also sufficient to induce chondrogenic differentiation, in a manner comparable to TGF-β1 (10 ng/mL). Like TGF-β1, BMP-2 and GDF-5 are reported to stimulate chondrogenic differentiation of BMSCs, but the effects of transient or brief use in culture have not been explored. Hypertrophy is an unwanted outcome in BMSC chondrogenic differentiation that renders engineered tissues unsuitable for use in clinical cartilage repair. Using three BMSC donors, we observed that all GFs facilitated chondrogenesis, although the efficiency and the necessary duration of stimulation differed. Microtissues treated with 2 days or 14 days of TGF-β1 were both superior at producing extracellular matrix and expression of chondrogenic gene markers compared to BMP-2 and GDF-5 with the same exposure times. Hypertrophic markers increased proportionally with chondrogenic differentiation, suggesting that these processes are intertwined for all three GFs. The rapid action, or "temporal potency", of these GFs to induce BMSC chondrogenesis was found to be as follows: TGF-β1 > BMP-2 > GDF-5. Whether briefly or continuously supplied in culture, TGF-β1 was the most potent GF for inducing chondrogenesis in BMSCs.

Hyperplastic Human Macromass Cartilage for Joint Regeneration

Cartilage damage affects millions of people worldwide. Tissue engineering strategies hold the promise to provide off-the-shelf cartilage analogs for tissue transplantation in cartilage repair. However, current strategies hardly generate sufficient grafts, as tissues cannot maintain size growth and cartilaginous phenotypes simultaneously. Herein, a step-wise strategy is developed for fabricating expandable human macromass cartilage (macro-cartilage) in a 3D condition by employing human polydactyly chondrocytes and a screen-defined serum-free customized culture (CC). CC-induced chondrocytes demonstrate improved cell plasticity, expressing chondrogenic biomarkers after a 14.59-times expansion. Crucially, CC-chondrocytes form large-size cartilage tissues with average diameters of 3.25 ± 0.05 mm, exhibiting abundant homogenous matrix and intact structure without a necrotic core. Compared with typical culture, the cell yield in CC increases 2.57 times, and the expression of cartilage marker collagen type II increases 4.70 times. Transcriptomics reveal that this step-wise culture drives a proliferation-to-differentiation process through an intermediate plastic stage, and CC-chondrocytes undergo a chondral lineage-specific differentiation with an activated metabolism. Animal studies show that CC macro-cartilage maintains a hyaline-like cartilage phenotype in vivo and significantly promotes the healing of large cartilage defects. Overall, an efficient expansion of human macro-cartilage with superior regenerative plasticity is achieved, providing a promising strategy for joint regeneration.

Rethinking of TEER measurement reporting for epithelial cells grown on permeable inserts

Transepithelial electrical resistance (TEER) measures electrical resistance across epithelial tissue barriers involving confluent layer(s) of cells. TEER values act as a prerequisite for determining the barrier integrity of cells, which play a key role in evaluating the transport of drugs, materials or chemicals of interest across an epithelial barrier. The measurements can be performed non-invasively by measuring ohmic resistance across a defined area. Thus, the TEER values are reported in Ω·cm2. In vitro epithelial models are typically assembled on semi-permeable inserts providing two-chamber compartments, and the majority of the studies use inserts with polyethylene terephthalate (PET) membranes. Recently, new inserts with different membrane types and properties have been introduced. However, the TEER values presented so far did not allow a direct comparison. This study presents the characterization of selected epithelial tissues, i.e., lung, retina, and intestine, grown on an ultra-thin ceramic microporous permeable insert (SiMPLI) and PET membranes with different properties, i.e., thickness, material, and pore numbers. We verified the epithelial cell growth on both inserts via phase-contrast and confocal laser scanning microscope imaging. Barrier characteristics were assessed by TEER measurements and also by evaluating the permeability of fluorescein isothiocyanate through cell layers. The findings indicated that background TEER value calculations and the available surface area for cell growth must be thoroughly assessed when new inserts are introduced, as the values cannot be directly compared without re-calculations. Finally, we proposed electrical circuit models highlighting the contributors to TEER recordings on PET and SiMPLI insert membranes. This study paves the way for making the ohmic-based evaluation of epithelial tissues' permeability independent of the material and geometry of the insert membrane used for cell growth.

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.

SP7 gene silencing dampens bone marrow stromal cell hypertrophy, but it also dampens chondrogenesis

For bone marrow stromal cells (BMSC) to be useful in cartilage repair their propensity for hypertrophic differentiation must be overcome. A single day of TGF-β1 stimulation activates intrinsic signaling cascades in BMSCs which subsequently drives both chondrogenic and hypertrophic differentiation. TGF-β1 stimulation upregulates SP7, a transcription factor known to contribute to hypertrophic differentiation, and SP7 remains upregulated even if TGF-β1 is subsequently withdrawn from the chondrogenic induction medium. Herein, we stably transduced BMSCs to express an shRNA designed to silence SP7, and assess the capacity of SP7 silencing to mitigate hypertrophy. SP7 silencing dampened both hypertrophic and chondrogenic differentiation processes, resulting in diminished microtissue size, impaired glycosaminoglycan production and reduced chondrogenic and hypertrophic gene expression. Thus, while hypertrophic features were dampened by SP7 silencing, chondrogenic differentation was also compromised. We further investigated the role of SP7 in monolayer osteogenic and adipogenic cultures, finding that SP7 silencing dampened characteristic mineralization and lipid vacuole formation, respectively. Overall, SP7 silencing affects the trilineage differentiation of BMSCs, but is insufficient to decouple BMSC hypertrophy from chondrogenesis. These data highlight the challenge of promoting BMSC chondrogenesis whilst simultaneously reducing hypertrophy in cartilage tissue engineering strategies.

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.

Articular Cartilage Regeneration through Bioassembling Spherical Micro-Cartilage Building Blocks

Articular cartilage lesions are prevalent and affect one out of seven American adults and many young patients. Cartilage is not capable of regeneration on its own. Existing therapeutic approaches for articular cartilage lesions have limitations. Cartilage tissue engineering is a promising approach for regenerating articular neocartilage. Bioassembly is an emerging technology that uses microtissues or micro-precursor tissues as building blocks to construct a macro-tissue. We summarize and highlight the application of bioassembly technology in regenerating articular cartilage. We discuss the advantages of bioassembly and present two types of building blocks: multiple cellular scaffold-free spheroids and cell-laden polymer or hydrogel microspheres. We present techniques for generating building blocks and bioassembly methods, including bioprinting and non-bioprinting techniques. Using a data set of 5069 articles from the last 28 years of literature, we analyzed seven categories of related research, and the year trends are presented. The limitations and future directions of this technology are also discussed.

Scalable Formation of Highly Viable and Functional Hepatocellular Carcinoma Spheroids in an Oxygen-Permeable Microwell Device for Anti-Tumor Drug Evaluation

For high-throughput anti-cancer drug screening, microwell arrays may serve as an effective tool to generate uniform and scalable tumor spheroids. However, microwell arrays are commonly anchored in non-oxygen-permeable culture plates, leading to limited oxygen supply for avascular spheroids. Herein, a polydimethylsiloxane (PDMS)-based oxygen-permeable microwell device is introduced for generating highly viable and functional hepatocellular carcinoma (HCC) spheroids. The PDMS sheets at the bottom of the microwell device provide a high flux of oxygen like in vivo neighboring hepatic sinusoids. Owing to the better oxygen supply, the generated HepG2 spheroids are larger in size and exhibit higher viability and proliferation with less cell apoptosis and necrosis. These spheroids also exhibit lower levels of anaerobic cellular respiration and express higher levels of liver-related functions. In anti-cancer drug testing, spheroids cultured in PDMS plates show a significantly stronger resistance against doxorubicin because of the stronger stem-cell and multidrug resistance phenotype. Moreover, higher expression of vascular endothelial growth factor-A produces a stronger angiogenesis capability of the spheroids. Overall, compared to the spheroids cultured in conventional non-oxygen-permeable plates, these spheroids can be used as a more favorable model for early-stage HCCs and be applied in high-throughput anti-cancer drug screening.

A Review on Techniques and Biomaterials Used in 3D Bioprinting

Three-dimensional (3D) bioprinting is a cutting-edge technology that has come to light recently and shows a promising potential whose progress will change the face of medicine. This article reviews the most commonly used techniques and biomaterials for 3D bioprinting. We will also look at the advantages and limitations of various techniques and biomaterials and get a comparative idea about them. In addition, we will also look at the recent applications of these techniques in different industries. This article aims to get a basic idea of the techniques and biomaterials used in 3D bioprinting, their advantages and limitations, and their recent applications in various fields.

Direct bone marrow injection of human bone marrow-derived stromal cells into mouse femurs results in greater prostate cancer PC-3 cell proliferation, but not specifically proliferation within the injected femurs

Background

While prostate cancer (PCa) cells most often metastasize to bone in men, species-specific differences between human and mouse bone marrow mean that this pattern is not faithfully replicated in mice. Herein we evaluated the impact of partially humanizing mouse bone marrow with human bone marrow-derived stromal cells (BMSC, also known as "mesenchymal stem cells") on human PCa cell behaviour.

Methods

BMSC are key cellular constituents of marrow. We used intrafemoral injection to transplant 5 × 10⁵ luciferase (Luc) and green fluorescence protein (GFP) expressing human BMSC (hBMSC-Luc/GFP) into the right femur of non-obese diabetic (NOD)-severe combined immunodeficiency (scid) interleukin (IL)-2γ^−/− (NSG) mice. Two weeks later, 2.5 × 10⁶ PC-3 prostate cancer cells expressing DsRed (PC-3-DsRed) were delivered into the mice via intracardiac injection. PC-3-DsRed cells were tracked over time using an In Vivo Imaging System (IVIS) live animal imaging system, X-ray and IVIS imaging performed on harvested organs, and PC-3 cell numbers in femurs quantified using flow cytometry and histology.

Results

Flow cytometry analysis revealed greater PC-3-DsRed cell numbers within femurs of the mice that received hBMSC-Luc/GFP. However, while there were overall greater PC-3-DsRed cell numbers in these animals, there were not more PC-3-DsRed in the femurs injected with hBMSC-Luc/GFP than in contralateral femurs. A similar proportion of mice in with or without hBMSC-Luc/GFP had bone lessions, but the absolute number of bone lesions was greater in mice that had received hBMSC-Luc/GFP.

Conclusion

PC-3-DsRed cells preferentially populated bones in mice that had received hBMSC-Luc/GFP, although PC-3-DsRed cells not specifically localize in the bone marrow cavity where hBMSC-Luc/GFP had been transplanted. hBMSC-Luc/GFP appear to modify mouse biology in a manner that supports PC-3-DsRed tumor development, rather than specifically influencing PC-3-DsRed cell homing. This study provides useful insights into the role of humanizing murine bone marrow with hBMSC to study human PCa cell biology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09430-6.

Trapping of Aqueous Droplets under Surface Acoustic Wave-Driven Streaming in Oil-Filled Microwells

Microwell arrays are ideal platforms for cell culturing, cell separation, and low-volume liquid handling. The ability to manipulate droplets in microwells could open up the opportunity for developing new biochemical assays. Here, we study the trapping of aqueous droplets in an oil-filled microwell driven by the application of nanometer amplitude vibrations called surface acoustic waves (SAW). We elucidate the dynamics of the droplet within the vortex toward the final trapping location and the physics of the trapping phenomenon using a theoretical model by considering the relevant forces. Our study revealed that the combined effect of acoustic radiation and hydrodynamic forces leads to droplet migration and trapping. We demarcate the trapping and nontrapping regimes in terms of the minimum critical input power required for the trapping of droplets of different sizes and densities. We find that the critical power varies as the square of the droplet size and is higher for a denser droplet. The effects of input power and droplet size on the trapping location and trapping time are also studied.

Two-Dimensional and Three-Dimensional Cartilage Model Platforms for Drug Evaluation and High-Throughput Screening Assays

Osteoarthritis (OA) is a severely painful and debilitating disease of the joint, which brings about degradation of the articular cartilage and currently has few therapeutic solutions. Two-dimensional (2D) high-throughput screening (HTS) assays have been widely used to identify candidate drugs with therapeutic potential for the treatment of OA. A number of small molecules which improve the chondrogenic differentiation of progenitor cells for tissue engineering applications have also been discovered in this way. However, due to the failure of these models to accurately represent the native joint environment, the efficacy of these drugs has been limited in vivo. Screening systems utilizing three-dimensional (3D) models, which more closely reflect the tissue and its complex cell and molecular interactions, have also been described. However, the vast majority of these systems fail to recapitulate the complex, zonal structure of articular cartilage and its unique cell population. This review summarizes current 2D HTS techniques and addresses the question of how to use existing 3D models of tissue-engineered cartilage to create 3D drug screening platforms with improved outcomes. Impact statement Currently, the use of two-dimensional (2D) screening platforms in drug discovery is common practice. However, these systems often fail to predict efficacy in vivo, as they do not accurately represent the complexity of the native three-dimensional (3D) environment. This article describes existing 2D and 3D high-throughput systems used to identify small molecules for osteoarthritis treatment or in vitro chondrogenic differentiation, and suggests ways to improve the efficacy of these systems based on the most recent research.

Inhibition of BMP signaling with LDN 193189 can influence bone marrow stromal cell fate but does not prevent hypertrophy during chondrogenesis

Bone morphogenetic protein (BMP) cascades are upregulated during bone marrow-derived stromal cell (BMSC) chondrogenesis, contributing to hypertrophy and preventing effective BMSC-mediated cartilage repair. Previous work demonstrated that a proprietary BMP inhibitor prevented BMSC hypertrophy, yielding stable cartilage tissue. Because of the significant therapeutic potential of a molecule capable of hypertrophy blockade, we evaluated the capacity of a commercially available BMP type I receptor inhibitor with similar properties, LDN 193189, to prevent BMSC hypertrophy. Using 14-day microtissue chondrogenic induction cultures we found that LDN 193189 permitted BMSC chondrogenesis but did not prevent hypertrophy. LDN 193189 was sufficiently potent to counter mineralization and adipogenesis in response to exogenous BMP-2 in osteogenic induction cultures. LDN 193189 did not modify BMSC behavior in adipogenic induction cultures. Although LDN 193189 is effective in countering BMP signaling in a manner that influences BMSC fate, this blockade is insufficient to prevent hypertrophy.

Effects of Growth Factor Combinations TGFβ3, GDF5 and GDF6 on the Matrix Synthesis of Nucleus Pulposus and Nasoseptal Chondrocyte Self-Assembled Microtissues

There has been significant interest in identifying alternative cell sources and growth factor stimulation to improve matrix synthesis for disc repair. Recent work has identified nasoseptal chondrocytes (NC) as a possible alternative cell source with significant matrix-forming abilities. While various growth factors such as members of the TGFβ superfamily have been explored to enhance matrix formation, no consensus exists as to the optimum growth factor needed to induce cells towards a discogenic phenotype. This study assessed both nucleus pulposus (NP) and NC microtissues of different densities (1000, 2500 or 5000 cells/microtissue) stimulated by individual or combinations of the growth factors TGFβ3, GDF5, and GDF6. Lower cell densities result in increased sGAG/DNA and collagen/DNA levels due to higher nutrient availability levels. Our findings suggest that growth factors exert differential effects on matrix synthesis depending on the cell type. NP cells were found to be relatively insensitive to the different growth factor types examined in isolation or in combination. Overall, NCs exhibited a higher propensity to form extracellular matrix compared to NP cells. In addition, stimulating NC-microtissues with GDF5 or TGFβ3 alone induced enhanced matrix formation and may be an appropriate growth factor to stimulate this cell type for disc regeneration.

Collagenase treatment appears to improve cartilage tissue integration but damage to collagen networks is likely permanent

When repairing cartilage defects a major challenge is achieving high-quality integration between the repair tissue and adjacent native cartilage. Matrix-rich cartilage is not easily remodeled, motivating several studies to trial enzyme treatment of the tissue interface to facilitate remodeling and integration. Studying and optimizing such processes is tedious, as well as potentially expensive, and thus simpler models are needed to evaluate the merits of enzyme treatment on cartilage tissue integration. Herein, we used engineered cartilage microtissues formed from bone marrow-derived stromal cells (BMSC) or expanded articular chondrocytes (ACh) to study the impact of enzyme treatment on cartilage tissue integration and matrix remodeling. A 5-min treatment with collagenase appeared to improve cartilage microtissue integration, while up to 48 h treatment with hyaluronidase did not. Alcian blue and anti-collagen II staining suggested that collagenase treatment did facilitate near seamless integration of cartilage microtissues. Microtissue sections were stained with Picrosirius red and characterized using polarized light microscopy, revealing that individual microtissues contained a collagen network organized in concentric shells. While collagenase treatment appeared to improve tissue integration, assessment of the collagen fibers with polarized light indicated that enzymatically damaged networks were not remodeled nor restored during subsequent culture. This model and these data paradoxically suggest that collagen network disruption is required to improve cartilage tissue integration, but that the disrupted collagen networks are unlikely to subsequently be restored. Future studies should attempt to limit collagen network disruption to the surface of the cartilage, and we recommend using Picrosirius red staining and polarized light to assess the quality of matrix remodeling and integration.

3D printed silk-gelatin hydrogel scaffold with different porous structure and cell seeding strategy for cartilage regeneration

Hydrogel scaffolds are attractive for tissue defect repair and reorganization because of their human tissue-like characteristics. However, most hydrogels offer limited cell growth and tissue formation ability due to their submicron- or nano-sized gel networks, which restrict the supply of oxygen, nutrients and inhibit the proliferation and differentiation of encapsulated cells. In recent years, 3D printed hydrogels have shown great potential to overcome this problem by introducing macro-pores within scaffolds. In this study, we fabricated a macroporous hydrogel scaffold through horseradish peroxidase (HRP)-mediated crosslinking of silk fibroin (SF) and tyramine-substituted gelatin (GT) by extrusion-based low-temperature 3D printing. Through physicochemical characterization, we found that this hydrogel has excellent structural stability, suitable mechanical properties, and an adjustable degradation rate, thus satisfying the requirements for cartilage reconstruction. Cell suspension and aggregate seeding methods were developed to assess the inoculation efficiency of the hydrogel. Moreover, the chondrogenic differentiation of stem cells was explored. Stem cells in the hydrogel differentiated into hyaline cartilage when the cell aggregate seeding method was used and into fibrocartilage when the cell suspension was used. Finally, the effect of the hydrogel and stem cells were investigated in a rabbit cartilage defect model. After implantation for 12 and 16 weeks, histological evaluation of the sections was performed. We found that the enzymatic cross-linked and methanol treatment SF5GT15 hydrogel combined with cell aggregates promoted articular cartilage regeneration. In summary, this 3D printed macroporous SF-GT hydrogel combined with stem cell aggregates possesses excellent potential for application in cartilage tissue repair and regeneration.

Fabrication approaches for high-throughput and biomimetic disease modeling

There is often a tradeoff between in vitro disease modeling platforms that capture pathophysiologic complexity and those that are amenable to high-throughput fabrication and analysis. However, this divide is closing through the application of a handful of fabrication approaches-parallel fabrication, automation, and flow-driven assembly-to design sophisticated cellular and biomaterial systems. The purpose of this review is to highlight methods for the fabrication of high-throughput biomaterial-based platforms and showcase examples that demonstrate their utility over a range of throughput and complexity. We conclude with a discussion of future considerations for the continued development of higher-throughput in vitro platforms that capture the appropriate level of biological complexity for the desired application. STATEMENT OF SIGNIFICANCE: There is a pressing need for new biomedical tools to study and understand disease. These platforms should mimic the complex properties of the body while also permitting investigation of many combinations of cells, extracellular cues, and/or therapeutics in high-throughput. This review summarizes emerging strategies to fabricate biomimetic disease models that bridge the gap between complex tissue-mimicking microenvironments and high-throughput screens for personalized medicine.

Perichondrium-inspired permeable nanofibrous tube well promoting differentiation of hiPSC-derived pellet toward hyaline-like cartilage pellet

The pellet formation has been regarded as a golden standard forin vitrochondrogenic differentiation. However, a spatially inhomogeneous chondrogenic microenvironment around a pellet resulted from the use of a traditional impermeable narrow tube, such as the conical tube, undermines the differentiation performance and therapeutic potential of differentiated cartilage pellet in defective articular cartilage treatment. To address this drawback, a perichondrium-inspired permeable nanofibrous tube (PINaT) well with a nanofibrous wall permeable to gas and soluble molecules is proposed. The PINaT well was fabricated with a micro deep drawing process where a flat thin nanofibrous membrane was transformed to a 3.5 mm deep tube well with a ∼50µm thick nanofibrous wall. Similar toin vivoperichondrium, the PINaT well was found to allow oxygen and growth factor diffusion required for chondrogenic differentiation across the entire nanofibrous wall. Analyses of gene expressions (COL2A1, COL10A1, ACAN, and SOX9), proteins (type II and X collagen), and glycosaminoglycans contents were conducted to assess the differentiation performance and clinical efficacy of differentiated cartilage pellet. The regulated spatially homogeneous chondrogenic microenvironment around the human induced pluripotent stem cell-derived pellet (3 × 10⁵cells per pellet) in the PINaT well remarkably improved the quality of the differentiated pellet toward a more hyaline-like cartilage pellet. Furthermore, an accelerated chondrogenic differentiation process of the pellet produced by the PINaT well was achieved for 14 days, demonstrating a hyaline cartilage-specific marker similar to the control pellet differentiated for 20 days. Finally, the enhanced clinical efficacy of the hyaline-like cartilage pellet was confirmed using an osteochondral defect rat model, with the repaired tissue resembling hyaline cartilage rather than fibrous cartilage after 8 weeks of regeneration.

3D printed gelatin-genipin scaffolds for temporomandibular joint cartilage regeneration

Gelatin has emerged as a biocompatible polymer with high printability in scaffold-based tissue engineering. The aim of the current study was to investigate the potential of genipin-crosslinked 3D printed gelatin scaffolds for temporomandibular joint (TMJ) cartilage regeneration. Crosslinking with genipin increased the stability and mechanical properties, without any cytotoxic effects. Chondrogenic differentiation of human bone marrow-derived mesenchymal stem cells (hBMSC) on the scaffolds were compared to cell pellets and spheres. Although hBMSC seeded scaffolds showed a lower expression of chondrogenesis-related genes compared to cell pellets and spheres, they demonstrated a significantly reduced expression of collagen (COL) 10, suggesting a decreased hypertrophic tendency. After 21 days, staining with Alcian blue and immunofluorescence for SOX9 and COL1 confirmed the chondrogenic differentiation of hBMSC on genipin-crosslinked gelatin scaffolds. In summary, 3D printed gelatin-genipin scaffolds supported the viability, attachment and chondrogenic differentiation of hBMSC, thus, demonstrating potential for TMJ cartilage regeneration applications.

Advanced Strategies of Biomimetic Tissue-Engineered Grafts for Bone Regeneration

The failure to repair critical-sized bone defects often leads to incomplete regeneration or fracture non-union. Tissue-engineered grafts have been recognized as an alternative strategy for bone regeneration due to their potential to repair defects. To design a successful tissue-engineered graft requires the understanding of physicochemical optimization to mimic the composition and structure of native bone, as well as the biological strategies of mimicking the key biological elements during bone regeneration process. This review provides an overview of engineered graft-based strategies focusing on physicochemical properties of materials and graft structure optimization from macroscale to nanoscale to further boost bone regeneration, and it summarizes biological strategies which mainly focus on growth factors following bone regeneration pattern and stem cell-based strategies for more efficient repair. Finally, it discusses the current limitations of existing strategies upon bone repair and highlights a promising strategy for rapid bone regeneration.

Pharmaceutical electrospinning and 3D printing scaffold design for bone regeneration

Bone regenerative engineering provides a great platform for bone tissue regeneration covering cells, growth factors and other dynamic forces for fabricating scaffolds. Diversified biomaterials and their fabrication methods have emerged for fabricating patient specific bioactive scaffolds with controlled microstructures for bridging complex bone defects. The goal of this review is to summarize the points of scaffold design as well as applications for bone regeneration based on both electrospinning and 3D bioprinting. It first briefly introduces biological characteristics of bone regeneration and summarizes the applications of different types of material and the considerations for bone regeneration including polymers, ceramics, metals and composites. We then discuss electrospinning nanofibrous scaffold applied for the bone regenerative engineering with various properties, components and structures. Meanwhile, diverse design in the 3D bioprinting scaffolds for osteogenesis especially in the role of drug and bioactive factors delivery are assembled. Finally, we discuss challenges and future prospects in the development of electrospinning and 3D bioprinting for osteogenesis and prominent strategies and directions in future.

Human bone marrow-derived stromal cell behavior when injected directly into the bone marrow of NOD-scid-gamma mice pre-conditioned with sub-lethal irradiation

BACKGROUND

Direct bone marrow injection of cells into murine marrow cavities is used in a range of cell characterization assays and to develop disease models. While human bone marrow-derived stromal cells (hBMSC, also known as mesenchymal stem cells (MSC)) are frequently described in therapeutic applications, or disease modeling, their behavior following direct injection into murine bone marrow is poorly characterized. Herein, we characterized hBMSC engraftment and persistence within the bone marrow of NOD-scid interleukin (IL)-2γ^-/- (NSG) mice with or without prior 2 Gy total-body γ-irradiation of recipient mice.

METHODS

One day after conditioning NSG mice with sublethal irradiation, 5 × 10⁵ luciferase (Luc) and green fluorescent protein (GFP)-expressing hBMSC (hBMSC-Luc/GFP) were injected into the right femurs of animals. hBMSC-Luc/GFP were tracked in live animals using IVIS imaging, and histology was used to further characterize hBMSC location and behavior in tissues.

RESULTS

hBMSC-Luc/GFP number within injected marrow cavities declined rapidly over 4 weeks, but prior irradiation of animals delayed this decline. At 4 weeks, hBMSC-Luc/GFP colonized injected marrow cavities and distal marrow cavities at rates of 2.5 ± 2.2% and 1.7 ± 1.9% of total marrow nucleated cells, respectively in both irradiated and non-irradiated mice. In distal marrow cavities,  hBMSC were not uniformly distributed and appeared to be co-localized in clusters, with the majority found in the endosteal region.

CONCLUSIONS

While significant numbers of hBMSC-Luc/GFP could be deposited into the mouse bone marrow via direct bone marrow injection, IVIS imaging indicated that the number of hBMSC-Luc/GFP in that bone marrow cavity declined with time. Irradiation of mice prior to transplant only delayed the rate of hBMSC-Luc/GFP population decline in injected femurs. Clusters of hBMSC-Luc/GFP were observed in the histology of distal marrow cavities, suggesting that some transplanted cells actively homed to distal marrow cavities. Individual cell clusters may have arisen from discrete clones that homed to the marrow, and then underwent modest proliferation. The transient high-density population of hBMSC within the injected femur, or the longer-term low-density population of hBMSC in distal marrow cavities, offers useful models for studying disease or regenerative processes. Experimental designs should consider how relative hBMSC distribution and local hBMSC densities evolve over time.

Using the Microwell-mesh to culture microtissues in vitro and as a carrier to implant microtissues in vivo into mice

Prostate cancer (PCa) patient-derived xenografts (PDXs) are commonly propagated by serial transplantation of "pieces" of tumour in mice, but the cellular composition of pieces is not standardised. Herein, we optimised a microwell platform, the Microwell-mesh, to aggregate precise numbers of cells into arrays of microtissues, and then implanted the Microwell-mesh into NOD-scid IL2γ^-/- (NSG) mice to study microtissue growth. First, mesh pore size was optimised using microtissues assembled from bone marrow-derived stromal cells, with mesh opening dimensions of 100×100 μm achieving superior microtissue vascularisation relative to mesh with 36×36 μm mesh openings. The optimised Microwell-mesh was used to assemble and implant PCa cell microtissue arrays (hereafter microtissues formed from cancer cells are referred to as microtumours) into mice. PCa cells were enriched from three different PDX lines, LuCaP35, LuCaP141, and BM18. 3D microtumours showed greater in vitro viability than 2D cultures, but neither proliferated. Microtumours were successfully established in mice 81% (57 of 70), 67% (4 of 6), 76% (19 of 25) for LuCaP35, LuCaP141, and BM18 PCa cells, respectively. Microtumour growth was tracked using live animal imaging for size or bioluminescence signal. If augmented with further imaging advances and cell bar coding, this microtumour model could enable greater resolution of PCa PDX drug response, and lead to the more efficient use of animals. The concept of microtissue assembly in the Microwell-mesh, and implantation in vivo may also have utility in implantation of islets, hair follicles or other organ-specific cells that self-assemble into 3D structures, providing an important bridge between in vitro assembly of mini-organs and in vivo implantation.

Development and Angiogenic Potential of Cell-Derived Microtissues Using Microcarrier-Template

Tissue engineering and regenerative medicine approaches use biomaterials in combination with cells to regenerate lost functions of tissues and organs to prevent organ transplantation. However, most of the current strategies fail in mimicking the tissue's extracellular matrix properties. In order to mimic native tissue conditions, we developed cell-derived matrix (CDM) microtissues (MT). Our methodology uses poly-lactic acid (PLA) and Cultispher® S microcarriers' (MCs') as scaffold templates, which are seeded with rat bone marrow mesenchymal stem cells (rBM-MSCs). The scaffold template allows cells to generate an extracellular matrix, which is then extracted for downstream use. The newly formed CDM provides cells with a complex physical (MT architecture) and biochemical (deposited ECM proteins) environment, also showing spontaneous angiogenic potential. Our results suggest that MTs generated from the combination of these two MCs (mixed MTs) are excellent candidates for tissue vascularization. Overall, this study provides a methodology for in-house fabrication of microtissues with angiogenic potential for downstream use in various tissue regenerative strategies.

Turning Nature's own processes into design strategies for living bone implant biomanufacturing: a decade of Developmental Engineering

A decade after the term developmental engineering (DE) was coined to indicate the use of developmental processes as blueprints for the design and development of engineered living implants, a myriad of proof-of-concept studies demonstrate the potential of this approach in small animal models. This review provides an overview of DE work, focusing on applications in bone regeneration. Enabling technologies allow to quantify the distance between in vitro processes and their developmental counterpart, as well as to design strategies to reduce that distance. By embedding Nature's robust mechanisms of action in engineered constructs, predictive large animal data and subsequent positive clinical outcomes can be gradually achieved. To this end, the development of next generation biofabrication technologies should provide the necessary scale and precision for robust living bone implant biomanufacturing.

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.

Engineered Microsystems for Spheroid and Organoid Studies

3D in vitro model systems such as spheroids and organoids provide an opportunity to extend the physiological understanding using recapitulated tissues that mimic physiological characteristics of in vivo microenvironments. Unlike 2D systems, 3D in vitro systems can bridge the gap between inadequate 2D cultures and the in vivo environments, providing novel insights on complex physiological mechanisms at various scales of organization, ranging from the cellular, tissue-, to organ-levels. To satisfy the ever-increasing need for highly complex and sophisticated systems, many 3D in vitro models with advanced microengineering techniques have been developed to answer diverse physiological questions. This review summarizes recent advances in engineered microsystems for the development of 3D in vitro model systems. The relationship between the underlying physics behind the microengineering techniques, and their ability to recapitulate distinct 3D cellular structures and functions of diverse types of tissues and organs are highlighted and discussed in detail. A number of 3D in vitro models and their engineering principles are also introduced. Finally, current limitations are summarized, and perspectives for future directions in guiding the development of 3D in vitro model systems using microengineering techniques are provided.

A combined 3D printing/CNC micro-milling method to fabricate a large-scale microfluidic device with the small size 3D architectures: an application for tumor spheroid production

The fabrication of a large-scale microfluidic mold with 3D microstructures for manufacturing of the conical microwell chip using a combined projection micro-stereolithography (PµSL) 3D printing/CNC micro-milling method for tumor spheroid formation is presented. The PµSL technique is known as the most promising method of manufacturing microfluidic chips due to the possibility of creating complex three-dimensional microstructures with high resolution in the range of several micrometers. The purpose of applying the proposed method is to investigate the influence of microwell depths on the formation of tumor spheroids. In the conventional methods, the construction of three-dimensional microstructures and multi-height chips is difficult, time-consuming, and is performed using a multi-step lithography process. Microwell depth is an essential parameter for microwell design since it directly affects the shear stress of the fluid flow and the diffusion of nutrients, respiratory gases, and growth factors. In this study, a chip was made with microwells of different depth varying from 100 to 500 µm. The mold of the microwell section is printed by the lab-made PµSL printer with 6 and 1 µm lateral and vertical resolutions. Other parts of the mold, such as the main chamber and micro-channels, were manufactured using the CNC micro-milling method. Finally, different parts of the master mold were assembled and used for PDMS casting. The proposed technique drastically simplifies the fabrication and rapid prototyping of large-scale microfluidic devices with high-resolution microstructures by combining 3D printing with the CNC micro-milling method.

Stromal cells cultivated from the choroid of human eyes display a mesenchymal stromal cell (MSC) phenotype and inhibit the proliferation of choroidal vascular endothelial cells in vitro

Mesenchymal stromal cells (MSC), with progenitor cell and immunological properties, have been cultivated from numerous vascularized tissues including bone marrow, adipose tissue and the corneal-limbus of the eye. After observing mesenchymal cells as contaminants in primary cultures of vascular endothelial cells derived from the choroidal tunic of the human eye, we investigated whether the choroid might also provide a source of cultured MSC. Moreover, we examined the effect of the choroidal stromal cells (Ch-SC) on the proliferation of freshly isolated choroidal vascular endothelial cells (ChVEC) in vitro. The phenotype of cultures established from five choroidal tissue donors was examined by flow cytometry and immunocytochemistry. The potential for mesenchymal cell differentiation was examined in parallel with MSC established from human bone marrow. Additional cultures were growth-arrested by treatment with mitomycin-C, before being tested as a potential feeder layer for ChVEC. The five unique cultures established from choroidal stroma displayed a phenotype consistent with the accepted definition for MSC (CD34^-, CD45^-, HLA-DR^-, CD73⁺, CD90⁺, and CD105⁺), including the capacity for mesenchymal differentiation when cultivated under osteogenic, adipogenic and chondrogenic conditions. Growth-arrested Ch-SC inhibited the proliferation of ChVEC derived from five separate donors. Cultures of Ch-SC secreted approximately 40-fold higher concentrations of the anti-angiogenic factor pigment epithelium derived factor (PEDF/serpin F1) compared to the pro-angiogenic factor, vascular endothelial growth factor (VEGF), regardless of normal or growth-arrested state. Our results provide first evidence of a resident MSC cell type within the choroid and encourage investigation of new mechanisms for altering the growth of ChVEC.

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.

Transforming growth factor-beta stimulates human bone marrow-derived mesenchymal stem/stromal cell chondrogenesis more so than kartogenin

A previous study identified kartogenin (KGN) as a potent modulator of bone marrow mesenchymal stem/stromal cell (BMSC) chondrogenesis. This initial report did not contrast KGN directly against transforming growth factor-beta 1 (TGF-β1), the most common growth factor used in chondrogenic induction medium. Herein, we directly compared the in vitro chondrogenic potency of TGF-β1 and KGN using a high resolution micropellet model system. Micropellets were cultured for 7–14 days in medium supplemented with TGF-β1, KGN, or both TGF-β1 + KGN. Following 14 days of induction, micropellets exposed to TGF-β1 alone or TGF-β1 + KGN in combination were larger and produced more glycosominoglycan (GAG) than KGN-only cultures. When TGF-β1 + KGN was used, GAG quantities were similar or slightly greater than the TGF-β1-only cultures, depending on the BMSC donor. BMSC micropellet cultures supplemented with KGN alone contracted in size over the culture period and produced minimal GAG. Indicators of hypertrophy were not mitigated in TGF-β1 + KGN cultures, suggesting that KGN does not obstruct BMSC hypertrophy. KGN appears to have weak chondrogenic potency in human BMSC cultures relative to TGF-β1, does not obstruct hypertrophy, and may not be a viable alternative to growth factors in cartilage tissue engineering.

Rapid Cartilage Regeneration of Spheroids Composed of Human Nasal Septum-Derived Chondrocyte in Rat Osteochondral Defect Model

BACKGROUND

Cell-based therapies have been studied for articular cartilage regeneration. Articular cartilage defects have little treatments because articular cartilage was limited regenerative capacity. Damaged articular cartilage is difficult to obtain a successful therapeutic effect. In additionally these articular cartilage defects often cause osteoarthritis. Chondrocyte implantation is a widely available therapy used for regeneration of articular cartilage because this tissue has poor repair capacity after injury. Human nasal septum-drived chondrocytes (hNCs) from the septum show greater proliferation ability and chondrogenic capacity than human articular chondrocytes (hACs), even across different donors with different ages. Moreover, the chondrogenic properties of hNCs can be maintained after extensive culture expansion.

METHODS

In this study, 2 dimensional (2D) monolayer cultured hNCs (hNCs-2D) and 3 dimensional (3D) spheroids cultured hNCs (hNCs-3D) were examined for chondrogenic capacity in vitro by PCR and immunofluorescence staining for chondrogenic marker, cell survival during cultured and for cartilage regeneration ability in vivo in a rat osteochondral defect model.

RESULTS

hNCs-3D showed higher viability and more uniform morphology than 3D spheroids cultured hACs (hACs-3D) in culture. hNCs-3D also showed greater expression levels of the chondrocyte-specific marker Type II collagen (COL2A1) and sex-determining region Y (SRY)-box 9 (SOX9) than hNCs-2D. hNCs-3D also expressed chondrogenic markers in collagen. Specially, in the osteochondral defect model, implantation of hNCs-3D led to greater chondrogenic repair of focal cartilage defects in rats than implantation of hNCs-2D.

CONCLUSION

These data suggest that hNCs-3D are valuable therapeutic agents for repair and regeneration of cartilage defects.

Developmentally Engineered Callus Organoid Bioassemblies Exhibit Predictive In Vivo Long Bone Healing

Clinical translation of cell-based products is hampered by their limited predictive in vivo performance. To overcome this hurdle, engineering strategies advocate to fabricate tissue products through processes that mimic development and regeneration, a strategy applicable for the healing of large bone defects, an unmet medical need. Natural fracture healing occurs through the formation of a cartilage intermediate, termed "soft callus," which is transformed into bone following a process that recapitulates developmental events. The main contributors to the soft callus are cells derived from the periosteum, containing potent skeletal stem cells. Herein, cells derived from human periosteum are used for the scalable production of microspheroids that are differentiated into callus organoids. The organoids attain autonomy and exhibit the capacity to form ectopic bone microorgans in vivo. This potency is linked to specific gene signatures mimicking those found in developing and healing long bones. Furthermore, callus organoids spontaneously bioassemble in vitro into large engineered tissues able to heal murine critical-sized long bone defects. The regenerated bone exhibits similar morphological properties to those of native tibia. These callus organoids can be viewed as a living "bio-ink" allowing bottom-up manufacturing of multimodular tissues with complex geometric features and inbuilt quality attributes.

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.

Constraints to counting bioluminescence producing cells by a commonly used transgene promoter and its implications for experimental design

It is routine to genetically modify cells to express fluorescent or bioluminescent reporter proteins to enable tracking or quantification of cells in vitro and in vivo. Herein, we characterized the stability of luciferase reporter systems in C4-2B prostate cancer cells in mono-culture and in co-culture with bone marrow-derived mesenchymal stem/stromal cells (BMSC). An assumption made when employing the luciferase reporter is that the luciferase expressing cell number and bioluminescence signal are linearly proportional. We observed instances where luciferase expression was significantly upregulated in C4-2B cell populations when co-cultured with BMSC, resulting in a significant disconnect between bioluminescence signal and cell number. We subsequently characterized luciferase reporter stability in a second C4-2B reporter cell line, and six other cancer cell lines. All but the single C4-2B reporter cell population had stable luciferase reporter expression in mono-culture and BMSC co-culture. Whole-genome sequencing revealed that relative number of luciferase gene insertions per genome in the unstable C4-2B reporter cell population was lesser than stable C4-2B, PC3 and MD-MBA-231 luciferase reporter cell lines. We reasoned that the low luciferase gene copy number and genome insertion locations likely contributed to the reporter gene expression being exquisitely sensitive BMSC paracrine signals. In this study, we show that it is possible to generate a range of stable and reliable luciferase reporter prostate- and breast- cancer cell populations but advise not to assume stability across different culture conditions. Reporter stability should be validated, on a case-by-case basis, for each cell line and culture condition.

Bone marrow-derived stem/stromal cells (BMSC) 3D microtissues cultured in BMP-2 supplemented osteogenic induction medium are prone to adipogenesis

Bone marrow-derived mesenchymal stem/stromal cells (BMSC) may facilitate bone repair through secretion of factors that stimulate endogenous repair processes or through direct contribution to new bone through differentiation into osteoblast-like cells. BMSC microtissue culture and differentiation has been widely explored recently, with high-throughput platforms making large-scale manufacture of microtissues increasingly feasible. Bone-like BMSC microtissues could offer an elegant method to enhance bone repair, especially in small-volume non-union defects, where small diameter microtissues could be delivered orthoscopically. Using a high-throughput microwell platform, our data demonstrate that (1) BMSC in 3D microtissue culture result in tissue compaction, rather than growth, (2) not all mineralised bone-like matrix is incorporated in the bulk microtissue mass and (3) a significant amount of lipid vacuole formation is observed in BMSC microtissues exposed to BMP-2. These factors should be considered when optimising BMSC osteogenesis in microtissues or developing BMSC microtissue-based therapeutic delivery processes.

Design and fabrication of a liver-on-a-chip platform for convenient, highly efficient, and safe in situ perfusion culture of 3D hepatic spheroids

Spheroid-based three-dimensional (3D) liver culture models, offering a desirable biomimetic microenvironment, are useful for recapitulating liver functions in vitro. However, a user-friendly, robust and specially optimized method has not been well developed for a convenient, highly efficient, and safe in situ perfusion culture of spheroid-based 3D liver models. Here, we have developed a biomimetic and reversibly assembled liver-on-a-chip (3D-LOC) platform and presented a proof of concept for long-term perfusion culture of 3D human HepG2/C3A spheroids for building a 3D liver spheroid model. On the basis of a fast and reversible seal of concave microwell-based PDMS-membrane-PDMS sandwich multilayer chips, it enables a high-throughput and parallel perfusion culture of 1080 cell spheroids in a high mass transfer and low fluid shear stress biomimetic microenvironment as well as allowing the convenient collection and analysis of the cell spheroids. In terms of reducing spheroid loss and maintaining cell morphology and viability in long-term perfusion culture, the cell spheroids in the 3D-LOC were more safe and efficient. Notably, the polarisation, liver-specific functions, and metabolic activity of the cell spheroids in 3D-LOC were also remarkably improved and exhibited better long-term maintenance over conventional perfusion methods. Additionally, a robust micromilling method that incorporates secondary PDMS coating techniques (SPCs) for fabricating V-shaped concave microwells was also developed. The V-shaped concave microwell arrays exhibited a higher distribution density and aperture ratio, making it easy to form large-scale and uniform-sized cell spheroids with minimum cell loss. In summary, the proposed 3D-LOC could provide a convenient and robust solution for the long-term safe perfusion culture of hepatic spheroids and be beneficial for a variety of potential applications including development of bio-artificial livers, disease modeling, and drug toxicity screening.

Using high throughput microtissue culture to study the difference in prostate cancer cell behavior and drug response in 2D and 3D co-cultures

Background

There is increasing appreciation that non-cancer cells within the tumour microenvironment influence cancer progression and anti-cancer drug efficacy. For metastatic prostate cancer (PCa), the bone marrow microenvironment influences metastasis, drug response, and possibly drug resistance.

Methods

Using a novel microwell platform, the Microwell-mesh, we manufactured hundreds of 3D co-culture microtissues formed from PCa cells and bone marrow stromal cells. We used luciferase-expressing C42B PCa cells to enable quantification of the number of PCa cells in complex microtissue co-cultures. This strategy enabled us to quantify specific PCa cell growth and death in response to drug treatment, in different co-culture conditions. In parallel, we used Transwell migration assays to characterize PCa cell migration towards different 2D and 3D stromal cell populations.

Results

Our results reveal that PCa cell migration varied depending on the relative aggressiveness of the PCa cell lines, the stromal cell composition, and stromal cell 2D or 3D geometry. We found that C42B cell sensitivity to Docetaxel varied depending on culture geometry, and the presence or absence of different stromal cell populations. By contrast, the C42B cell response to Abiraterone Acetate was dependent on geometry, but not on the presence or absence of stromal cells.

Conclusion

In summary, stromal cell composition and geometry influences PCa cell migration, growth and drug response. The Microwell-mesh and microtissues are powerful tools to study these complex 3D interactions.

Electronic supplementary material

The online version of this article (10.1186/s12885-018-4473-8) contains supplementary material, which is available to authorized users.

Combination of Heparin Binding Peptide and Heparin Cell Surface Coatings for Mesenchymal Stem Cell Spheroid Assembly

Microtissues containing multiple cell types have been used in both in vitro models and in vivo tissue repair applications. However, to improve throughput, there is a need to develop a platform that supports self-assembly of a large number of 3D microtissues containing multiple cell types in a dynamic suspension system. Thus, the objective of this study was to exploit the binding interaction between the negatively charged glycosaminoglycan, heparin, and a known heparin binding peptide to establish a method that promotes assembly of mesenchymal stem cell (MSC) spheroids into larger aggregates. We characterized heparin binding peptide (HEPpep) and heparin coatings on cell surfaces and determined the specificity of these coatings in promoting assembly of MSC spheroids in dynamic culture. Overall, combining spheroids with both coatings promoted up to 70 ± 11% of spheroids to assemble into multiaggregate structures, as compared to only 10 ± 4% assembly when cells having the heparin coating were cultured with cells coated with a scrambled peptide. These results suggest that this self-assembly method represents an exciting approach that may be applicable for a wide range of applications in which cell aggregation is desired.

The Microwell-mesh: A high-throughput 3D prostate cancer spheroid and drug-testing platform

Treatment following early diagnosis of Prostate cancer (PCa) is increasingly successful, whilst the treatment of advanced and metastatic PCa remains challenging. A major limitation in the development of new therapies is the prediction of drug efficacy using in vitro models. Classic in vitro 2-dimensional (2D) cell monolayer cultures are hypersensitive to anti-cancer drugs. As a result, there has been a surge in the development of platforms that enable three dimensional (3D) cultures thought to better replicate natural physiology and better predict drug efficacy. A deficiency associated with most 3D culture systems is that their complexity reduces the number of replicates and combination therapies that can be feasibly evaluated. Herein, we describe the use of a microwell platform that utilises a nylon mesh to retain 3D micro-tumours in discrete microwells; termed the Microwell-mesh. The Microwell-mesh enables the manufacture of ~150 micro-tumours per well in a 48-well plate, and response to anti-tumour drugs can be readily quantified. Our results demonstrate that 3D micro-tumours, unlike 2D monolayers, are not hypersensitive to Docetaxel or Abiraterone Acetate, providing a superior platform for the evaluation of sequential drug treatment. In summary, the Microwell-mesh provides an efficient 3D micro-tumour platform for single and sequential drug screening.

Self-Renewal and CSCs In Vitro Enrichment: Growth as Floating Spheres

Cancer stem cells (CSC) are a vital component to the progression and reoccurrence of cancers, making them a primary target of study for both fundamental understanding of cancer biology and the development of effective and targeted treatments. CSCs reside in a complex 3D microenvironment, and the 3D spheroids are an indispensable tool in tumor biology due to their 3D structure and replication of the tumor microenvironment. Within this chapter the methodology for CSC isolation, suspension culture in hanging drop model, and characterization assays for CSC are described. First, the methodology for identifying and isolating CSCs from patient tumors, ascites, or cancer cell lines is described through the use of FACS analysis. Next, a detailed description of 3D hanging drop model for generating CSC spheroids is provided, followed by maintenance and monitoring techniques for extended 3D culture. Analysis methods are described for the quantification of CSC spheroid proliferation and viability tracking, throughout culture by on-plate alamarBlue fluorescence. Additional viability assays are described utilizing confocal microscopy with Live/Dead Viability/Cytotoxicity Kit. The characterization of CSCs populations within spheroids is described through FACS analysis. Further, an immunohistochemistry procedure is described for cell-cell and cell-matrix interaction assessment. Finally, several notes and tips for successful experiments with 3D CSC spheroids on the hanging drop model are provided. These methods are not only applicable to CSCs within a variety of tumor cell types, for not only understanding the fundamental tumor biology, but also for drug screening and development of preclinical chemotherapeutic strategies.

Honing Cell and Tissue Culture Conditions for Bone and Cartilage Tissue Engineering

An avenue of tremendous interest and need in health care encompasses the regeneration of bone and cartilage. Over the years, numerous tissue engineering strategies have contributed substantial progress toward the realization of clinically relevant therapies. Cell and tissue culture protocols, however, show many variations that make experimental results among different publications challenging to compare. This collection surveys prevalent cell sources, soluble factors, culture medium formulations, environmental factors, and genetic modification approaches in the literature. The intent of consolidating this information is to provide a starting resource for scientists considering how to optimize the parameters for cell differentiation and tissue culture procedures within the context of bone and cartilage tissue engineering.

Spheroid Coculture of Hematopoietic Stem/Progenitor Cells and Monolayer Expanded Mesenchymal Stem/Stromal Cells in Polydimethylsiloxane Microwells Modestly Improves In Vitro Hematopoietic Stem/Progenitor Cell Expansion

While two-dimensional (2D) monolayers of mesenchymal stem/stromal cells (MSCs) have been shown to enhance hematopoietic stem/progenitor cell (HSPC) expansion in vitro, expanded cells do not engraft long term in human recipients. This outcome is attributed to the failure of 2D culture to recapitulate the bone marrow (BM) niche signal milieu. Herein, we evaluated the capacity of a novel three-dimensional (3D) coculture system to support HSPC expansion in vitro. A high-throughput polydimethylsiloxane (PDMS) microwell platform was used to manufacture thousands of uniform 3D multicellular coculture spheroids. Relative gene expression in 3D spheroid versus 2D adherent BM-derived MSC cultures was characterized and compared with literature reports. We evaluated coculture spheroids, each containing 25-400 MSCs and 10 umbilical cord blood (CB)-derived CD34⁺ progenitor cells. At low exogenous cytokine concentrations, 2D and 3D MSC coculture modestly improved overall hematopoietic cell and CD34⁺ cell expansion outcomes. By contrast, a substantial increase in CD34⁺CD38^- cell yield was observed in PDMS microwell cultures, regardless of the presence or absence of MSCs. This outcome indicated that CD34⁺CD38^- cell culture yield could be increased using the microwell platform alone, even without MSC coculture support. We found that the increase in CD34⁺CD38^- cell yield observed in PDMS microwell cultures did not translate to enhanced engraftment in NOD/SCID gamma (NSG) mice or a modification in the relative human hematopoietic lineages established in engrafted mice. In summary, there was no statistical difference in CD34⁺ cell yield from 2D or 3D cocultures, and MSC coculture support provided only modest benefit in either geometry. While the high-throughput 3D microwell platform may provide a useful model system for studying cells in coculture, further optimization will be required to generate HSPC yields suitable for use in clinical applications.

Distributed vasculogenesis from modular agarose-hydroxyapatite-fibrinogen microbeads

Critical limb ischemia impairs circulation to the extremities, causing pain, disrupted wound healing, and potential tissue necrosis. Therapeutic angiogenesis seeks to repair the damaged microvasculature directly to restore blood flow. In this study, we developed modular, micro-scale constructs designed to possess robust handling qualities, allow in vitro pre-culture, and promote microvasculature formation. The microbead matrix consisted of an agarose (AG) base to prevent aggregation, combined with cell-adhesive components of fibrinogen (FGN) and/or hydroxyapatite (HA). Microbeads encapsulating a co-culture of human umbilical vein endothelial cells (HUVEC) and fibroblasts were prepared and characterized. Microbeads were generally 80-100µm in diameter, and the size increased with the addition of FGN and HA. Addition of HA increased the yield of microbeads, as well as the homogeneity of distribution of FGN within the matrix. Cell viability was high in all microbead types. When cell-seeded microbeads were embedded in fibrin hydrogels, HUVEC sprouting and inosculation between neighboring microbeads were observed over seven days. Pre-culture of microbeads for an additional seven days prior to embedding in fibrin resulted in significantly greater HUVEC network length in AG+HA+FGN microbeads, as compared to AG, AG+HA or AG+FGN microbeads. Importantly, composite microbeads resulted in more even and widespread endothelial network formation, relative to control microbeads consisting of pure fibrin. These results demonstrate that AG+HA+FGN microbeads support HUVEC sprouting both within and between adjacent microbeads, and can promote distributed vascularization of an external matrix. Such modular microtissues may have utility in treating ischemic tissue by rapidly re-establishing a microvascular network.

STATEMENT OF SIGNIFICANCE

Critical limb ischemia (CLI) is a chronic disease that can lead to tissue necrosis, amputation, and death. Cell-based therapies are being explored to restore blood flow and prevent the complications of CLI. In this study, we developed small, non-aggregating agarose-hydroxyapatite-fibrinogen microbeads that contained endothelial cells and fibroblasts. Microbeads were easy to handle and culture, and endothelial sprouts formed within and between microbeads. Our data demonstrates that the composition of the microbead matrix altered the degree of endothelial sprouting, and that the addition of hydroxyapatite and fibrinogen resulted in more distributed sprouting compared to pure fibrin microbeads. The microbead format and control of the matrix formulation may therefore be useful in developing revascularization strategies for the treatment of ischemic disease.

Development of demineralized bone matrix-based implantable and biomimetic microcarrier for stem cell expansion and single-step tissue-engineered bone graft construction

Tissue engineered bone grafts (TEBG) using mesenchymal stem cells (MSCs) demonstrate great potential for bone defect treatment.