Cell spheroid fusion: beyond liquid drops model

Generating human skeletal myoblast spheroids for vascular myogenic tissue engineering

Engineered myogenic microtissues derived from human skeletal myoblasts offer unique opportunities for varying skeletal muscle tissue engineering applications, such asin vitrodrug-testing and disease modelling. However, more complex models require the incorporation of vascular structures, which remains to be challenging. In this study, myogenic spheroids were generated using a high-throughput, non-adhesive micropatterned surface. Since monoculture spheroids containing human skeletal myoblasts were unable to remain their integrity, co-culture spheroids combining human skeletal myoblasts and human adipose-derived stem cells were created. When using the optimal ratio, uniform and viable spheroids with enhanced myogenic properties were achieved. Applying a pre-vascularization strategy, through addition of endothelial cells, resulted in the formation of spheroids containing capillary-like networks, lumina and collagen in the extracellular matrix, whilst retaining myogenicity. Moreover, sprouting of endothelial cells from the spheroids when encapsulated in fibrin was allowed. The possibility of spheroids, from different maturation stages, to assemble into a more large construct was proven by doublet fusion experiments. The relevance of using three-dimensional microtissues with tissue-specific microarchitecture and increased complexity, together with the high-throughput generation approach, makes the generated spheroids a suitable tool forin vitrodrug-testing and human disease modeling.

Mini-bones: miniaturized bone in vitro models

In bone tissue engineering (TE) and regeneration, miniaturized, (sub)millimeter-sized bone models have become a popular trend since they bring about physiological biomimicry, precise orchestration of concurrent stimuli, and compatibility with high-throughput setups and high-content imaging. They also allow efficient use of cells, reagents, materials, and energy. In this review, we describe the state of the art of miniaturized in vitro bone models, or 'mini-bones', describing these models based on their characteristics of (multi)cellularity and engineered extracellular matrix (ECM), and elaborating on miniaturization approaches and fabrication techniques. We analyze the performance of 'mini-bone' models according to their applications for studying basic bone biology or as regeneration models, disease models, and screening platforms, and provide an outlook on future trends, challenges, and opportunities.

Bioassembly of multicellular spheroids to mimic complex tissue structure using surface-modified magnetized nanofibers

Advancements in biofabrication have led to major strides toward creating authentic organ models; however, replicating intricate organ structures without scaffolds remains challenging. In this study, we introduce a method utilizing surface-modifiable magnetic nanofibers to achieve precise control over spheroid functions and geometrical features, allowing the creation of multiple functional domains within a single microtissue. We generated magnetized nanofibers by electrospinning magnetic nanoparticles dispersed in poly-L-lactic acid solution. These fibers were then coated with polydopamine (PD) to enhance their biological functions, particularly reactive oxygen species (ROS) scavenging. These PD-coated magnetic fibers (PMFs) had magnetic-responsive properties when incorporated into human dermal fibroblast spheroids (0.019 ± 0.001 emu g-1). Furthermore, PMFs within the spheroids effectively regulated ROS levels by upregulating the expression of key anti-oxidative genes such assuperoxide dismutase-1(2.2 ± 0.1) andglutathione peroxidase-1(2.6 ± 0.1). By exploiting the magnetic responsiveness of spheroids, we were able to assemble them into various structures such as linear, triangular, and square structures using remotely applied magnetic forces. Within the assembled three-dimensional constructs, the cells in spheroids incorporating PMFs demonstrated resistance to ROS regulatory activity in the presence of hydrogen peroxide, while spheroids composed of bare fibers exhibited high ROS levels. Furthermore, we assembled spheroids containing fibroblasts and endothelial cells into complex tissue structures resembling vessels under magnetic manipulation. This innovative method holds tremendous promise for organ modeling and regenerative medicine due to the unprecedented control it allows in developing microtissues that closely emulate real organs.

Scaffolded spheroids as building blocks for bottom-up cartilage tissue engineering show enhanced bioassembly dynamics

Due to the capability of cell spheroids (SPH) to assemble into large high cell density constructs, their use as building blocks attracted a lot of attention in the field of biofabrication. Nevertheless, upon maturation, the composition along with the size of such building blocks change, affecting their fusiogenic ability to form a cohesive tissue construct of controllable size. This natural phenomenon remains a limitation for the standardization of spheroid-based therapies in the clinical setting. We recently showed that scaffolded spheroids (S-SPH) can be produced by forming spheroids directly within porous PCL-based microscaffolds fabricated using multiphoton lithography (MPL). In this new study, we compare the bioassembly potential of conventional SPHs versus S-SPHs depending on their degree of maturation. Doublets of both types of building blocks were cultured and their fusiogenicity was compared by measuring the intersphere angle, the length of the fusing spheroid pairs (referred to as doublet length) as well as their spreading behaviour. Finally, the possibility to fabricate macro-sized tissue constructs (i.e. cartilage-like) from both chondrogenic S-SPHs and SPHs was analyzed. This study revealed that, in contrast to conventional SPHs, S-SPHs exhibit robust and stable fusiogenicity, independently from their degree of maturation. In order to understand this behavior, we further analyze the intersection area of doublets, looking at the kinetic of cell migration and at the mechanical stability of the formed tissue using dissection measurements. Our findings indicate that the presence of microscaffolds enhances the ability of spheroids to be used as building blocks for bottom-up tissue engineering, which is an important advantage compared to conventional spheroid-based therapy approaches. STATEMENT OF SIGNIFICANCE: The approach of using SPHs as building blocks for bottom-up tissue engineering offers a variety of advantages. At the same time the self-assembly of large tissues remains challenging due to several intrinsic properties of SPHs, such as for instance the shrinkage of tissues assembled from SPHs, or the reduced fusiogenicity commonly observed with mature SPHs. In this work, we demonstrate the capability of scaffolded spheroids (S-SPH) to fuse and recreate cartilage-like tissue constructs despite their advanced maturation stage. In this regard, the presence of microscaffolds compensates for some of the intrinsic limitations of SPHs and can help to overcome current limitations of spheroid-based tissue engineering.

Multifunctional carbon dots originated from waste garlic peel for rapid sensing of heavy metals and fluorescent imaging of 2D and 3D spheroids cultured fibroblast cells

Here, we prepared sulfur and nitrogen self-doped carbon dots derived from garlic peel extract (GPSNCDs) using a hydrothermal method. The as-synthesized GPSNCDs were confirmed using Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The analytical techniques indicate that the resulting GPSNCDs exhibit distinct emissive carbon-core with functionalities (owing to various ligands in the GPSNCDs). These functionalities are responsible for excellent hydrophilic and optical properties, including excitation-dependent emission and anti-photobleaching. Fluorescence intensities of GPSNCDs were quenched in the existence of Mn2+ and Fe3+ ions. This indicates that the GPSNCDs were sensitive to Fe3+ and Mn2+ ions with a limited range from 5 to 50 µM and showed lower recognition at ∼0.75 and 0.95 µM, respectively. In addition, the sensing results were generated in a short time (20 s). The cytotoxicity of GPSNCDs was tested to demonstrate that they are sufficiently safe to use for cellular imaging. The novel fluorescent GPSNCDs-based sensor can be used as a high-performance sensor for environmental monitoring. Further, GPSNCDs showed greater biocompatibility with normal fibroblast cells, and In Vitro fluorescent imaging of GPSNCDs revealed strong fluorescence signals in the two-dimensional (2D) and three-dimensional (3D) spheroids cultured fibroblast cells. The properties mentioned above demonstrate that the GPSNCDs can be applied to imaging normal cells without further modifications.

Incorporation of human iPSC-derived stromal cells creates a pancreatic cancer organoid with heterogeneous cancer-associated fibroblasts

The aggressiveness of pancreatic ductal adenocarcinoma (PDAC) is affected by the tumor microenvironment (TME). In this study, to recapitulate the PDAC TME ex vivo, we cocultured patient-derived PDAC cells with mesenchymal and vascular endothelial cells derived from human induced pluripotent stem cells (hiPSCs) to create a fused pancreatic cancer organoid (FPCO) in an air-liquid interface. FPCOs were further induced to resemble two distinct aspects of PDAC tissue. Quiescent FPCOs were drug resistant, likely because the TME consisted of abundant extracellular matrix proteins that were secreted from the various types of cancer-associated fibroblasts (CAFs) derived from hiPSCs. Proliferative FPCOs could re-proliferate after anticancer drug treatment, suggesting that this type of FPCO would be useful for studying PDAC recurrence. Thus, we generated PDAC organoids that recapitulate the heterogeneity of PDAC tissue and are a potential platform for screening anticancer drugs.

Recapitulating the Drifting and Fusion of Two-Generation Spheroids on Concave Agarose Microwells

Cells with various structures and proteins naturally come together to cooperate in vivo. This study used cell spheroids cultured in agarose micro-wells as a 3D model to study the movement of cells or spheroids toward other spheroids. The formation dynamics of tumor spheroids and the interactions of two batches of cells in the agarose micro-wells were studied. The results showed that a concave bottom micro-well (diameter: 2 mm, depth: 2 mm) prepared from 3% agarose could be used to study the interaction of two batches of cells. The initial tumor cell numbers from 5 × 103 cells/well to 6 × 104 cells/well all could form 3D spheroids after 3 days of incubation. Adding the second batch of DU 145 cells to the existing DU 145 spheroid resulted in the formation of satellite cell spheroids around the existing parental tumor spheroid. Complete fusion of two generation cell spheroids was observed when the parental spheroids were formed from 1 × 104 and 2 × 104 cells, and the second batch of cells was 5 × 103 per well. A higher amount of the second batch of cells (1 × 104 cell/well) led to the formation of independent satellite spheroids after 48 h of co-culture, suggesting the behavior of the second batch of cells towards existing parental spheroids depended on various factors, such as the volume of the parental spheroids and the number of the second batch cells. The interactions between the tumor spheroids and Human Umbilical Vein Endothelial Cells (HUVECs) were modeled on concave agarose micro-wells. The HUVECs (3 × 103 cell/well) were observed to gather around the parental tumor spheroids formed from 1 × 104, 2 × 104, and 3 × 104 cells per well rather than aggregate on their own to form HUVEC spheroids. This study highlights the importance of analyzing the biological properties of cells before designing experimental procedures for the sequential fusion of cell spheroids. The study further emphasizes the significant roles that cell density and the volume of the spheroids play in determining the location and movement of cells.

Engineered biomaterials to guide spheroid formation, function, and fabrication into 3D tissue constructs

Cellular spheroids are aggregates of cells that are being explored to address fundamental biological questions and as building blocks for engineered tissues. Spheroids possess distinct advantages over cellular monolayers or cell encapsulation in 3D natural and synthetic hydrogels, including direct cell-cell interactions and high cell densities, which better mimic aspects of many tissues. Despite these advantages, spheroid cultures often exhibit uncontrollable growth and may be too simplistic to mimic complex tissue structures. To address this, biomaterials are being leveraged to further expand the use of cellular spheroids for biomedical applications. In this review, we provide an overview of recent studies that utilize engineered biomaterials to guide spheroid formation and function, as well as their fabrication into tissues for use as tissue models and for therapeutic applications. First, we describe biomaterial strategies that allow the high-throughput fabrication of homogeneously-sized spheroids. Next, we summarize how engineered biomaterials are introduced into spheroid cultures either internally as microparticles or externally as hydrogel microenvironments to influence spheroid behavior (e.g., differentiation, fusion). Lastly, we discuss a variety of biofabrication strategies (e.g., 3D bioprinting, melt electrowriting) that have been used to develop macroscale tissue models and implantable constructs through the guided assembly of spheroids. Overall, the goal of this review is to provide a summary of how biomaterials are currently being engineered and leveraged to support spheroids in biomedical applications, as well as to provide a future outlook of the field. STATEMENT OF SIGNIFICANCE: Cellular spheroids are becoming increasingly used as in vitro tissue models or as 'building blocks' for tissue engineering and repair strategies. Engineered biomaterials and their processing through biofabrication approaches are being leveraged to structurally support and guide spheroid processes. This review summarizes current approaches where such biomaterials are being used to guide spheroid formation, function, and fabrication into tissue constructs. As the field is rapidly expanding, we also provide an outlook on future directions and how new engineered biomaterials can be implemented to further the development of biofabricated spheroid-based tissue constructs.

MSCs' conditioned media cytokine and growth factor profiles and their impact on macrophage polarization

BACKGROUND

There is a growing body of evidence that multipotent mesenchymal stromal cells' (MSCs') remarkable therapeutic potential is attributed not only to their differentiation and regenerative capacity, but also to the paracrine effect, underlying their immunomodulatory properties. MSCs' secretome (i.e., cytokines, growth factors, and extracellular vesicles) is therefore increasingly discussed in the context of their ability to modulate inflammatory response and promote regeneration. There is evidence that 2D or 3D culturing conditions have an impact on the cells' secretome, and here we aimed to compare the secretion of cytokines and growth factors in human MSCs from different sources cultured in 2D and 3D conditions and assess their effect on human macrophages polarization in vitro.

METHODS

MSCs were derived from human adipose tissue, bone marrow, gingiva, placenta, and umbilical cord, cultured as monolayers or as cell spheroids. Their cytokine profiles were analyzed, and data standardization was carried out using a z-score. Human peripheral blood mononuclear cells-derived macrophages were then treated with umbilical cord-derived MSCs' conditioned media and their effect on macrophages polarization was assessed.

RESULTS

Our findings suggest that umbilical cord-derived MSCs' conditioned media demonstrated the highest cytokine and growth factor levels and despite mostly pro-inflammatory cytokine profile were able to promote anti-inflammatory macrophage polarization.

CONCLUSIONS

Umbilical cord-derived MSCs' conditioned media hold great potential for therapeutic use, demonstrating significant anti-inflammatory effect on human macrophages.

Contractility-induced self-organization of smooth muscle cells: from multilayer cell sheets to dynamic three-dimensional clusters

Smooth muscle cells (SMCs) are mural cells that play a vital contractile function in many tissues. Abnormalities in SMC organization are associated with many diseases including atherosclerosis, asthma, and uterine fibroids. Various studies have reported that SMCs cultured on flat surfaces can spontaneously form three-dimensional clusters whose organization resembles that encountered in some of these pathological settings. Remarkably, how these structures form remains unknown. Here we combine in vitro experiments and physical modeling to show that three-dimensional clusters initiate when cellular contractile forces induce a hole in a flat SMC sheet, a process that can be modeled as the brittle fracture of a viscoelastic material. The subsequent evolution of the nascent cluster can be modeled as an active dewetting process with cluster shape evolution driven by a balance between cluster surface tension, arising from both cell contractility and adhesion, and cluster viscous dissipation. The description of the physical mechanisms governing the spontaneous emergence of these intriguing three-dimensional clusters may offer insight into SMC-related disorders.

Human Astrocyte Spheroids as Suitable In Vitro Screening Model to Evaluate Synthetic Cannabinoid MAM2201-Induced Effects on CNS

There is growing concern about the consumption of synthetic cannabinoids (SCs), one of the largest groups of new psychoactive substances, its consequence on human health (general population and workers), and the continuous placing of new SCs on the market. Although drug-induced alterations in neuronal function remain an essential component for theories of drug addiction, accumulating evidence indicates the important role of activated astrocytes, whose essential and pleiotropic role in brain physiology and pathology is well recognized. The study aims to clarify the mechanisms of neurotoxicity induced by one of the most potent SCs, named MAM-2201 (a naphthoyl-indole derivative), by applying a novel three-dimensional (3D) cell culture model, mimicking the physiological and biochemical properties of brain tissues better than traditional two-dimensional in vitro systems. Specifically, human astrocyte spheroids, generated from the D384 astrocyte cell line, were treated with different MAM-2201 concentrations (1-30 µM) and exposure times (24-48 h). MAM-2201 affected, in a concentration- and time-dependent manner, the cell growth and viability, size and morphological structure, E-cadherin and extracellular matrix, CB1-receptors, glial fibrillary acidic protein, and caspase-3/7 activity. The findings demonstrate MAM-2201-induced cytotoxicity to astrocyte spheroids, and support the use of this human 3D cell-based model as species-specific in vitro tool suitable for the evaluation of neurotoxicity induced by other SCs.

Building a tissue: mesenchymal and epithelial cell spheroids' mechanical properties at micro- and nanoscale

Cell transitions between the epithelial and mesenchymal phenotypes provide the regulated morphogenesis and regeneration throughout the ontogenesis. The tissue mechanics and mechanotransduction play an essential role in these processes. Cell spheroids reproduce the cell density of native tissues and represent simple building blocks for the tissue engineering purposes. The mechanical properties of mesenchymal and epithelial cells have been extensively studied in 2D monolayer cultures, but have not been sufficiently compared in spheroids. Here, we have simultaneously applied several techniques to assess the mechanical parameters of such spheroids. The local surface mechanical properties were measured by AFM, and the bulk properties were analyzed with parallel-plate compression, as well as by observing cut opening after microdissection. The comparison of the collected data allowed us to apply the model of a solid body with surface tension, and estimate the parameters of this model. We found an expectedly higher surface tension in mesenchymal spheroids, as well as a higher bulk modulus and relaxation time. The two latter parameters agree with the bulk poroelastic behavior of spheroids, and with the higher cell density and extracellular matrix content in mesenchymal spheroids. The higher tension of the surface layer cells in mesenchymal cell spheroids was also confirmed by the viscoelastic AFM characterization. The cell phenotype affected the self-organization during the spheroid formation, as well as the structure, biomechanical properties, and spreading of spheroids. The obtained results will contribute to a more detailed description of spheroid and tissue biomechanics, and will help in controlling the tissue regeneration and morphogenesis. STATEMENT OF SIGNIFICANCE: Spheroids are widely used as building blocks for scaffold-based and scaffold-free strategies in tissue engineering. In the majority of the past studies, either the concept of a solid body or a liquid with surface tension was used to describe the biomechanical behavior of spheroids. Here, we have used a model which combines both aspects, a solid body with surface tension. The "solid" aspect was described as a visco-poroelastic material, affected by the liquid redistribution through the cells and ECM at the scale of the whole spheroid. A higher surface tension was found for mesenchymal spheroids than that for epithelial spheroids, observed as a higher stiffness of the spheroid surface, as well as a larger spontaneous opening of the cut edges after microdissection.

Correlation of the regenerative potential of dermal fibroblasts in 2D culture with the biological properties of fibroblast-derived tissue spheroids

In situ 3D bioprinting is a new emerging therapeutic modality for treating human skin diseases. The tissue spheroids have been previously suggested as a powerful tool in rapidly expanding bioprinting technology. It has been demonstrated that the regenerative potential of human dermal fibroblasts could be quantitatively evaluated in 2D cell culture and confirmed after implantation in vivo. However, the development of unbiassed quantitative criteria of the regenerative potential of 3D tissue spheroids in vitro before their in situ bioprinting remains to be investigated. Here it has been demonstrated for the first time that specific correlations exist between the regenerative potential of human dermal fibroblasts cultured in vitro as 2D cell monolayer with biological properties of 3D tissue spheroids fabricated from these fibroblasts. In vitro assessment of biological properties included diameter, spreading and fusion kinetics, and biomechanical properties of 3D tissue spheroids. This comprehensive characterization could be used to predict tissue spheroids' regenerative potential in vivo.

A platform for automated and label-free monitoring of morphological features and kinetics of spheroid fusion

Spheroids are widely applied as building blocks for biofabrication of living tissues, where they exhibit spontaneous fusion toward an integrated structure upon contact. Tissue fusion is a fundamental biological process, but due to a lack of automated monitoring systems, the in-depth characterization of this process is still limited. Therefore, a quantitative high-throughput platform was developed to semi-automatically select doublet candidates and automatically monitor their fusion kinetics. Spheroids with varying degrees of chondrogenic maturation (days 1, 7, 14, and 21) were produced from two different cell pools, and their fusion kinetics were analyzed via the following steps: (1) by applying a novel spheroid seeding approach, the background noise was decreased due to the removal of cell debris while a sufficient number of doublets were still generated. (2) The doublet candidates were semi-automatically selected, thereby reducing the time and effort spent on manual selection. This was achieved by automatic detection of the microwells and building a random forest classifier, obtaining average accuracies, sensitivities, and precisions ranging from 95.0% to 97.4%, from 51.5% to 92.0%, and from 66.7% to 83.9%, respectively. (3) A software tool was developed to automatically extract morphological features such as the doublet area, roundness, contact length, and intersphere angle. For all data sets, the segmentation procedure obtained average sensitivities and precisions ranging from 96.8% to 98.1% and from 97.7% to 98.8%, respectively. Moreover, the average relative errors for the doublet area and contact length ranged from 1.23% to 2.26% and from 2.30% to 4.66%, respectively, while the average absolute errors for the doublet roundness and intersphere angle ranged from 0.0083 to 0.0135 and from 10.70 to 13.44°, respectively. (4) The data of both cell pools were analyzed, and an exponential model was used to extract kinetic parameters from the time-series data of the doublet roundness. For both cell pools, the technology was able to characterize the fusion rate and quality in an automated manner and allowed us to demonstrate that an increased chondrogenic maturity was linked with a decreased fusion rate. The platform is also applicable to other spheroid types, enabling an increased understanding of tissue fusion. Finally, our approach to study spheroid fusion over time will aid in the design of controlled fabrication of “assembloids” and bottom-up biofabrication of living tissues using spheroids.

A defined road to tracheal reconstruction: laser structuring and cell support for rapid clinic translation

One of the severe complications occurring because of the patient's intubation is tracheal stenosis. Its incidence has significantly risen because of the COVID-19 pandemic and tends only to increase. Here, we propose an alternative to the donor trachea and synthetic prostheses-the tracheal equivalent. To form it, we applied the donor trachea samples, which were decellularized, cross-linked, and treated with laser to make wells on their surface, and inoculated them with human gingiva-derived mesenchymal stromal cells. The fabricated construct was assessed in vivo using nude (immunodeficient), immunosuppressed, and normal mice and rabbits. In comparison with the matrix ones, the tracheal equivalent samples demonstrated the thinning of the capsule, the significant vessel ingrowth into surrounding tissues, and the increase in the submucosa resorption. The developed construct was shown to be highly biocompatible and efficient in trachea restoration. These results can facilitate its clinical translation and be a base to design clinical trials.

Mathematical and computational models in spheroid-based biofabrication

Ubiquitous in embryonic development, tissue fusion is of interest to tissue engineers who use tissue spheroids or organoids as building blocks of three-dimensional (3D) multicellular constructs. This review presents mathematical models and computer simulations of the fusion of tissue spheroids. The motivation of this study stems from the need to predict the post-printing evolution of 3D bioprinted constructs. First, we provide a brief overview of differential adhesion, the main morphogenetic mechanism involved in post-printing structure formation. It will be shown that clusters of cohesive cells behave as an incompressible viscous fluid on the time scale of hours. The discussion turns then to mathematical models based on the continuum hydrodynamics of highly viscous liquids and on statistical mechanics. Next, we analyze the validity and practical use of computational models of multicellular self-assembly in live constructs created by tissue spheroid bioprinting. Finally, we discuss the perspectives of the field as machine learning starts to reshape experimental design, and modular robotic workstations tend to alleviate the burden of repetitive tasks in biofabrication. STATEMENT OF SIGNIFICANCE: Bioprinted constructs are living systems, which evolve via morphogenetic mechanisms known from developmental biology. This review presents mathematical and computational tools devised for modeling post-printing structure formation. They help achieving a desirable outcome without expensive optimization experiments. While previous reviews mainly focused on assumptions, technical details, strengths, and limitations of computational models of multicellular self-assembly, this article discusses their validity and practical use in biofabrication. It also presents an overview of mathematical models that proved to be useful in the evaluation of experimental data on tissue spheroid fusion, and in the calibration of computational models. Finally, the perspectives of the field are discussed in the advent of robotic biofabrication platforms and bioprinting process optimization by machine learning.

Biomanufacturing human tissues via organ building blocks

The construction of human organs on demand remains a tantalizing vision to solve the organ donor shortage. Yet, engineering tissues that recapitulate the cellular and architectural complexity of native organs is a grand challenge. The use of organ building blocks (OBBs) composed of multicellular spheroids, organoids, and assembloids offers an important pathway for creating organ-specific tissues with the desired cellular-to-tissue-level organization. Here, we review the differentiation, maturation, and 3D assembly of OBBs into functional human tissues and, ultimately, organs for therapeutic repair and replacement. We also highlight future challenges and areas of opportunity for this nascent field.

Fusion Dynamics of Hybrid Cell-Microparticle Aggregates: A Jelly Pearl Model

We study the fusion of homogeneous cell aggregates and of hybrid aggregates combining cells and microparticles. In all cases, we find that the contact area does not vary linearly over time, as observed for liquid drops, but rather it follows a power law in t^2/3. This result is interpreted by generalizing the fusion model of soft viscoelastic solid balls to viscoelastic liquid balls, akin to jelly pearls. We also explore the asymmetric fusion between a homogeneous aggregate and a hybrid aggregate. This latter experiment allows the determination of the self-diffusion coefficient of the cells in a tissue by following the spatial distribution of internalized particles in the cells.

Spatially arranged encapsulation of stem cell spheroids within hydrogels for the regulation of spheroid fusion and cell migration

Mesenchymal stem cell spheroids have been encapsulated in hydrogels for various applications because spheroids demonstrate higher cell activity than individual cells in suspension. However, there is limited information on the effect of distance between spheroids (inter-spheroid distance) on fusion or migration in a hydrogel. In this study, we developed temperature-responsive hydrogels with surface microwell patterns to culture adipose-derived stem cell (ASC) spheroids and deliver them into a Matrigel for the investigation of the effect of inter-spheroid distance on spheroid behavior. The ASC spheroids were encapsulated successfully in a Matrigel, denoted as sandwich culture, with a specific inter-spheroid distance ranging from 100 to 400 µm. Interestingly, ASCs migrated from the host spheroid and formed a bridge-like structure between spheroids, denoted as a cellular bridge, only when the inter-spheroid distance was 200 µm. Thus, we performed a sandwich culture of human umbilical vein endothelial cells (HUVECs) and ASCs in co-cultured spheroids in the Matrigel to create a homogeneous endothelial cell network in the hydrogel. The HUVECs sprouted through the ASC cellular bridge and directly interacted with the adjacent spheroid when the inter-spheroid distance was 200 µm. Similar results were obtained from an in vivo study. Thus, our study suggests the appropriate inter-spheroid distance for effective spheroid encapsulation in a hydrogel. STATEMENT OF SIGNIFICANCE: Recently, spheroid-based 3D tissue culture techniques such as spheroid encapsulation or 3D printing are being intensively investigated for various purposes. However, there is limited research regarding the effect of the inter-spheroid distance on spheroid communication. Here, we demonstrate a spatially arranged spheroid encapsulation method within a Matrigel by using a temperature-responsive hydrogel. Human adipose-derived stem cell spheroids are encapsulated with a precisely controlled inter-spheroid distance from 100 to 400 µm and show different tendencies in cell migration and spheroid fusion. Our results suggest that the inter-spheroid distance affects spheroid communication, and thus, the inter-spheroid distance needs to be considered carefully according to the purpose.

Hybrid spheroid microscaffolds as modular tissue units to build macro-tissue assemblies for tissue engineering

Since its inception, tissue engineering and regenerative medicine (TERM) has been relying on either scaffold-based or scaffold-free strategies. Recent reports outlined the possibility of a synergistic, convergence approach, referred to as the third TERM strategy, which could alleviate bottlenecks of the two previous options. This strategy requires the fabrication of highly porous microscaffolds, allowing to create single spheroids within each of them. The resulting tissue units can then be combined and used as modular building blocks for creating tissue constructs through a bottom-up self-assembly. Such strategy can have a significant impact for the future of TERM, but so far, no reports have assessed its feasibility in detail. This work reports a first systematic study, which includes a comparison of the in vitro behavior of tissue units based on adipose derived stem cell spheroids cultured within microscaffolds versus conventional spheroids. We first proved that the presence of the microscaffold neither impairs the cells 'ability to form spheroids nor impacts their viability. Importantly, the fusiogenic and the differentiation potential (i.e. chondrogenesis and osteogenesis), which are important features for cellularized building blocks to be used in TERM, are preserved when spheroids are cultured within microscaffolds. Significant benefits of microscaffold-based tissue units include the enhanced cell retention, the decreased compaction and the better control over the size observed when larger tissue constructs are formed through self-assembly. The proof of concept study presented here demonstrates the great potential offered by those microsize tissue units to be used as building blocks for directed tissue self-assembly. STATEMENT OF SIGNIFICANCE: One of the most exciting and recent advances in tissue engineering and regenerative medicine (TERM) is to combine together multiple micro-size cellularized units, which are able to self-assemble altogether to recreate larger tissue constructs. In this work, we produce such modules by forming single spheroids within highly porous microscaffolds, and study how this new microenvironment impacts on the spheroid's behavior and stemness potential. This work highlights as well that such novel route is enabled by two-photon polymerization, which is an additive manufacturing technique offering high spatial resolution down to 100 nm. These findings provide a first scientific evidence about the utilization of hybrid spheroid microscaffold-based tissue units with great perspective as a modular tool for TERM.

Aspiration-assisted freeform bioprinting of mesenchymal stem cell spheroids within alginate microgels

Aspiration-assisted freeform bioprinting (AAfB) has emerged as a promising technique for precise placement of tissue spheroids in three-dimensional (3D) space enabling tissue fabrication. To achieve success in embedded bioprinting using AAfB, an ideal support bath should possess shear-thinning behavior and yield-stress to facilitate tight fusion and assembly of bioprinted spheroids forming tissues. Several studies have demonstrated support baths for embedded bioprinting in the past few years, yet a majority of these materials poses challenges due to their low biocompatibility, opaqueness, complex and prolonged preparation procedures, and limited spheroid fusion efficacy. In this study, to circumvent the aforementioned limitations, we present the feasibility of AAfB of human mesenchymal stem cell (hMSC) spheroids in alginate microgels as a support bath. Alginate microgels were first prepared with different particle sizes modulated by blending time and concentration, followed by determination of the optimal bioprinting conditions by the assessment of rheological properties, bioprintability, and spheroid fusion efficiency. The bioprinted and consequently self-assembled tissue structures made of hMSC spheroids were osteogenically induced for bone tissue formation. Alongside, we investigated the effects of peripheral blood monocyte-derived osteoclast incorporation into the hMSC spheroids in heterotypic bone tissue formation. We demonstrated that alginate microgels enabled unprecedented positional accuracy (∼5%), transparency for visualization, and improved fusion efficiency (∼97%) of bioprinted hMSC spheroids for bone fabrication. This study demonstrates the potential of using alginate microgels as a support bath for many different applications including but not limited to freeform bioprinting of spheroids, cell-laden hydrogels, and fugitive inks to form viable tissue constructs.

Biocompatible micro tweezers for 3D hydrogel organoid array mechanical characterization

This study presents novel biocompatible Polydimethylsiloxane (PDMS)-based micromechanical tweezers (μTweezers) capable of the stiffness characterization and manipulation of hydrogel-based organoids. The system showed great potential for complementing established mechanical characterization methods such as Atomic Force Microscopy (AFM), parallel plate compression (PPC), and nanoindentation, while significantly reducing the volume of valuable hydrogels used for testing. We achieved a volume reduction of ~0.22 μl/sample using the μTweezers vs. ~157 μl/sample using the PPC, while targeting high-throughput measurement of widely adopted micro-mesoscale (a few hundred μm-1500 μm) 3D cell cultures. The μTweezers applied and measured nano-millinewton forces through cantilever' deflection with high linearity and tunability for different applications; the assembly is compatible with typical inverted optical microscopes and fit on standard tissue culture Petri dishes, allowing mechanical compression characterization of arrayed 3D hydrogel-based organoids in a high throughput manner. The average achievable output per group was 40 tests per hour, where 20 organoids and 20 reference images in one 35 mm petri dish were tested, illustrating efficient productivity to match the increasing demand on 3D organoids' applications. The changes in stiffness of collagen I hydrogel organoids in four conditions were measured, with ovarian cancer cells (SKOV3) or without (control). The Young's modulus of the control group (Control-day 0, E = 407± 146, n = 4) measured by PPC was used as a reference modulus, where the relative elastic compressive modulus of the other groups based on the stiffness measurements was also calculated (control-day 0, E = 407 Pa), (SKOV3-day 0, E = 318 Pa), (control-day 5, E = 528 Pa), and (SKOV3-day 5, E = 376 Pa). The SKOV3-embedded hydrogel-based organoids had more shrinkage and lowered moduli on day 0 and day 5 than controls, consistently, while SKOV3 embedded organoids increased in stiffness in a similar trend to the collagen I control from day 0 to day 5. The proposed method can contribute to the biomedical, biochemical, and regenerative engineering fields, where bulk mechanical characterization is of interest. The μTweezers will also provide attractive design and application concepts to soft membrane-micro 3D robotics, sensors, and actuators.

Directed Regeneration of Osteochondral Tissue by Hierarchical Assembly of Spatially Organized Composite Spheroids

The use of engineered scaffolds or stem cells is investigated widely in the repair of injured musculoskeletal tissue. However, the combined regeneration of hierarchical osteochondral tissue remains a challenge due to delamination between cartilage and subchondral bone or difficulty in spatial control over differentiation of transplanted stem cells. Here, two types of composite spheroids are prepared using adipose-derived stem cells (hADSCs) and nanofibers coated with either transforming growth factor-β3 or bone morphogenetic growth factor-2 for chondrogenesis or osteogenesis, respectively. Each type of spheroid is then cultured within a 3D-printed microchamber in a spatially arranged manner to recapitulate the bilayer structure of osteochondral tissue. The presence of inductive factors regionally modulates in vitro chondrogenic or osteogenic differentiation of hADSCs within the biphasic construct without dedifferentiation. Furthermore, hADSCs from each spheroid proliferate and sprout and successfully connect the two layers mimicking the osteochondral interface without apertures. In vivo transplantation of the biphasic construct onto a femoral trochlear groove defect in rabbit knee joint results in 21.2 ± 2.8% subchondral bone volume/total volume and a cartilage score of 25.0 ± 3.7. The present approach can be an effective therapeutic platform to engineer complex tissue.

Multiocular organoids from human induced pluripotent stem cells displayed retinal, corneal, and retinal pigment epithelium lineages

BACKGROUND

Recently, great efforts have been made to design protocols for obtaining ocular cells from human stem cells to model diseases or for regenerative purposes. Current protocols generally focus on isolating retinal cells, retinal pigment epithelium (RPE), or corneal cells and fail to recapitulate the complexity of the tissue during eye development. Here, the generation of more advanced in vitro multiocular organoids from human induced pluripotent stem cells (hiPSCs) is demonstrated.

METHODS

A 2-step method was established to first obtain self-organized multizone ocular progenitor cells (mzOPCs) from 2D hiPSC cultures within three weeks. Then, after the cells were manually isolated and grown in suspension, 3D multiocular organoids were generated to model important cellular features of developing eyes.

RESULTS

In the 2D culture, self-formed mzOPCs spanned the neuroectoderm, surface ectoderm, neural crest, and RPE, mimicking early stages of eye development. After lifting, mzOPCs developed into different 3D multiocular organoids composed of multiple cell lineages including RPE, retina, and cornea, and interactions between the different cell types and regions of the eye system were observed. Within these organoids, the retinal regions exhibited correct layering and contained all major retinal cell subtypes as well as retinal morphological cues, whereas the corneal regions closely resembled the transparent ocular-surface epithelium and contained of corneal, limbal, and conjunctival epithelial cells. The arrangement of RPE cells also formed organoids composed of polarized pigmented epithelial cells at the surface that were completely filled with collagen matrix.

CONCLUSIONS

This approach clearly demonstrated the advantages of the combined 2D-3D construction tissue model as it provided a more ocular native-like cellular environment than that of previous models. In this complex preparations, multiocular organoids may be used to model the crosstalk between different cell types in eye development and disease.

Numerical Modelling of Multicellular Spheroid Compression: Viscoelastic Fluid vs. Viscoelastic Solid

Parallel-plate compression of multicellular spheroids (MCSs) is a promising and popular technique to quantify the viscoelastic properties of living tissues. This work presents two different approaches to the simulation of the MCS compression based on viscoelastic solid and viscoelastic fluid models. The first one is the standard linear solid model implemented in ABAQUS/CAE. The second one is the new model for 3D viscoelastic free surface fluid flow, which combines the Oldroyd-B incompressible fluid model and the incompressible neo-Hookean solid model via incorporation of an additional elastic tensor and a dynamic equation for it. The simulation results indicate that either approach can be applied to model the MCS compression with reasonable accuracy. Future application of the viscoelastic free surface fluid model is the MCSs fusion highly-demanded in bioprinting.

Upscaling of pneumatic membrane valves for the integration of 3D cell cultures on chip

Microfluidic large-scale integration (mLSI) technology enables the automation of two-dimensional (2D) cell culture processes in a highly parallel manner. Despite the wide range of biological applications of mLSI chips, manufacturing limitations of the central functional element, the pneumatic membrane valve (PMV), make the technology inaccessible for integrating tissue cultures and organoids with dimensions larger than tens of microns. In this study, we developed microtechnology processes to upscale PMVs for mLSI chips by combining 3D printing and soft lithography. Therefore, we developed a robust soft lithography protocol for the production of polydimethylsiloxane chips with PMVs from 3D-printed acrylate and wax molds. While scaled-up PMVs manufactured from acrylate-printed molds exhibited channel profiles with staircases, owing to the inherent 3D stereolithography printing process, PMVs manufactured from reflowed wax molds exhibited a semi-half-rounded channel profile. PMVs with different channel profiles showed closing pressures between 130 and 22.5 kPa, respectively. We demonstrated the functionality of the scaled-up PMVs by forming and maintaining 3D cell cultures from mouse fibroblasts (NIH3T3), human induced pluripotent stem cells (hiPSCs), and human adipose-derived adult stem cells (hASCs), with a narrow size distribution between 124 and 136 μm. Further, parallel and serial design of PMVs on an mLSI chip is used to first form and culture 3D cell cultures before fusing them within a defined flow process. Unit cell designs with upscaled PMVs enabled parallel formation, culturing, trapping, retrieval, and fusion of 3D cell cultures. Thus, the presented additive manufacturing strategy for mLSI chips will foster new developments for highly parallel 3D cell culture screening applications.

Mechanical properties of cell sheets and spheroids: the link between single cells and complex tissues

Cell aggregates, including sheets and spheroids, represent a simple yet powerful model system to study both biochemical and biophysical intercellular interactions. However, it is becoming evident that, although the mechanical properties and behavior of multicellular structures share some similarities with individual cells, yet distinct differences are observed in some principal aspects. The description of mechanical phenomena at the level of multicellular model systems is a necessary step for understanding tissue mechanics and its fundamental principles in health and disease. Both cell sheets and spheroids are used in tissue engineering, and the modulation of mechanical properties of cell constructs is a promising tool for regenerative medicine. Here, we review the data on mechanical characterization of cell sheets and spheroids, focusing both on advances in the measurement techniques and current understanding of the subject. The reviewed material suggest that interplay between the ECM, intercellular junctions, and cellular contractility determines the behavior and mechanical properties of the cell aggregates.

Organotypic 3D decellularized matrix tumor spheroids for high-throughput drug screening

Decellularized extracellular matrix (dECM) is emerging as a valuable tool for generating 3D in vitro tumor models that better recapitulate tumor-stroma interactions. However, the development of dECM-3D heterotypic microtumors exhibiting a controlled morphology is yet to be materialized. Precisely controlling microtumors morphologic features is key to avoid an inaccurate evaluation of therapeutics performance during preclinical screening. To address this, herein we employed ultra-low adhesion surfaces for bioengineering organotypic 3D metastatic breast cancer-fibroblast models enriched with dECM microfibrillar fragments, as a bottom-up strategy to include major matrix components and their associated biomolecular cues during the early stages of 3D microtissue spheroids assembly, simulating pre-existing ECM presence in the in vivo setting. This biomimetic approach enabled the self-assembly of dECM-3D tumor-stroma spheroids with tunable size and reproducible morphology. Along time, dECM enriched and stroma-rich microtumors exhibited necrotic core formation, secretion of key biomarkers and higher cancer-cell specific resistance to different chemotherapeutics in comparison to standard spheroids. Exometabolomics profiling of dECM-Spheroid in vitro models further identified important breast cancer metabolic features including glucose/pyruvate consumption and lactate excretion, which suggest an intense glycolytic activity, recapitulating major hallmarks of the native microenvironment. Such organotypic dECM-enriched microtumors overcome the morphologic variability generally associated with cell-laden dECM models, while providing a scalable testing platform that can be foreseeable leveraged for high-throughput screening of candidate therapeutics.

Modeling Liver Organogenesis by Recreating Three-Dimensional Collective Cell Migration: A Role for TGFβ Pathway

Three-dimensional (3D) collective cell migration (CCM) is critical for improving liver cell therapies, eliciting mechanisms of liver disease, and modeling human liver development and organogenesis. Mechanisms of CCM differ in 2D vs. 3D systems, and existing models are limited to 2D or transwell-based systems, suggesting there is a need for improved 3D models of CCM. To recreate liver 3D CCM, we engineered in vitro 3D models based upon a morphogenetic transition that occurs during liver organogenesis, which occurs rapidly between E8.5 and E9.5 (mouse). During this morphogenetic transition, 3D CCM exhibits co-migration (multiple cell types), thick-strand interactions with surrounding septum transversum mesenchyme (STM), branching morphogenesis, and 3D interstitial migration. Here, we engineer several 3D in vitro culture systems, each of which mimics one of these processes in vitro. In mixed spheroids bearing both liver cells and uniquely MRC-5 (fetal lung) fibroblasts, we observed evidence of co-migration, and a significant increase in length and number of liver spheroid protrusions, which was highly sensitive to transforming growth factor beta 1 (TGFβ1) stimulation. In MRC-5-conditioned medium (M-CM) experiments, we observed dose-dependent branching morphogenesis associated with an upregulation of Twist1, which was inhibited by a broad TGFβ inhibitor. In models in which liver spheroids and MRC-5 spheroids were co-cultured, we observed complex strand morphogenesis, whereby thin, linear, 3D liver cell strands attach to the MRC-5 spheroid, anchor and thicken to form permanent and thick anchoring contacts between the two spheroids. In these spheroid co-culture models, we also observed spheroid fusion and strong evidence for interstitial migration. In conclusion, we present several novel cultivation systems that recreate distinct features of liver 3D CCM. These methodologies will greatly improve our molecular, cellular, and tissue-scale understanding of liver organogenesis, liver diseases like cancer, and liver cell therapy, and will also serve as a tool to bridge conventional 2D studies and preclinical in vivo studies.