Long-Term Culturing of Undifferentiated Embryonic Stem Cells in Conditioned Media and Three-Dimensional Fibrous Matrices Without Extracellular Matrix Coating

Stochastic biological system-of-systems modelling for iPSC culture

Large-scale manufacturing of induced pluripotent stem cells (iPSCs) is essential for cell therapies and regenerative medicines. Yet, iPSCs form large cell aggregates in suspension bioreactors, resulting in insufficient nutrient supply and extra metabolic waste build-up for the cells located at the core. Since subtle changes in micro-environment can lead to a heterogeneous cell population, a novel Biological System-of-Systems (Bio-SoS) framework is proposed to model cell-to-cell interactions, spatial and metabolic heterogeneity, and cell response to micro-environmental variation. Building on stochastic metabolic reaction network, aggregation kinetics, and reaction-diffusion mechanisms, the Bio-SoS model characterizes causal interdependencies at individual cell, aggregate, and cell population levels. It has a modular design that enables data integration and improves predictions for different monolayer and aggregate culture processes. In addition, a variance decomposition analysis is derived to quantify the impact of factors (i.e., aggregate size) on cell product health and quality heterogeneity.

Dialysis based-culture medium conditioning improved the generation of human induced pluripotent stem cell derived-liver organoid in a high cell density

Human pluripotent stem cell-derived liver organoids (HLOs) have recently become a promising alternative for liver regenerative therapy. To realize this application, a large amount of human-induced pluripotent stem cells (hiPSCs) derived-liver cells are required for partial liver replacement during transplantation. This method requires stepwise induction using costly growth factors to direct the hiPSCs into the hepatic lineage. Therefore, we developed a simple dialysis-based medium conditioning that fully utilized growth factors accumulation to improve hepatic differentiation of hiPSCs at a high cell density. The results demonstrated that the dialysis culture system could accumulate the four essential growth factors required in each differentiation stage: activin A, bone morphogenetic protein 4 (BMP4), hepatocyte growth factor (HGF), and oncostatin M (OSM). As a result, this low lactate culture environment allowed high-density bipotential hepatic differentiation of up to 4.5 × 10⁷ cells/mL of human liver organoids (HLOs), consisting of hiPSC derived-hepatocyte like cells (HLCs) and cholangiocyte like-cells (CLCs). The differentiated HLOs presented a better or comparable hepatic marker and hepatobiliary physiology to the one that differentiated in suspension culture with routine daily medium replacement at a lower cell density. This simple miniaturized dialysis culture system demonstrated the feasibility of cost-effective high-density hepatic differentiation with minimum growth factor usage.

Production of homogenous size-controlled human induced pluripotent stem cell aggregates using ring-shaped culture vessel

Aggregate size is an important parameter that determines the cell fate and quality of the resulting human-induced pluripotent stem cells (hiPSCs). Nowadays, large-scale suspension culture is a common method for scaling-up the biomanufacturing of hiPSCs to realize their practical application. However, this culture system exhibits a complex hydrodynamic condition resulting from the different mixing conditions of culture media, which potentially produce non-uniform aggregates, which may decrease the quality of the cell yield. Here, we performed expansion in a ring-shaped culture vessel and compared it with three other suspension-based culture systems to evaluate the uniformity and characteristics of hiPSC aggregates. Morphologically, the hiPSC aggregates formed and expanded in the ring-shaped culture vessel, resulting in small and uniform aggregates compared to the other culture systems. This aggregate population showed a decent mass transfer required for the exchange of biochemical substances, such as nutrients, growth factors, oxygen, and waste metabolic products, inside the aggregates. Thus, better metabolic performance and pluripotency markers were achieved in this system. Interestingly, all culture systems used in this study showed different tendencies in embryoid body differentiation. The smaller aggregates produced by sphere ring and dish bag tended to differentiate toward ectodermal and mesodermal lineages, while predominantly larger aggregates from the 6-well plates and spinner flask exhibited more potential for endodermal lineage. Our study demonstrates the production of a decent homogenous aggregate population by providing equal hydrodynamic force through the ring-shaped culture vessel design, which may be further upscaled to produce a large number of hiPSCs for clinical applications.

A miniature dialysis-culture device allows high-density human-induced pluripotent stem cells expansion from growth factor accumulation

Three-dimensional aggregate-suspension culture is a potential biomanufacturing method to produce a large number of human induced pluripotent stem cells (hiPSCs); however, the use of expensive growth factors and method-induced mechanical stress potentially result in inefficient production costs and difficulties in preserving pluripotency, respectively. Here, we developed a simple, miniaturized, dual-compartment dialysis-culture device based on a conventional membrane-culture insert with deep well plates. The device improved cell expansion up to approximately ~3.2 to 4×10⁷ cells/mL. The high-density expansion was supported by reduction of excessive shear stress and agglomeration mediated by the addition of the functional polymer FP003. The results revealed accumulation of several growth factors, including fibroblast growth factor 2 and insulin, along with endogenous Nodal, which acts as a substitute for depleted transforming growth factor-β1 in maintaining pluripotency. Because we used the same growth-factor formulation per volume in the upper culture compartment, the cost reduced in inverse proportional manner with the cell density. We showed that growth-factor-accumulation dynamics in a low-shear-stress environment successfully improved hiPSC proliferation, pluripotency, and differentiation potential. This miniaturised dialysis-culture system demonstrated the feasibility of cost-effective mass production of hiPSCs in high-density culture.

3D Cell Culture Systems: Tumor Application, Advantages, and Disadvantages

The traditional two-dimensional (2D) in vitro cell culture system (on a flat support) has long been used in cancer research. However, this system cannot be fully translated into clinical trials to ideally represent physiological conditions. This culture cannot mimic the natural tumor microenvironment due to the lack of cellular communication (cell-cell) and interaction (cell-cell and cell-matrix). To overcome these limitations, three-dimensional (3D) culture systems are increasingly developed in research and have become essential for tumor research, tissue engineering, and basic biology research. 3D culture has received much attention in the field of biomedicine due to its ability to mimic tissue structure and function. The 3D matrix presents a highly dynamic framework where its components are deposited, degraded, or modified to delineate functions and provide a platform where cells attach to perform their specific functions, including adhesion, proliferation, communication, and apoptosis. So far, various types of models belong to this culture: either the culture based on natural or synthetic adherent matrices used to design 3D scaffolds as biomaterials to form a 3D matrix or based on non-adherent and/or matrix-free matrices to form the spheroids. In this review, we first summarize a comparison between 2D and 3D cultures. Then, we focus on the different components of the natural extracellular matrix that can be used as supports in 3D culture. Then we detail different types of natural supports such as matrigel, hydrogels, hard supports, and different synthetic strategies of 3D matrices such as lyophilization, electrospiding, stereolithography, microfluid by citing the advantages and disadvantages of each of them. Finally, we summarize the different methods of generating normal and tumor spheroids, citing their respective advantages and disadvantages in order to obtain an ideal 3D model (matrix) that retains the following characteristics: better biocompatibility, good mechanical properties corresponding to the tumor tissue, degradability, controllable microstructure and chemical components like the tumor tissue, favorable nutrient exchange and easy separation of the cells from the matrix.

Induced pluripotent stem cells can utilize lactate as a metabolic substrate to support proliferation

Human-induced pluripotent stem cells (iPSCs) hold the promise to improve cell-based therapies. Yet, to meet rising demands and become clinically impactful, sufficient high-quality iPSCs in quantity must be generated, a task that exceeds current capabilities. In this study, K3 iPSCs cultures were examined using parallel-labeling metabolic flux analysis (¹³ C-MFA) to quantify intracellular fluxes at relevant bioprocessing stages: glucose concentrations representative of initial media concentrations and high lactate concentrations representative of fed-batch culture conditions, prior to and after bolus glucose feeds. The glucose and lactate concentrations are also representative of concentrations that might be encountered at different locations within 3D cell aggregates. Furthermore, a novel method was developed to allow the isotopic tracer [U-¹³ C₃ ] lactate to be used in the ¹³ C-MFA model. The results indicated that high extracellular lactate concentrations decreased glucose consumption and lactate production, while glucose concentrations alone did not affect rates of aerobic glycolysis. Moreover, for the high lactate cultures, lactate was used as a metabolic substrate to support oxidative mitochondrial metabolism. These results demonstrate that iPSCs have metabolic flexibility and possess the capacity to metabolize lactate to support exponential growth, and that high lactate concentrations alone do not adversely impact iPSC proliferation.

Generation of embryonic stem cells derived from the inner cell mass of blastocysts of outbred ICR mice

Embryonic stem cells (ESCs) derived from outbred mice which share several genetic characteristics similar to humans have been requested for developing stem cell-based bioengineering techniques directly applicable to humans. Here, we report the generation of ESCs derived from the inner cell mass of blastocysts retrieved from 9-week-old female outbred ICR mice mated with 9-week-old male outbred ICR mice (_ICRESCs). Similar to those from 129/Ola mouse blastocysts (_E14ESCs), the established _ICRESCs showed inherent characteristics of ESCs except for partial and weak protein expression and activity of alkaline phosphatase. Moreover, _ICRESCs were not originated from embryonic germ cells or pluripotent cells that may co-exist in outbred ICR strain-derived mouse embryonic fibroblasts (_ICRMEFs) used for deriving colonies from inner cell mass of outbred ICR mouse blastocysts. Furthermore, instead of outbred _ICRMEFs, hybrid _B6CBAF1MEFs as feeder cells could sufficiently support in vitro maintenance of _ICRESC self-renewal. Additionally, _ICRESC-specific characteristics (self-renewal, pluripotency, and chromosomal normality) were observed in _ICRESCs cultured for 40th subpassages (164 days) on _B6CBAF1MEFs without any alterations. These results confirmed the successful establishment of ESCs derived from outbred ICR mice, and indicated that self-renewal and pluripotency of the established _ICRESCs could be maintained on _B6CBAF1MEFs in culture.

Fabrication and evaluation of modified poly(ethylene terephthalate) microfibrous scaffolds for hepatocyte growth and functionality maintenance

For hepatocyte culture in vitro, the surface feature of utilized scaffolds exerts a direct impact on cell adhesion, growth and differentiated functionality. Herein, to regulate hepatocyte growth and differentiated functionality, modified microfibrous scaffolds were fabricated by surface grafting monoamine terminated lactobionic lactone (L-NH₂) and gelatin onto non-woven poly(ethylene terephthalate) (PET) fibrous substrate (PET-Gal and PET-Gel), respectively. The physicochemical properties of PET scaffolds before and after modification were characterized. Upon 15-day culture, the effects of modified PET scaffolds on growth and differentiated functionality of human induced hepatocytes (hiHeps) were evaluated, compared with that of control without modification. Results demonstrated that both L-NH₂ and gelatin modifications improved scaffold properties including hydrophilicity, water uptake ratio, stiffness and roughness, resulting in efficient cell adhesion, ~20-fold cell expansion and enhanced differentiated functionality. After culture for 15 days, PET-Gal cultured cells formed aggregates, displaying better cell viability and significantly higher differentiated functionality regarding albumin secretion, urea synthesis, phases I (cytochrome P450, CYP1A1/2 and CYP3A4) and II (uridine 5'-diphosphate glucuronosyltransferases, UGT) enzyme activity, biliary excretion and detoxification ability (ammonia elimination and bilirubin conjugation), compared with PET and PET-Gel cultured ones. Hence, as a three-dimensional (3D) microfibrous scaffold, PET-Gal promotes hiHeps growth and differentiated functionality maintenance, which is promisingly utilized in bioartificial liver (BAL) bioreactors.

In vitro differentiation of conjunctiva mesenchymal stem cells into insulin producing cells on natural and synthetic electrospun scaffolds

Polymers are used in tissue engineering as a scaffold. In this study the differentiation capability of conjunctiva mesenchymal stem cells (CJMSCs) on natural and synthetic nanofibrous electrospun scaffolds into insulin producing cells (IPCs) were studied. Natural Silk fibroin and synthetic PLLA polymers were used to fabricate electrospun scaffolds. These scaffolds are characterized by SEM and CJMSCs were differentiated into IPCs on these scaffolds. The differentiation efficiency was measured by analysis the expression of specific pancreatic markers by RT-qPCR and insulin release capacity via ELISA. Microscopy analysis showed the fabrication of uniform nanofibers and the formation of the islet-like clusters at the end of differentiation period. Significant differences in expression of Pdx-1 and glucagon were observed in PLLA scaffold compared to Silk scaffold (Fold: 1.625 and 1.434, respectively; P-value ≤ 0.0001 for both). Furthermore, insulin secretion at high glucose concentration was significantly higher in cells differentiated on PLLA scaffold than those cultured on Silk scaffold (P-value: 0.012). The scaffolds can enhance the differentiation of IPCs from CJMSCs. In this way, PLLA synthetic scaffold was more efficient than Silk natural scaffold. We conclude that the nanofibrous scaffolds reported herein could be used as a potential supportive matrix for islet tissue engineering.

Fabrication of a Novel Culture Dish Adapter with a Small Recess Structure for Flow Control in a Closed Environment

Cell culture medium replacement is necessary to replenish nutrients and remove waste products, and perfusion and batch media exchange methods are available. The former can establish an environment similar to that in vivo, and microfluidic devices are frequently used. However, these methods are hampered by incompatibility with commercially available circular culture dishes and the difficulty in controlling liquid flow. Here, we fabricated a culture dish adapter using polydimethylsiloxane that has a small recess structure for flow control compatible with commercially available culture dishes. We designed U-shaped and I-shaped recess structure adapters and we examined the effects of groove structure on medium flow using simulation. We found that the U-shaped and I-shaped structures allowed a uniform and uneven flow of medium, respectively. We then applied these adaptors to 293T cell culture and examined the effects of recess structures on cell proliferation. As expected, cell proliferation was similar in each area of a dish in the U-shaped structure adapter, whereas in the early flow area in the I-shaped structure adapter, it was significantly higher. In summary, we succeeded in controlling liquid flow in culture dishes with the fabricated adapter, as well as in applying the modulation of culture medium flow to control cell culture.

Therapeutic effects of adipose-derived stem cells-based microtissues on erectile dysfunction in streptozotocin-induced diabetic rats

This study aimed to explore the therapeutic effects of adipose-derived stem cells (ADSCs)-based microtissues (MTs) on erectile dysfunction (ED) in streptozotocin (STZ)-induced diabetic rats. Fifty-six 8-week-old Sprague-Dawley rats received intraperitoneal injection of STZ (60 mg kg⁻¹), and 8 weeks later, the determined diabetic rats randomly received intracavernous (IC) injection of phosphate buffer solution (PBS), ADSCs, or MTs. Another eight normal rats equally got IC injection of PBS. MTs were generated with a hanging drop method, and the injected cells were tracked in ADSC- and MT-injected rats. Four weeks after the treatments, intracavernous pressure (ICP), histopathological changes in corpus cavernosum (CC), and functional proteins were measured. Rat cytokine antibody array was used to detect ADSCs or MTs lysate. The results showed that MTs expressed vascular endothelial growth factor (VEGF), nerve growth factor (NGF), and tumor necrosis factor-stimulated gene-6 (TSG-6). MTs injection had a higher retention than ADSCs injection and MTs treatment improved ICP, neuronal nitric oxide synthase (nNOS) expression, smooth muscle, and endothelial contents in diabetic rats, ameliorated local inflammation in CC better. Thus, our findings demonstrate that IC injection of MTs improves erectile function and histopathological changes in STZ-induced diabetic rats and appears to be more promising than traditional ADSCs. The underlying mechanisms involve increased cell retention accompanied with neuroprotection and anti-inflammatory behaviors of the paracrine factors.

Culture medium refinement by dialysis for the expansion of human induced pluripotent stem cells in suspension culture

Human induced pluripotent stem cells (hiPSCs) secrete essential autocrine factors that are removed along with toxic metabolites when the growth medium is exchanged daily. In this study, after determining the minimum inhibitory level of lactic acid for hiPSCs, a medium refining system was constructed by which toxic metabolites were removed from used culture medium and autocrine factors as well as other growth factors were recycled. Specifically, about 87 % of the basic fibroblast growth factor and 80 % of transforming growth factor beta 1 were retained in the refined medium after dialysis. The refined medium efficiently potentiated the proliferation of hiPS cells in adherent culture. When the refining system was used to refresh medium in suspension culture, a final cell density of (1.1 ± 0.1) × 10⁶ cells mL^-1 was obtained, with 99.5 ± 0.2 % OCT 3/4 and 78.3 ± 1.1 % TRA-1-60 expression, on day 4 of culture. These levels of expression were similar to those observed in the conventional suspension culture. With this method, culture medium refinement by dialysis was established to remove toxic metabolites, recycle autocrine factors as well as other growth factors, and reduce the use of macromolecules for the expansion of hiPSCs in suspension culture.

Real-time monitoring of specific oxygen uptake rates of embryonic stem cells in a microfluidic cell culture device

Oxygen plays a key role in stem cell biology as a signaling molecule and as an indicator of cell energy metabolism. Quantification of cellular oxygen kinetics, i.e. the determination of specific oxygen uptake rates (sOURs), is routinely used to understand metabolic shifts. However current methods to determine sOUR in adherent cell cultures rely on cell sampling, which impacts on cellular phenotype. We present real‐time monitoring of cell growth from phase contrast microscopy images, and of respiration using optical sensors for dissolved oxygen. Time‐course data for bulk and peri‐cellular oxygen concentrations obtained for Chinese hamster ovary (CHO) and mouse embryonic stem cell (mESCs) cultures successfully demonstrated this non‐invasive and label‐free approach. Additionally, we confirmed non‐invasive detection of cellular responses to rapidly changing culture conditions by exposing the cells to mitochondrial inhibiting and uncoupling agents. For the CHO and mESCs, sOUR values between 8 and 60 amol cell⁻¹ s⁻¹, and 5 and 35 amol cell⁻¹ s⁻¹ were obtained, respectively. These values compare favorably with literature data. The capability to monitor oxygen tensions, cell growth, and sOUR, of adherent stem cell cultures, non‐invasively and in real time, will be of significant benefit for future studies in stem cell biology and stem cell‐based therapies.

Advances in 3D cell culture technologies enabling tissue-like structures to be created in vitro

Research in mammalian cell biology often relies on developing in vitro models to enable the growth of cells in the laboratory to investigate a specific biological mechanism or process under different test conditions. The quality of such models and how they represent the behavior of cells in real tissues plays a critical role in the value of the data produced and how it is used. It is particularly important to recognize how the structure of a cell influences its function and how co-culture models can be used to more closely represent the structure of real tissue. In recent years, technologies have been developed to enhance the way in which researchers can grow cells and more readily create tissue-like structures. Here we identify the limitations of culturing mammalian cells by conventional methods on two-dimensional (2D) substrates and review the popular approaches currently available that enable the development of three-dimensional (3D) tissue models in vitro. There are now many ways in which the growth environment for cultured cells can be altered to encourage 3D cell growth. Approaches to 3D culture can be broadly categorized into scaffold-free or scaffold-based culture systems, with scaffolds made from either natural or synthetic materials. There is no one particular solution that currently satisfies all requirements and researchers must select the appropriate method in line with their needs. Using such technology in conjunction with other modern resources in cell biology (e.g. human stem cells) will provide new opportunities to create robust human tissue mimetics for use in basic research and drug discovery. Application of such models will contribute to advancing basic research, increasing the predictive accuracy of compounds, and reducing animal usage in biomedical science.

The importance of three-dimensional scaffold structure on stemness maintenance of mouse embryonic stem cells

Revealing the mechanisms of cell fate regulation is important for scientific research and stem cell-based therapy. The traditional two-dimensional (2D) cultured mES cells are in a very different 2D niche from the in vivo equivalent-inner cell mass (ICM). Because the cell fate decision could be regulated by many cues which could be impacted by geometry, the traditional 2D culture system would hamper us from understanding the in vivo situations correctly. Three-dimensional (3D) scaffold was believed to provide a 3D environment closed to the in vivo one. In this work, three different scaffolds were prepared for cell culture. Several characters of mES cells were changed under 3D scaffolds culture compared to 2D, and these changes were mainly due to the alteration in geometry but not the matrix. The self-renewal of mES cells was promoted by the introducing of dimensionality. The stemness maintenance of mES was supported by all three 3D scaffolds without feeder cells in the long-time culture. Our findings demonstrated that the stemness maintenance of mES cells was promoted by the 3D geometry of scaffolds and this would provide a promising platform for ES cell research.

Evaluation of biocompatibility using in vitro methods: interpretation and limitations

The in vitro biocompatibility of novel materials has to be proven before a material can be used as component of a medical device. This must be done in cell culture tests according to internationally recognized standard protocols. Subsequently, preclinical and clinical tests must be performed to verify the safety of the new material and device. The present chapter focuses on the first step, the in vitro testing according to ISO 10993-5, and critically discusses its limited significance. Alternative strategies and a brief overview of activities to improve the current in vitro tests are presented in the concluding section.

Improving embryonic stem cell expansion through the combination of perfusion and Bioprocess model design

BACKGROUND

High proliferative and differentiation capacity renders embryonic stem cells (ESCs) a promising cell source for tissue engineering and cell-based therapies. Harnessing their potential, however, requires well-designed, efficient and reproducible expansion and differentiation protocols as well as avoiding hazardous by-products, such as teratoma formation. Traditional, standard culture methodologies are fragmented and limited in their fed-batch feeding strategies that afford a sub-optimal environment for cellular metabolism. Herein, we investigate the impact of metabolic stress as a result of inefficient feeding utilizing a novel perfusion bioreactor and a mathematical model to achieve bioprocess improvement.

METHODOLOGY/PRINCIPAL FINDINGS

To characterize nutritional requirements, the expansion of undifferentiated murine ESCs (mESCs) encapsulated in hydrogels was performed in batch and perfusion cultures using bioreactors. Despite sufficient nutrient and growth factor provision, the accumulation of inhibitory metabolites resulted in the unscheduled differentiation of mESCs and a decline in their cell numbers in the batch cultures. In contrast, perfusion cultures maintained metabolite concentration below toxic levels, resulting in the robust expansion (>16-fold) of high quality 'naïve' mESCs within 4 days. A multi-scale mathematical model describing population segregated growth kinetics, metabolism and the expression of selected pluripotency ('stemness') genes was implemented to maximize information from available experimental data. A global sensitivity analysis (GSA) was employed that identified significant (6/29) model parameters and enabled model validation. Predicting the preferential propagation of undifferentiated ESCs in perfusion culture conditions demonstrates synchrony between theory and experiment.

CONCLUSIONS/SIGNIFICANCE

The limitations of batch culture highlight the importance of cellular metabolism in maintaining pluripotency, which necessitates the design of suitable ESC bioprocesses. We propose a novel investigational framework that integrates a novel perfusion culture platform (controlled metabolic conditions) with mathematical modeling (information maximization) to enhance ESC bioprocess productivity and facilitate bioprocess optimization.

Three-dimensional neural differentiation of embryonic stem cells with ACM induction in microfibrous matrices in bioreactors

The clinical use of pluripotent stem cell (PSC)-derived neural cells requires an efficient differentiation process for mass production in a bioreactor. Toward this goal, neural differentiation of murine embryonic stem cells (ESCs) in three-dimensional (3D) polyethylene terephthalate microfibrous matrices was investigated in this study. To streamline the process and provide a platform for process integration, the neural differentiation of ESCs was induced with astrocyte-conditioned medium without the formation of embryoid bodies, starting from undifferentiated ESC aggregates expanded in a suspension bioreactor. The 3D neural differentiation was able to generate a complex neural network in the matrices. When compared to 2D differentiation, 3D differentiation in microfibrous matrices resulted in a higher percentage of nestin-positive cells (68% vs. 54%) and upregulated gene expressions of nestin, Nurr1, and tyrosine hydroxylase. High purity of neural differentiation in 3D microfibrous matrix was also demonstrated in a spinner bioreactor with 74% nestin + cells. This study demonstrated the feasibility of a scalable process based on 3D differentiation in microfibrous matrices for the production of ESC-derived neural cells.

Cell-based screening of traditional Chinese medicines for proliferation enhancers of mouse embryonic stem cells

A high-throughput cell-based method was developed for screening traditional Chinese herbal medicines (TCHMs) for potential stem cell growth promoters. Mouse embryonic stem (mES) cells expressing enhanced green fluorescent protein (EGFP) were cultured in growth media supplemented with various TCHM extracts. The dosage-dependent effects of TCHM extracts on cell growth, including proliferation and cytotoxicity, were assessed via EGFP fluorescence measurement. Seven TCHMs were investigated, and among them Panax notoginseng (PN), Rhizoma Atractylodis macrocephalae, Rhizoma chuanxiong, and Ganoderma lucidum spores (GLS) showed potential to improve mES cell proliferation. Eleven mixtures of these four TCHMs were then studied, and the results showed that the mixture of PN and GLS had the strongest growth promoting effect, increasing the specific growth rate of mES cells by 29.5% at a low dosage of 0.01% (wt/vol) PN/GLS (P<0.01) and 34.2% at 0.1% (wt/vol) PN/GLS (P<0.05) compared to the control. The growth promoting effect of PN/GLS was further confirmed with ES cells cultured in spinner flasks. A 29.3-fold increase in the total cell number was achieved in the medium supplemented with 0.01% PN/GLS after 5 days, while the control culture only gave a 16.8-fold increase. This cell-based screening method thus can provide an efficient and high-throughput way to explore potential stem cell growth promoters from TCHMs.

Engineering of the embryonic and adult stem cell niches

CONTEXT

Stem cells have the potential to generate a renewable source of cells for regenerative medicine due to their ability to self-renew and differentiate to various functional cell types of the adult organism. The extracellular microenvironment plays a pivotal role in controlling stem cell fate responses. Therefore, identification of appropriate environmental stimuli that supports cellular proliferation and lineage-specific differentiation is critical for the clinical application of the stem cell therapies.

EVIDENCE ACQUISITION

Traditional methods for stem cells culture offer limited manipulation and control of the extracellular microenvironment. Micro engineering approaches are emerging as powerful tools to control stem cell-microenvironment interactions and for performing high-throughput stem cell experiments.

RESULTS

In this review, we provided an overview of the application of technologies such as surface micropatterning, microfluidics, and engineered biomaterials for directing stem cell behavior and determining the molecular cues that regulate cell fate decisions.

CONCLUSIONS

Stem cells have enormous potential for therapeutic and pharmaceutical applications, because they can give rise to various cell types. Despite their therapeutic potential, many challenges, including the lack of control of the stem cell microenvironment remain. Thus, a greater understanding of stem cell biology that can be used to expand and differentiate embryonic and adult stem cells in a directed manner offers great potential for tissue repair and regenerative medicine.

Bone scaffold architecture modulates the development of mineralized bone matrix by human embryonic stem cells

Decellularized bone has been widely used as a scaffold for bone formation, due to its similarity to the native bone matrix and excellent osteoinductive and biomechanical properties. We have previously shown that human mesenchymal and embryonic stem cells form functional bone matrix on such scaffolds, without the use of growth factors. In this study, we focused on differences in bone matrix that exist even among identical harvesting sites, and the effects of the matrix architecture and mineral content on bone formation by human embryonic stem cells (hESC). Mesenchymal progenitors derived from hESCs were cultured for 5 weeks in decellularized bone scaffolds with three different densities: low (0.281 ± 0.018 mg/mm(3)), medium (0.434 ± 0.015 mg/mm(3)) and high (0.618 ± 0.027 mg/mm(3)). The medium-density group yielded highest densities of cells and newly assembled bone matrix, presumably due to the best balance between the transport of nutrients and metabolites to and from the cells, space for cell infiltration, surface for cell attachment and the mechanical strength of the scaffolds, all of which depend on the scaffold density. Bone mineral was beneficial for the higher expression of bone markers in cultured cells and more robust accumulation of the new bone matrix.

Bioreactor strategy in bone tissue engineering: pre-culture and osteogenic differentiation under two flow configurations

Since robust osteogenic differentiation and mineralization are integral to the engineering of bone constructs, understanding the impact of the cellular microenvironments on human mesenchymal stem cell (hMSCs) osteogenic differentiation is crucial to optimize bioreactor strategy. Two perfusion flow conditions were utilized in order to understand the impact of the flow configuration on hMSC construct development during both pre-culture (PC) in growth media and its subsequent osteogenic induction (OI). The media in the in-house perfusion bioreactor was controlled to perfuse either around (termed parallel flow [PF]) the construct surfaces or penetrate through the construct (termed transverse flow [TF]) for 7 days of the PC followed by 7 days of the OI. The flow configuration during the PC not only changed growth kinetics but also influenced cell distribution and potency of osteogenic differentiation and mineralization during the subsequent OI. While shear stress resulted from the TF stimulated cell proliferation during PC, the convective removal of de novo extracellular matrix (ECM) proteins and growth factors (GFs) reduced cell proliferation on OI. In contrast, the effective retention of de novo ECM proteins and GFs in the PC constructs under the PF maintained cell proliferation under the OI but resulted in localized cell aggregations, which influenced their osteogenic differentiation. The results revealed the contrasting roles of the convective flow as a mechanical stimulus, the redistribution of the cells and macromolecules in 3D constructs, and their divergent impacts on cellular events, leading to bone construct formation. The results suggest that the modulation of the flow configuration in the perfusion bioreactor is an effective strategy that regulates the construct properties and maximizes the functional outcome.

Microfibrous substrate geometry as a critical trigger for organization, self-renewal, and differentiation of human embryonic stem cells within synthetic 3-dimensional microenvironments

Substrates used to culture human embryonic stem cells (hESCs) are typically 2-dimensional (2-D) in nature, with limited ability to recapitulate in vivo-like 3-dimensional (3-D) microenvironments. We examined critical determinants of hESC self-renewal in poly-d-lysine-pretreated synthetic polymer-based substrates with variable microgeometries, including planar 2-D films, macroporous 3-D sponges, and microfibrous 3-D fiber mats. Completely synthetic 2-D substrates and 3-D macroporous scaffolds failed to retain hESCs or support self-renewal or differentiation. However, synthetic microfibrous geometries made from electrospun polymer fibers were found to promote cell adhesion, viability, proliferation, self-renewal, and directed differentiation of hESCs in the absence of any exogenous matrix proteins. Mechanistic studies of hESC adhesion within microfibrous scaffolds indicated that enhanced cell confinement in such geometries increased cell-cell contacts and altered colony organization. Moreover, the microfibrous scaffolds also induced hESCs to deposit and organize extracellular matrix proteins like laminin such that the distribution of laminin was more closely associated with the cells than the Matrigel treatment, where the laminin remained associated with the coated fibers. The production of and binding to laminin was critical for formation of viable hESC colonies on synthetic fibrous scaffolds. Thus, synthetic substrates with specific 3-D microgeometries can support hESC colony formation, self-renewal, and directed differentiation to multiple lineages while obviating the stringent needs for complex, exogenous matrices. Similar scaffolds could serve as tools for developmental biology studies in 3-D and for stem cell differentiation in situ and transplantation using defined humanized conditions.

Biological characterization of woven fabric using two- and three-dimensional cell cultures

The integration and long-term functional retention of tissue implants are both strongly linked to the implant material characteristics. As a first approach, the cytocompatibility and bioactivity of such materials are evaluated using in vitro-based cell culture models. Typically, in vitro bioactivity is assessed by seeding single cells onto the test material to evaluate certain parameters such as cell adhesion, survival, proliferation, and functional differentiation. Probably, due to the reduction from three dimensional (3D) toward the two dimensional (2D) situation the data obtained from 2D culture models falls short of predicting the in vivo behavior of the biomaterial in question. In this study, a three dimensional (3D) in vitro cell culture model was applied to evaluate the bioactivity of well characterized fiber-based scaffolds using scaffold colonization as a bioactivity indicator. Cell behavior in this culture model was evaluated against a classical comparable, 2D cell culture system using polyethylene terephthalat and polyamide 6.6 fabrics. By using the 3D culture model, however, differences in cell population performance as a function of fiber diameter and mesh angle were evident. The use of 3D cell culture model clearly outperformed typical cell culture setup as means to evaluate cell population-scaffold interaction.

Perfusion regulation of hMSC microenvironment and osteogenic differentiation in 3D scaffold

The combination of hMSCs with 3D scaffolds has become an important approach to creating functional bone constructs. Bioreactors are important tools to mitigate mass transfer limitations and to provide controlled physiochemical and biomechanical environments for the 3D bone construct development. Media flow in the bioreactor systems is generally controlled either parallel or transverse with respect to the 3D construct, creating different cellular and biomechanical microenvironments in the 3D constructs. In this study, a custom designed modular perfusion bioreactor system was operated under either the parallel or transverse flow. The influence of the flow patterns on the characteristics of the hMSCs' cellular microenvironment and subsequent construct development was investigated. The parallel flow configuration retained ECM proteins and mitogenic growth factors within the scaffold, effectively preserving hMSC progenicity and proliferation potential (e.g., CFU-F, proliferation, and OCT-4), whereas the transverse flow induced hMSC osteogenic differentiation with higher ALP activity and calcium deposition and up-regulation of osteogenic bone markers (e.g., BMP-2, ALP, RUNX2, OSX, and OC). These results demonstrate the regulatory role of the macroscopic flow on the cellular microenvironment of the 3D hMSC construct, and suggest configuring media flow as a strategy for directing hMSC fate and 3D bone construct development in the perfusion bioreactor.

Kidney spheroids recapitulate tubular organoids leading to enhanced tubulogenic potency of human kidney-derived cells

Cell-based approaches utilizing autologous human renal cells require their isolation, expansion in vitro, and reintroduction back into the host for renal tissue regeneration. Nevertheless, human kidney epithelial cells (hKEpCs) lose their phenotype, dedifferentiate, and assume the appearance of fibroblasts after relatively few passages in culture. We hypothesized that growth conditions may influence hKEpC phenotype and function. hKEpCs retrieved from human nephrectomy tissue samples showed the ability to reproducibly form kidney spheres when grown in suspension culture developed in nonadherent conditions. Genetic labeling and time-lapse microscopy indicated, at least in part, the aggregation of hKEpCs into 3D spheroids rather than formation of pure clonally expanded spheres. Characterization of hKEpC spheroids by real-time polymerase chain reaction and FACS analysis showed upregulation of some renal developmental and "stemness" markers compared with monolayer and mostly an EpCAM(+)CD24(+)CD133(+)CD44(+) spheroid cell phenotype. Oligonucleotide microarrays, which were used to identify global transcriptional changes accompanying spheroid formation, showed predominantly upregulation of cell matrix/cell contact molecules and cellular biogenesis processes and downregulation of cell cycle, growth, and locomotion. Accordingly, hKEpC spheroids slowly proliferated as indicated by low Ki-67 staining, but when grafted in low cell numbers onto the chorioallantoic membrane (CAM) of the chick embryo, they exclusively reconstituted various renal tubular epithelia. Moreover, efficient generation of kidney spheroids was observed after long-term monolayer culture resulting in reestablishment of tubulogenic capacity upon CAM grafting. Thus, generation of a tubular organoid in hKEpC spheroids may provide a functional benefit for kidney-derived cells in vivo.

Cardiospheres recapitulate a niche-like microenvironment rich in stemness and cell-matrix interactions, rationalizing their enhanced functional potency for myocardial repair

Cardiac stem cells (CSCs) are promising candidates for use in myocardial regenerative therapy. We test the hypothesis that growing cardiac-derived cells as three-dimensional cardiospheres may recapitulate a stem cell niche-like microenvironment, favoring cell survival and enhancing functional benefit after transplantation into the injured heart. CSCs and supporting cells from human endomyocardial biopsies were grown as cardiospheres and compared with cells cultured under traditional monolayer condition or dissociated from cardiospheres. Cardiospheres self-assembled into stem cell niche-like structures in vitro in suspension culture, while exhibiting greater proportions of c-kit(+) cells and upregulated expression of SOX2 and Nanog. Pathway-focused polymerase chain reaction (PCR) array, quantitative real-time PCR, and immunostaining revealed enhanced expression of stem cell-relevant factors and adhesion/extracellular-matrix molecules (ECM) in cardiospheres including IGF-1, histone deacetylase 2 (HDAC2), Tert, integrin-α(2), laminin-β(1), and matrix metalloproteinases (MMPs). Implantation of cardiospheres in severe combined immunodeficiency (SCID) mouse hearts with acute infarction disproportionately improved cell engraftment and myocardial function, relative to monolayer-cultured cells. Dissociation of cardiospheres into single cells decreased the expression of ECM and adhesion molecules and undermined resistance to oxidative stress, negating the improved cell engraftment and functional benefit in vivo. Growth of cardiac-derived cells as cardiospheres mimics stem cell niche properties with enhanced "stemness" and expression of ECM and adhesion molecules. These changes underlie an increase in cell survival and more potent augmentation of global function following implantation into the infarcted heart.

Three-dimensional culture systems for the expansion of pluripotent embryonic stem cells

Mouse embryonic stem cell (ESC) lines, and more recently human ESC lines, have become valuable tools for studying early mammalian development. Increasing interest in ESCs and their differentiated progeny in drug discovery and as potential therapeutic agents has highlighted the fact that current two-dimensional (2D) static culturing techniques are inadequate for large-scale production. The culture of mammalian cells in three-dimensional (3D) agitated systems has been shown to overcome many of the restrictions of 2D and is therefore likely to be effective for ESC proliferation. Using murine ESCs as our initial model, we investigated the effectiveness of different 3D culture environments for the expansion of pluripotent ESCs. Solohill Collagen, Solohill FACT, and Cultispher-S microcarriers were employed and used in conjunction with stirred bioreactors. Initial seeding parameters, including cell number and agitation conditions, were found to be critical in promoting attachment to microcarriers and minimizing the size of aggregates formed. While all microcarriers supported the growth of undifferentiated mESCs, Cultispher-S out-performed the Solohill microcarriers. When cultured for successive passages on Cultispher-S microcarriers, mESCs maintained their pluripotency, demonstrated by self-renewal, expression of pluripotency markers and the ability to undergo multi-lineage differentiation. When these optimized conditions were applied to unweaned human ESCs, Cultispher-S microcarriers supported the growth of hESCs that retained expression of pluripotency markers including SSEA4, Tra-1–60, NANOG, and OCT-4. Our study highlights the importance of optimization of initial seeding parameters and provides proof-of-concept data demonstrating the utility of microcarriers and bioreactors for the expansion of hESCs. Biotechnol. Bioeng. 2010;107:683–695. © 2010 Wiley Periodicals, Inc.

High-throughput 3-D cell-based proliferation and cytotoxicity assays for drug screening and bioprocess development

We have designed, built and tested a three-dimensional (3-D) cell culture system on modified microplates for high-throughput, real-time, proliferation and cytotoxicity assays. In this 3-D culture system, cells expressing the enhanced green fluorescent protein (EGFP) were cultured in nonwoven polyethylene terephthalate (PET) fibrous scaffolds. Compared to 2-D cultures in conventional microplates, 3-D cultures gave more than 10-fold higher fluorescence signals with significantly increased signal-to-noise ratio (SNR), thus extending the application of conventional fluorescence microplate readers for online monitoring of culture fluorescence. The 3-D system was successfully used to demonstrate the effects of fetal bovine serum, fibronectin coating of PET fibers, and cytotoxicity of dexamethasone on recombinant murine embryonic stem D3 cells. The dosage effects of 5-fluorouracil and gemcitabine on high-density colon cancer HT-29 cells were also tested. These studies demonstrated that the 3-D culture microplate system with EGFP expressing cells can be used as a high-throughput system in drug discovery and bioprocess development.

Biocompatibility of synthetic poly(ester urethane)/polyhedral oligomeric silsesquioxane matrices with embryonic stem cell proliferation and differentiation

Incorporation of polyhedral oligomeric silsesquioxanes (POSS) into poly(ester urethanes) (PEU) as a building block results in a PEU/POSS hybrid polymer with increased mechanical strength and thermostability. An attractive feature of the new polymer is that it forms a porous matrix when cast in the form of a thin film, making it potentially useful in tissue engineering. In this study, we present detailed microscopic analysis of the PEU/POSS matrix and demonstrate its biocompatibility with cell culture. The PEU/POSS polymer forms a continuous porous matrix with open pores and interconnected grooves. From SEM image analysis, it is calculated that there are about 950 pores/mm(2) of the matrix area with pore diameter size in the range 1-15 µm. The area occupied by the pores represents approximately 7.6% of the matrix area. Using mouse embryonic stem cells (ESCs), we demonstrate that the PEU/POSS matrix provides excellent support for cell proliferation and differentiation. Under the cell culture condition optimized to maintain self-renewal, ESCs grown on a PEU/POSS matrix exhibit undifferentiated morphology, express pluripotency markers and have a similar growth rate to cells grown on gelatin. When induced for differentiation, ESCs underwent dramatic morphological change, characterized by the loss of clonogenecity and increased cell size, with well-expanded cytoskeleton networks. Differentiated cells are able to form a continuous monolayer that is closely embedded in the matrix. The excellent compatibility between the PEU/POSS matrix and ESC proliferation/differentiation demonstrates the potential of using PEU/POSS polymers in future ESC-based tissue engineering.

Maintenance of pluripotency in mouse embryonic stem cells cultivated in stirred microcarrier cultures

The development of efficient and reproducible culture systems for embryonic stem (ES) cells is an essential pre-requisite for regenerative medicine. Culture scale-up ensuring maintenance of cell pluripotency is a central issue, because large amounts of pluripotent cells must be generated to warrant that differentiated cells deriving thereof are transplanted in great amounts and survive the procedure. This study aimed to develop a robust scalable cell expansion system, using a murine embryonic stem cell line that is feeder-dependent and adapted to serum-free medium, thus representing a more realistic model for human ES cells. We showed that high concentrations of murine ES cells can be obtained in stirred microcarrier-based spinner cultures, with a 10-fold concentration of cells per volume of medium and a 5-fold greater cell concentration per surface area, as compared to static cultures. No differences in terms of pluripotency and differentiation capability were observed between cells grown in traditional static systems and cells that were replated onto the traditional system after being expanded on microcarriers in the stirred system. This was verified by morphological analyses, quantification of cells expressing important pluripotency markers (Oct-4, SSEA-1, and SOX2), karyotype profile, and the ability to form embryoid bodies with similar sizes, and maintaining their intrinsic ability to differentiate into all three germ layers.

Centrifugal seeding of mammalian cells in nonwoven fibrous matrices

Three-dimensional (3D) cell cultures have many advantages over two-dimensional cultures. However, seeding cells in 3D scaffolds such as nonwoven fibrous polyethylene terephthalate (PET) matrices has been a challenge task in tissue engineering and cell culture bioprocessing. In this study, a centrifugal seeding method was investigated to improve the cell seeding efficiency in PET matrices with two different porosities (93% and 88%). Both the centrifugal force and centrifugation time were found to affect the seeding efficiency. With an appropriate centrifugation speed, a high 80-90% cell seeding efficiency was achieved and the time to reach this high seeding efficiency was less than 5 min. The seeding efficiency was similar for matrices with different porosities, although the optimal seeding time was significantly shorter for the low-porosity scaffold. Post seeding cell viability was demonstrated by culturing colon cancer cells seeded in PET matrices for over 5 days. The centrifugal seeding method developed in this work can be used to efficiently and uniformly seed small fibrous scaffolds for applications in 3D cell-based assays for high-throughput screening.

Endothelial progenitors encapsulated in bioartificial niches are insulated from systemic cytotoxicity and are angiogenesis competent

Intrinsic stem cells (SC) participate in tissue remodeling and regeneration in various diseases and following toxic insults. Failure of tissue regeneration is in part attributed to lack of SC protection from toxic stress of noxious stimuli, thus prompting intense research efforts to develop strategies for SC protection and functional preservation for in vivo delivery. One strategy is creation of artificial SC niches in an attempt to mimic the requirements of endogenous SC niches by generating scaffolds with properties of extracellular matrix. Here, we investigated the use of hyaluronic acid (HA) hydrogels as an artificial SC niche and examined regenerative capabilities of encapsulated embryonic endothelial progenitor cells (eEPC) in three different in vivo models. Hydrogel-encapsulated eEPC demonstrated improved resistance to toxic insult (adriamycin) in vitro, thus prompting in vivo studies. Implantation of HA hydrogels containing eEPC to mice with adriamycin nephropathy or renal ischemia resulted in eEPC mobilization to injured kidneys (and to a lesser extent to the spleen) and improvement of renal function, which was equal or superior to adoptively transferred EPC by intravenous infusion. In mice with hindlimb ischemia, EPC encapsulated in HA hydrogels dramatically accelerated the recovery of collateral circulation with the efficacy superior to intravenous infusion of EPC. In conclusion, HA hydrogels protect eEPC against adriamycin cytotoxicity and implantation of eEPC encapsulated in HA hydrogels supports renal regeneration in ischemic and cytotoxic (adriamycin) nephropathy and neovascularization of ischemic hindlimb, thus establishing their functional competence and superior capabilities to deliver stem cells stored in and released from this bioartificial niche.

Role of bioinspired polymers in determination of pluripotent stem cell fate

Human pluripotent stem cells, including embryonic and induced pluripotent stem cells, hold enormous potential for the treatment of many diseases, owing to their ability to generate cell types useful for therapeutic applications. Currently, many stem cell culture propagation and differentiation systems incorporate animal-derived components for promoting self-renewal and differentiation. However, use of these components is labor intensive, carries the risk of xenogeneic contamination and yields compromised experimental results that are difficult to duplicate. From a biomaterials perspective, the generation of an animal- and cell-free biomimetic microenvironment that provides the appropriate physical and chemical cues for stem cell self-renewal or differentiation into specialized cell types would be ideal. This review presents the use of natural and synthetic polymers that support propagation and differentiation of stem cells, in an attempt to obtain a clear understanding of the factors responsible for the determination of stem cell fate.