Characterization and modulation of fibroblast/endothelial cell co-cultures for the in vitro preformation of three-dimensional tubular networks

Improved Composite Hydrogel for Bioengineered Tracheal Graft Demonstrates Effective Early Angiogenesis

Background/Objectives: Collagen-agarose hydrogel blends currently used in tracheal graft bioengineering contain relatively high concentrations of collagen to withstand mechanical stresses associated with native trachea function (e.g., breathing). Unfortunately, the high collagen content restricts effective cell infiltration into the hydrogel. In this study, we created an improved hydrogel blend with lower concentrations of collagen (<5 mg/mL) and characterized its capacity for fibroblast invasion and angiogenesis. Methods: Four collagen-agarose hydrogel blends were created: 1 mg/mL type 1 collagen (T1C) and 0.25% agarose, 1 mg/mL T1C and 0.125% agarose, 2 mg/mL T1C and 0.25% agarose, and 2 mg/mL T1C and 0.125% agarose. The hydrogel surface was seeded with fibroblasts, while both endothelial cells and fibroblasts (3:1 ratio) were mixed within the hydrogel matrix. We assessed early angiogenesis by observing fibroblast migration and endothelial cell morphology (elongation and branching) at 7 days. In addition, we performed immunostaining for alpha-smooth muscle actin (aSMA) and explored the gene expression of various angiogenic markers (including vascular endothelial growth factor; VEGF). Results: Gels with lower agarose concentrations (0.125%) with 1 or 2 mg/mL T1C were more effective in allowing early attachment and migration of surface-applied fibroblasts compared to gels with higher (0.25%) agarose concentrations. The low-agarose gels also allowed cells to quickly adopt a spread morphology and self-assemble into elongated structures indicative of early angiogenesis, while demonstrating positive immunostaining for aSMA and increased gene expression of VEGF by day 7. Conclusions: Hydrogel blends with collagen and low agarose concentrations may be effective in allowing early cellular infiltration and angiogenesis, making such gels a suitable cell substrate for use in the development of composite bioengineered tracheal grafts. The collagen-agarose hydrogel blend is meant to be cast around a three-dimensional (3D) printed polycaprolactone support structure and wrapped in porcine small intestine submucosa ECM to create an off-the-shelf bioengineered tracheal implant.

Tumoroids, a valid preclinical screening platform for monitoring cancer angiogenesis

In recent years, biologists and clinicians have witnessed prominent advances in in vitro 3D culture techniques related to biomimetic human/animal tissue analogs. Numerous data have confirmed that unicellular and multicellular (tumoroids) tumor spheroids with dense native cells in certain matrices are sensitive and valid analytical tools for drug screening, cancer cell dynamic growth, behavior, etc. in laboratory settings. Angiogenesis/vascularization is a very critical biological phenomenon to support oxygen and nutrients to tumor cells within the deep layer of solid masses. It has been shown that endothelial cell (EC)-incorporated or -free spheroid/tumoroid systems provide a relatively reliable biological platform for monitoring the formation of nascent blood vessels in micron/micrometer scales. Besides, the paracrine angiogenic activity of cells within the spheroid/tumoroid systems can be monitored after being treated with different therapeutic approaches. Here, we aimed to collect recent advances and findings related to the monitoring of cancer angiogenesis using unicellular and multicellular tumor spheroids. Vascularized spheroids/tumoroids can help us in the elucidation of mechanisms related to cancer formation, development, and metastasis by monitoring the main influencing factors.

Development of an In Vitro Biomimetic Peripheral Neurovascular Platform

Nerves and blood vessels are present in most organs and are indispensable for their function and homeostasis. Within these organs, neurovascular (NV) tissue forms congruent patterns and establishes vital interactions. Several human pathologies, including diabetes type II, produce NV disruptions with serious consequences that are complicated to study using animal models. Complex in vitro organ platforms, with neural and vascular supply, allow the investigation of such interactions, whether in a normal or pathological context, in an affordable, simple, and direct manner. To date, a few in vitro models contain NV tissue, and most strategies report models with nonbiomimetic representations of the native environment. To this end, we have established here an NV platform that contains mature vasculature and neural tissue, composed of human microvascular endothelial cells (HMVECs), induced pluripotent stem cell (iPSCs)-derived sensory neurons, and primary rat Schwann cells (SCs) within a fibrin-embedded polymeric scaffold. First, we show that SCs can induce the formation of and stabilize vascular networks to the same degree as the traditional and more thoroughly studied human dermal fibroblasts (HDFs). We also show that through SC prepatterning, we are able to control vessel orientation. Using our NV platform, we demonstrate the concomitant formation of three-dimensional neural and vascular tissue, and the influence of different medium formulations and cell types on the NV tissue outcome. Finally, we propose a protocol to form mature NV tissue, via the integration of independent neural and vascular constituents. The platform described here provides a versatile and advanced model for in vitro research of the NV axis.

Before in vivo studies: In vitro screening of sphingomyelin nanosystems using a relevant 3D multicellular pancreatic tumor spheroid model

Sphingomyelin nanosystems have already shown to be promising carriers for efficient delivery of anticancer drugs. For further application in the treatment of pancreatic tumor, the investigation on relevant in vitro models able to reproduce its physio-pathological complexity, is mandatory. Accordingly, a 3D heterotype spheroid model of pancreatic tumor has been herein constructed to investigate the potential of bare and polyethylene glycol-modified lipids nanosystems in terms of their ability to penetrate the tumor mass and deliver drugs. Regardless of their surface properties, the lipid nanosystems successfully diffused through the spheroid without inducing toxicity, showing a clear safety profile. Loading of the bare nanosystems with a lipid prodrug of gemcitabine was used to evaluate their therapeutic potential. While the nanosystems were more effective than the free drug on 2D cell monocultures, this advantage, despite their efficient penetration capacity, was lost in the 3D tumor model. The latter, being able to mimic the tumor and its microenvironment, was capable to provide a more realistic information on the cell sensitivity to treatments. These results highlight the importance of using appropriate 3D tumour models as tools for proper in vitro evaluation of nanomedicine efficacy and their timely optimisation, so as to identify the best candidates for later in vivo evaluation.

Heterotypic Multicellular Spheroids as Experimental and Preclinical Models of Sprouting Angiogenesis

Sprouting angiogenesis is the common response of live tissues to physiological and pathological angiogenic stimuli. Its accurate evaluation is of utmost importance for basic research and practical medicine and pharmacology and requires adequate experimental models. A variety of assays for angiogenesis were developed, none of them perfect. In vitro approaches are generally less physiologically relevant due to the omission of essential components regulating the process. However, only in vitro models can be entirely non-xenogeneic. The limitations of the in vitro angiogenesis assays can be partially overcome using 3D models mimicking tissue O2 and nutrient gradients, the influence of the extracellular matrix (ECM), and enabling cell-cell interactions. Here we present a review of the existing models of sprouting angiogenesis that are based on the use of endothelial cells (ECs) co-cultured with perivascular or other stromal cells. This approach provides an excellent in vitro platform for further decoding of the cellular and molecular mechanisms of sprouting angiogenesis under conditions close to the in vivo conditions, as well as for preclinical drug testing and preclinical research in tissue engineering and regenerative medicine.

One-step harvest and delivery of micropatterned cell sheets mimicking the multi-cellular microenvironment of vascularized tissue

Techniques for harvest and delivery of cell sheets have been improving for decades. However, cell sheets with complicated patterns closely related to natural tissue architecture were hardly achieved. Here, we developed an efficient method to culture and harvest cell sheets with complex shape (noted as microtissues) using temperature-responsive hydrogel consisting of expandable polyethylene oxide polymer at low temperature. Firstly, a temperature-responsive hydrogel surface with honeycomb patterns (50 and 100 µm in width) were developed through microcontact printing of polydopamine (PD). The human dermal fibroblasts (HDFBs) and human umbilical vein endothelial cells (HUVECs) spontaneously formed honeycomb-shaped microtissues on the patterned hydrogel surface. The microtissues on the hydrogel were able to be harvested and directly delivered to the desired target through thermal expansion of the hydrogel at 4 °C with an efficiency close to 80% within 10 min which is faster than conventional method based on poly(N-isopropylacrylamide). The microtissues maintained their original honeycomb network and intact structures. Honeycomb-patterned cell sheets also were fabricated through serial seeding of various cell lines, including HDFBs, HUVECs, and human adipose-derived stem cells, in which cells were attached along the honeycomb pattern. The underlying honeycomb patterns in the cell sheets were successfully maintained for 3 days, even after delivery. In addition, patterned cell sheets were successfully delivered in vivo while maintaining an intact structure for 7 days. Together, our findings demonstrate that micropatterned temperature-responsive hydrogel is an efficient method of one-step culturing and delivery of complex microtissues and should prove useful in various tissue engineering applications. STATEMENT OF SIGNIFICANCE: Scaffold-free cell delivery techniques, including cell sheet engineering, have been developed for decades. However, there is limited research regarding culture and delivery of microtissues with complex architecture mimicking natural tissue. Herein, we developed a micro-patterned hydrogel platform for the culture and delivery of honeycomb-shaped microtissues. Honeycomb patterns were chemically engineered on the temperature-responsive hydrogel through microcontact printing of polydopamine to selectively allow for human dermal fibroblast or human umbilical vein endothelial cell adhesion. They spontaneously formed honeycomb-shaped microtissues within 24 hr upon cell seeding and directly delivered to various target area including in vivo via thermal expansion of the hydrogel at 4 °C, suggesting that the micro-patterned hydrogel can be an efficient tool for culture and delivery of complex microtissue.

Spheroid Coculture of Human Gingiva-Derived Progenitor Cells With Endothelial Cells in Modified Platelet Lysate Hydrogels

Cell coculture strategies can promote angiogenesis within tissue engineering constructs. This study aimed to test the angiogenic potential of human umbilical vein endothelial cells (HUVEC) cocultured with gingiva-derived progenitor cells (GPC) as spheroids in a xeno-free environment. Human platelet lysate (HPL) was used as a cell culture supplement and as a hydrogel matrix (HPLG) for spheroid encapsulation. HUVEC and HUVEC + GPC (1:1 or 5:1) spheroids were encapsulated in various HPLG formulations. Angiogenesis was assessed via in vitro sprouting and in vivo chick chorioallantoic membrane (CAM) assays. HUVEC revealed characteristic in vitro sprouting in HPL/HPLG and this was significantly enhanced in cocultures with GPC (p < 0.05). A trend for greater sprouting was observed in 5:1 vs 1:1 HUVEC + GPC spheroids and in certain HPLG formulations (p > 0.05). Both HUVEC and HUVEC + GPC spheroids in HPLG revealed abundant and comparable neoangiogenesis in the CAM assay (p > 0.05). Spheroid coculture of HUVEC + GPC in HPLG represents a promising strategy to promote angiogenesis.

Peripheral neurovascular link: an overview of interactions and in vitro models

Nerves and blood vessels (BVs) establish extensive arborized networks to innervate tissues and deliver oxygen/metabolic support. Developmental cues direct the formation of these intricate and often overlapping patterns, which reflect close interactions within the peripheral neurovascular system. Besides the mutual dependence to survive and function, nerves and BVs share several receptors and ligands, as well as principles of differentiation, growth and pathfinding. Neurovascular (NV) interactions are maintained in adult life and are essential for certain regenerative mechanisms, such as wound healing. In pathological situations (e.g., type 2 diabetes mellitus), the NV system can be severely perturbed and become dysfunctional. Unwanted neural growth and vascularization are also associated with the progression of some pathologies, such as cancer and endometriosis. In this review, we describe the fundamental NV interactions in development, highlighting the similarities between both networks and wiring mechanisms. We also describe the NV contribution to regenerative processes and potential pathological dysfunctions. Finally, we provide an overview of current in vitro models used to replicate and investigate the NV ecosystem, addressing present limitations and future perspectives.

Microenvironmentally-driven Plasticity of CD44 isoform expression determines Engraftment and Stem-like Phenotype in CRC cell lines

Theranostic biomarkers for putative cancer stem-like cells (CSC) in colorectal cancer (CRC) are of particular interest in translational research to develop patient-individualized treatment strategies. Surface proteins still under debate are CD44 and CD133. The structural and functional diversity of these antigens, as well as their plasticity, has only just begun to be understood. Our study aimed to gain novel insight into the plasticity of CD133/CD44, thereby proving the hypothesis of marker-associated tumorigenic and non-tumorigenic phenotypes to be environmentally driven.

Methods: CD133/CD44 profiles of 20 CRC cell lines were monitored; three models with distinct surface patterns in vitro were systematically examined. CD133/CD44 subpopulations were isolated by FACS and analyzed upon in vitro growth and/or in limiting dilution engraftment studies. The experimental setup included biomarker analyses on the protein (flow cytometry, Western blotting, immunofluorescence) and mRNA levels (RT-/qPCR) as well as CD44 gene sequencing.

Results: In general, we found that (i) the in vitro CD133/CD44 pattern never determined engraftment and (ii) the CD133/CD44 population distributions harmonized under in vivo conditions. The LS1034 cell line appeared as a unique model due to its de novo in vivo presentation of CD44. CD44v8-10 was identified as main transcript, which was stronger expressed in primary human CRC than in normal colon tissues. Biomarker pattern of LS1034 cells in vivo reflected secondary engraftment: the tumorigenic potential was highest in CD133⁺/CD44⁺, intermediate in CD133⁺/CD44^- and entirely lost in CD133^-/CD44^- subfractions. Both CD44⁺ and CD44^- LS1034 cells gave rise to tumorigenic and non-tumorigenic progeny and were convertible - but only as long as they expressed CD133 in vivo. The highly tumorigenic CD133⁺/CD44(v8-10)⁺ LS1034 cells were localized in well-oxygenated perivascular but not hypoxic regions. From a multitude of putative modulators, only the direct interaction with stromal fibroblasts triggered an essential, in vivo-like enhancement of CD44v8-10 presentation in vitro.

Conclusion: Environmental conditions modulate CD133/CD44 phenotypes and tumorigenic potential of CRC subpopulations. The identification of fibroblasts as drivers of cancer-specific CD44 expression profile and plasticity sheds light on the limitation of per se dynamic surface antigens as biomarkers. It can also explain the location of putative CD133/CD44-positive CRC CSC in the perivascular niche, which is likely to comprise cancer-associated fibroblasts. The LS1034 in vitro/in vivo model is a valuable tool to unravel the mechanism of stromal-induced CD44v8-10 expression and identify further therapeutically relevant, mutual interrelations between microenvironment and tumorigenic phenotype.

Human lung microvascular endothelial cells as potential alternatives to human umbilical vein endothelial cells in bio-3D-printed trachea-like structures

BACKGROUND

We have been trying to produce scaffold-free structures for airway regeneration using a bio-3D-printer with spheroids, to avoid scaffold-associated risks such as infection. Previous studies have shown that human umbilical vein endothelial cells (HUVECs) play an important role in such structures, but HUVECs cannot be isolated from adult humans. The aim of this study was to identify alternatives to HUVECs for use in scaffold-free structures.

METHODS

Three types of structure were compared, made of chondrocytes and mesenchymal stem cells with HUVECs, human lung microvascular endothelial cells (HMVEC-Ls), and induced pluripotent stem cell (iPSC)-derived endothelial cells.

RESULTS

No significant difference in tensile strength was observed between the three groups. Histologically, some small capillary-like tube formations comprising CD31-positive cells were observed in all groups. The number and diameters of such formations were significantly lower in the iPSC-derived endothelial cell group than in other groups. Glycosaminoglycan content was significantly lower in the iPSC-derived endothelial cell group than in the HUVEC group, while no significant difference was observed between the HUVEC and HMVEC-L groups.

CONCLUSIONS

HMVEC-Ls can replace HUVECs as a cell source for scaffold-free trachea-like structures. However, some limitations were associated with iPSC-derived endothelial cells.

Fabrication of core-shell spheroids as building blocks for engineering 3D complex vascularized tissue

Cell spheroids as building blocks for engineering micro-tissue should be able to mimic the complex structure of natural tissue. However, control of the distribution of multiple cell populations within cell spheroids is difficult to achieve with current spheroid-harvest methods such as hanging-drop and with the use of microwell plates. In this study, we report the fabrication of core-shell spheroids with the ultimate goal to form 3D complex micro-tissue. We used endothelial cells and two types of stem cells (human turbinate mesenchymal stem cells (hTMSCs)/adipose-derived stem cells (ADSCs)). The stem cells and endothelial cells formed layered micro-sized cell sheets (µCSs) on polydopamine micro-patterned temperature-responsive hydrogel surfaces by a sequential seeding method, and these layered µCSs self-assembled to form core-shell spheroids by expansion of the hydrogels. The co-cultured spheroids formed a core-shell structure irrespective of stem cell type. In addition, the size of the core-shell spheroids was controlled from 90 ± 1 to 144 ± 3 µm by changing pattern sizes (200, 300, and 400 µm). The shell thickness gradually increased from 12 ± 3 to 30 ± 6 µm by adjusting the endothelial cell seeding density. Finally, we fabricated the micro-tissue by fusion of the co-cultured spheroids, and the spheroids with the core-shell structure rapidly induced in vitro vessel-like network in 3 days. Thus, the position of endothelial cells in co-cultured spheroids may be an important factor for the modulation of the vascularization process, which can be useful for the production of 3D complex micro-tissues using spheroids as building blocks. STATEMENT OF SIGNIFICANCE: This manuscript describes our work on the fabrication of core-shell spheroids as building blocks to form 3D complex vascularized micro-tissue. Stem cells (human turbinate mesenchymal stem cells (hTMSCs) or adipose-derived stem cells (ADSCs)) and endothelial cells formed layered micro-sized cell sheets (µCSs) on micro-patterned temperature-responsive hydrogel surfaces by a sequential seeding method, and these layered µCSs self-assembled to form core-shell spheroids (core - stem cells, shell - endothelial cells), irrespective of stem cell type. In addition, the size and shell thickness of the core-shell spheroids were controlled by modifying pattern size and endothelial cell seeding density. We fabricated the vascularized micro-tissue by fusion of the spheroids and demonstrated that the spheroids with a core-shell structure rapidly induced vessel-like network.

Fabrication Techniques for Vascular and Vascularized Tissue Engineering

Impaired or damaged blood vessels can occur at all levels in the hierarchy of vascular systems from large vasculatures such as arteries and veins to meso- and microvasculatures such as arterioles, venules, and capillary networks. Vascular tissue engineering has become a promising approach for fabricating small-diameter vascular grafts for occlusive arteries. Vascularized tissue engineering aims to fabricate meso- and microvasculatures for the prevascularization of engineered tissues and organs. The ideal small-diameter vascular graft is biocompatible, bridgeable, and mechanically robust to maintain patency while promoting tissue remodeling. The desirable fabricated meso- and microvasculatures should rapidly integrate with the host blood vessels and allow nutrient and waste exchange throughout the construct after implantation. A number of techniques used, including engineering-based and cell-based approaches, to fabricate these synthetic vasculatures are herein explored, as well as the techniques developed to fabricate hierarchical structures that comprise multiple levels of vasculature.

Biofabrication of thick vascularized neo-pedicle flaps for reconstructive surgery

Wound chronicity due to intrinsic and extrinsic factors perturbs adequate lesion closure and reestablishment of the protective skin barrier. Immediate and proper care of chronic wounds is necessary for a swift recovery and a reduction of patient vulnerability to infection. Advanced therapies supplemented with standard wound care procedures have been clinically implemented to restore aberrant tissue; however, these treatments are ineffective if local vasculature is too compromised to support minimally-invasive strategies. Autologous "flaps", which are tissues equipped with their own hierarchical vascular supply, can be harvested from one region of the patient and transplanted to the wound where it is reperfused upon microsurgical anastomosis to appropriate recipient vessels. Despite the success of autologous flap transfer, these procedures are extremely invasive, incur obligatory donor-site morbidity, and require sufficient donor-tissue availability, microsurgical expertise, and specialized equipment. 3D-bioprinting modalities, such as extrusion-based bioprinting, can be used to address the clinical constraints of autologous flap transfer, primarily addressing donor-site morbidity and tissue availability. This advancement in regenerative medicine allows the biofabrication of heterogeneous tissue structures with high shape fidelity and spatial resolution to generate biomimetic constructs with the anatomically-precise geometries of native tissue to ensure tissue-specific function. Yet, meaningful progress toward this clinical application has been limited by the lack of vascularization required to meet the nutrient and oxygen demands of clinically relevant tissue volumes. Thus, various criteria for the fabrication of functional tissues with hierarchical, patent vasculature must be considered when implementing 3D-bioprinting technologies for deep, chronic wounds.

Regeneration of esophagus using a scaffold-free biomimetic structure created with bio-three-dimensional printing

Various strategies have been attempted to replace esophageal defects with natural or artificial substitutes using tissue engineering. However, these methods have not yet reached clinical application because of the high risks related to their immunogenicity or insufficient biocompatibility. In this study, we developed a scaffold-free structure with a mixture of cell types using bio-three-dimensional (3D) printing technology and assessed its characteristics in vitro and in vivo after transplantation into rats. Normal human dermal fibroblasts, human esophageal smooth muscle cells, human bone marrow-derived mesenchymal stem cells, and human umbilical vein endothelial cells were purchased and used as a cell source. After the preparation of multicellular spheroids, esophageal-like tube structures were prepared by bio-3D printing. The structures were matured in a bioreactor and transplanted into 10-12-week-old F344 male rats as esophageal grafts under general anesthesia. Mechanical and histochemical assessment of the structures were performed. Among 4 types of structures evaluated, those with the larger proportion of mesenchymal stem cells tended to show greater strength and expansion on mechanical testing and highly expressed α-smooth muscle actin and vascular endothelial growth factor on immunohistochemistry. Therefore, the structure with the larger proportion of mesenchymal stem cells was selected for transplantation. The scaffold-free structures had sufficient strength for transplantation between the esophagus and stomach using silicon stents. The structures were maintained in vivo for 30 days after transplantation. Smooth muscle cells were maintained, and flat epithelium extended and covered the inner surface of the lumen. Food had also passed through the structure. These results suggested that the esophagus-like scaffold-free tubular structures created using bio-3D printing could hold promise as a substitute for the repair of esophageal defects.

Multicellular spheroid based on a triple co-culture: A novel 3D model to mimic pancreatic tumor complexity

The preclinical drug screening of pancreatic cancer treatments suffers from the absence of appropriate models capable to reproduce in vitro the heterogeneous tumor microenvironment and its stiff desmoplasia. Driven by this pressing need, we describe in this paper the conception and the characterization of a novel 3D tumor model consisting of a triple co-culture of pancreatic cancer cells (PANC-1), fibroblasts (MRC-5) and endothelial cells (HUVEC), which assembled to form a hetero-type multicellular tumor spheroid (MCTS). By histological analyses and Selective Plain Illumination Microscopy (SPIM) we have monitored the spatial distribution of each cell type and the evolution of the spheroid composition. Results revealed the presence of a core rich in fibroblasts and fibronectin in which endothelial cells were homogeneously distributed. The integration of the three cell types enabled to reproduce in vitro with fidelity the influence of the surrounding environment on the sensitivity of cancer cells to chemotherapy. To our knowledge, this is the first time that a scaffold-free pancreatic cancer spheroid model combining both tumor and multiple stromal components has been designed. It holds the possibility to become an advantageous tool for a pertinent assessment of the efficacy of various therapeutic strategies.

STATEMENT OF SIGNIFICANCE

Pancreatic tumor microenvironment is characterized by abundant fibrosis and aberrant vasculature. Aiming to reproduce in vitro these features, cancer cells have been already co-cultured with fibroblasts or endothelial cells separately but the integration of both these essential components of the pancreatic tumor microenvironment in a unique system, although urgently needed, was still missing. In this study, we successfully integrated cellular and acellular microenvironment components (i.e., fibroblasts, endothelial cells, fibronectin) in a hetero-type scaffold-free multicellular tumor spheroid. This new 3D triple co-culture model closely mimicked the resistance to treatments observed in vivo, resulting in a reduction of cancer cell sensitivity to the anticancer treatment.

Xeno-free pre-vascularized spheroids for therapeutic applications

Spheroid culture has gained increasing popularity, arising as a promising tool for regenerative medicine applications. Importantly, spheroids may present advantages over single-cell suspensions in cell-based therapies (CT). Unfortunately, most growth media used for spheroid culture contain animal origin-components, such as fetal bovine serum (FBS). The presence of FBS compromises the safety of CT and presents economic and ethical constraints. SCC (supplement for cell culture) is a novel xeno-free (XF) industrial cell culture supplement, derived from well-controlled pooled human plasma and processed under good manufacturing practice rules. Here, we developed a XF SCC-based formulation for 2D-culture of outgrowth endothelial cells (OEC), and then used it for generating co-culture spheroids of OEC and mesenchymal stem cells (MSC). XF MSC-OEC spheroids were characterized in detail and compared to spheroids cultured in FBS-supplemented medium. XF spheroids presented comparable integrity, size and morphology as the reference culture. The use of both media resulted in spheroids with similar structure, abundant extracellular matrix deposition and specific patterns of OEC distribution and organization. Notably, XF spheroids presented significantly enhanced angiogenic potential, both in vitro (fibrin sprouting assay) and in vivo (CAM assay). These findings are particularly promising in the context of potential therapeutic applications.

A three-dimensional neural spheroid model for capillary-like network formation

BACKGROUND

In vitro three-dimensional neural spheroid models have an in vivo-like cell density, and have the potential to reduce animal usage and increase experimental throughput. The aim of this study was to establish a spheroid model to study the formation of capillary-like networks in a three-dimensional environment that incorporates both neuronal and glial cell types, and does not require exogenous vasculogenic growth factors.

NEW METHOD

We created self-assembled, scaffold-free cellular spheroids using primary-derived postnatal rodent cortex as a cell source. The interactions between relevant neural cell types, basement membrane proteins, and endothelial cells were characterized by immunohistochemistry. Transmission electron microscopy was used to determine if endothelial network structures had lumens.

RESULTS

Endothelial cells within cortical spheroids assembled into capillary-like networks with lumens. Networks were surrounded by basement membrane proteins, including laminin, fibronectin and collagen IV, as well as key neurovascular cell types.

COMPARISON WITH EXISTING METHOD(S)

Existing in vitro models of the cortical neurovascular environment study monolayers of endothelial cells, either on transwell inserts or coating cellular spheroids. These models are not well suited to study vasculogenesis, a process hallmarked by endothelial cell cord formation and subsequent lumenization.

CONCLUSIONS

The neural spheroid is a new model to study the formation of endothelial cell capillary-like structures in vitro within a high cell density three-dimensional environment that contains both neuronal and glial populations. This model can be applied to investigate vascular assembly in healthy or disease states, such as stroke, traumatic brain injury, or neurodegenerative disorders.

Integrating perfusable vascular networks with a three-dimensional tissue in a microfluidic device

Creating vascular networks in tissues is crucial for tissue engineering.

Accumulation of Multipotent Progenitor Cells on Polymethylpentene Membranes During Extracorporeal Membrane Oxygenation

Multipotent progenitor cells were mobilized during pediatric extracorporeal membrane oxygenation (ECMO). We hypothesize that these cells also adhered onto polymethylpentene (PMP) fibers within the membrane oxygenator (MO) during adult ECMO support. Mononuclear cells were removed from the surface of explanted PMP-MOs (n = 16). Endothelial-like outgrowth and mesenchymal-like cells were characterized by flow cytometric analysis using different surface markers. Spindle-shaped attaching cells were identified early, but without proliferative activity. After long-term cultivation palisading type or cobblestone-type outgrowth cells with high proliferative activity appeared and were characterized as (i) leukocytoid CD45+/CD31+ (CD133+/VEGFR-II+/CD90+/CD14+/CD146dim/CD105dim); (ii) endothelial-like CD45-/CD31+ (VEGF-RII+/CD146+/CD105+/CD133-/CD14-/CD90-); and (iii) mesenchymal-like cells CD45-/CD31- (CD105+/CD90+/CD133dim/VEGFR-II-/CD146-/CD14-). The distribution of the cell populations depended on the MO and cultivation time. Endothelial-like cells formed capillary-like structures and did uptake Dil-acetylated low-density lipoprotein. Endothelial- and mesenchymal-like cells adhered on the surface of PMP-MOs. Further research is needed to identify the clinical relevance of these cells.

Co-culture of 3D tumor spheroids with fibroblasts as a model for epithelial-mesenchymal transition in vitro

Epithelial-mesenchymal transition (EMT) acts as a facilitator of metastatic dissemination in the invasive margin of malignant tumors where active tumor-stromal crosstalks take place. Co-cultures of cancer cells with cancer-associated fibroblasts (CAFs) are often used as in vitro models of EMT. We established a tumor-fibroblast proximity co-culture using HT-29 tumor spheroids (TSs) with CCD-18 co fibroblasts. When co-cultured with TSs, CCD-18 co appeared activated, and proliferative activity as well as cell migration increased. Expression of fibronectin increased whereas laminin and type I collagen decreased in TSs co-cultured with fibroblasts compared to TSs alone, closely resembling the margin of in vivo xenograft tissue. Active TGFβ1 in culture media significantly increased in TS co-cultures but not in 2D co-cultures of cancer cells-fibroblasts, indicating that 3D context-associated factors from TSs may be crucial to crosstalks between cancer cells and fibroblasts. We also observed in TSs co-cultured with fibroblasts increased expression of α-SMA, EGFR and CTGF; reduced expression of membranous β-catenin and E-cadherin, together suggesting an EMT-like changes similar to a marginal region of xenograft tissue in vivo. Overall, our in vitro TS-fibroblast proximity co-culture mimics the EMT-state of the invasive margin of in vivo tumors in early metastasis.

Vasculogenesis and Angiogenesis in Modular Collagen-Fibrin Microtissues

The process of new blood vessel formation is critical in tissue development, remodeling and regeneration. Modular tissue engineering approaches have been developed to enable the bottom-up assembly of more complex tissues, including vascular networks. In this study, collagen-fibrin composite microbeads (100-300 μm in diameter) were fabricated using a water-in-oil emulsion technique. Human endothelial cells and human fibroblasts were embedded directly in the microbead matrix at the time of fabrication. Microbead populations were characterized and cultured for 14 days either as free-floating populations or embedded in a surrounding fibrin gel. The collagen-fibrin matrix efficiently entrapped cells and supported their viability and spreading. By 7 days in culture, endothelial cell networks were evident within microbeads, and these structures became more prominent by day 14. Fibroblasts co-localized with endothelial cells, suggesting a pericyte-like function, and laminin deposition indicated maturation of the vessel networks over time. Microbeads embedded in a fibrin gel immediately after fabrication showed the emergence of cells and the coalescence of vessel structures in the surrounding matrix by day 7. By day 14, inosculation of neighboring cords and prominent vessel structures were observed. Microbeads pre-cultured for 7 days prior to embedding in fibrin gave rise to vessel networks that emanated radially from the microbead by day 7, and developed into connected networks by day 14. Lumen formation in endothelial cell networks was confirmed using confocal sectioning. These data show that collagen-fibrin composite microbeads support vascular network formation. Microbeads embedded directly after fabrication emulated the process of vasculogenesis, while the branching and joining of vessels from pre-cultured microbeads resembled angiogenesis. This modular microtissue system has utility in studying the processes involved in new vessel formation, and may be developed into a therapy for the treatment of ischemic conditions.

Effects of carbon-ion beam irradiation on the angiogenic response in lung adenocarcinoma A549 cells

Radiotherapy has been focused mainly on killing cancer cells, and little attention has been paid to the process supporting tumor growth and metastasis, including the process of angiogenesis. To investigate the effects of carbon-ion irradiation on angiogenesis in lung cancer cells, we examined the expression of vascular endothelial growth factor and basic fibroblast growth factor in the tumor conditioned medium (TCM) of A549 cells exposed to carbon-ion or X-ray irradiation, as well as endothelial cell growth, invasion, and tube formation induced by TCM. No changes in vascular endothelial growth factor secretion were detected in the TCM of A549 cells exposed to carbon-ion irradiation at 2 or 4 Gy, whereas 1 Gy of irradiation significantly decreased vascular endothelial growth factor and basic fibroblast growth factor levels. Carbon-ion irradiation at 1 Gy inhibited endothelial cell invasion and tube formation. The TCM from A549 cells irradiated with X-ray promoted angiogenesis, whereas the TCM of A549 cells exposed to carbon-ion irradiation at 2 or 4 Gy had no effect. These findings suggest that carbon-ion irradiation at 1 Gy significantly suppressed the process of angiogenesis in vitro by inhibiting endothelial cell invasion and tube formation, which are related to vascular endothelial growth factor and basic fibroblast growth factor production.

Enzymatic conjugation of a bioactive peptide into an injectable hyaluronic acid-tyramine hydrogel system to promote the formation of functional vasculature

In this study, one-step enzyme-mediated preparation of a multi-functional injectable hyaluronic-acid-based hydrogel system is reported. Hydrogel was formed through the in situ coupling of phenol moieties by horseradish peroxidase (HRP) and hydrogen peroxide (H2O2), and bioactive peptides were simultaneously conjugated into the hydrogel during the gel formation process. The preparation of this multi-functional hydrogel was made possible by synthesizing peptides containing phenols which could couple with the phenol moieties of hyaluronic-acid-tyramine (HA-Tyr) during the HRP-mediated crosslinking reaction. Preliminary studies demonstrated that two phenol moieties per molecule resulted in a consistently high degree of conjugation into the HA-Tyr hydrogel network, unlike the one modified with one phenol moiety per molecule. Therefore, an Arg-Gly-Asp (RGD) peptide bearing two phenol moieties (phenol2-poly(ethylene glycol)-RGD) was designed for conjugation to endow the HA-Tyr hydrogel with adhesion signals and enhance its bioactivities. Human umbilical vein endothelial cells (HUVECs) cultured on or within the RGD-modified hydrogels showed significantly different adhesion behavior, from non-adherence on the HA-Tyr hydrogel to strong adhesion on hydrogels modified with phenol2-poly(ethylene glycol)-RGD. This altered cell adhesion behavior led to improved cell proliferation, migration and formation of capillary-like network in the hydrogel in vitro. More importantly, when HUVECs and human fibroblasts (HFF1) were encapsulated together in the RGD-modified HA-Tyr hydrogel, functional vasculature was observed inside the cell-laden gel after 2weeks in the subcutaneous tissue. Taken together, the in situ conjugation of phenol2-poly(ethylene glycol)-RGD into HA-Tyr hydrogel system, coupled with the ease of incorporating cells, offers a simple and effective means to introduce biological signals for preparation of multi-functional injectable hydrogels for tissue engineering application.

Biomaterials in co-culture systems: towards optimizing tissue integration and cell signaling within scaffolds

Most natural tissues consist of multi-cellular systems made up of two or more cell types. However, some of these tissues may not regenerate themselves following tissue injury or disease without some form of intervention, such as from the use of tissue engineered constructs. Recent studies have increasingly used co-cultures in tissue engineering applications as these systems better model the natural tissues, both physically and biologically. This review aims to identify the challenges of using co-culture systems and to highlight different approaches with respect to the use of biomaterials in the use of such systems. The application of co-culture systems to stimulate a desired biological response and examples of studies within particular tissue engineering disciplines are summarized. A description of different analytical co-culture systems is also discussed and the role of biomaterials in the future of co-culture research are elaborated on. Understanding the complex cell-cell and cell-biomaterial interactions involved in co-culture systems will ultimately lead the field towards biomaterial concepts and designs with specific biochemical, electrical, and mechanical characteristics that are tailored towards the needs of distinct co-culture systems.

Engineering three-dimensional stem cell morphogenesis for the development of tissue models and scalable regenerative therapeutics

The physiochemical stem cell microenvironment regulates the delicate balance between self-renewal and differentiation. The three-dimensional assembly of stem cells facilitates cellular interactions that promote morphogenesis, analogous to the multicellular, heterotypic tissue organization that accompanies embryogenesis. Therefore, expansion and differentiation of stem cells as multicellular aggregates provides a controlled platform for studying the biological and engineering principles underlying spatiotemporal morphogenesis and tissue patterning. Moreover, three-dimensional stem cell cultures are amenable to translational screening applications and therapies, which underscores the broad utility of scalable suspension cultures across laboratory and clinical scales. In this review, we discuss stem cell morphogenesis in the context of fundamental biophysical principles, including the three-dimensional modulation of adhesions, mechanics, and molecular transport and highlight the opportunities to employ stem cell spheroids for tissue modeling, bioprocessing, and regenerative therapies.

Cell-based approaches to the engineering of vascularized bone tissue

This review summarizes recent efforts to create vascularized bone tissue in vitro and in vivo through the use of cell-based therapy approaches. The treatment of large and recalcitrant bone wounds is a serious clinical problem, and in the United States approximately 10% of all fractures are complicated by delayed union or non-union. Treatment approaches with the use of growth factor and gene delivery have shown some promise, but results are variable and clinical complications have arisen. Cell-based therapies offer the potential to recapitulate key components of the bone-healing cascade, which involves concomitant regeneration of vasculature and new bone tissue. For this reason, osteogenic and vasculogenic cell types have been combined in co-cultures to capitalize on the function of each cell type and to promote heterotypic interactions. Experiments in both two-dimensional and three-dimensional systems have provided insight into the mechanisms by which osteogenic and vasculogenic cells interact to form vascularized bone, and these approaches have been translated to ectopic and orthotopic models in small-animal studies. The knowledge generated by these studies will inform and facilitate the next generation of pre-clinical studies, which are needed to move cell-based orthopaedic repair strategies into the clinic. The science and application of cytotherapy for repair of large and ischemic bone defects is developing rapidly and promises to provide new treatment methods for these challenging clinical problems.

The pivotal role of vascularization in tissue engineering

Vascularization is one of the great challenges that tissue engineering faces in order to achieve sizeable tissue and organ substitutes that contain living cells. There are instances, such as skin replacement, in which a tissue-engineered substitute does not absolutely need a preexisting vascularization. However, tissue or organ substitutes in which any dimension, such as thickness, exceeds 400 μm need to be vascularized to ensure cellular survival. Consistent with the wide spectrum of approaches to tissue engineering itself, which vary from acellular synthetic biomaterials to purely biological living constructs, approaches to tissue-engineered vascularization cover numerous techniques. Those techniques range from micropatterns engineered in biomaterials to microvascular networks created by endothelial cells. In this review, we strive to provide a critical overview of the elements that must be considered in the pursuit of this goal and the major approaches that are investigated in hopes of achieving it.

Inhibition of ATP-induced calcium influx by homocysteine in human umbilical vein endothelial cells

Mechanisms involved in the association between hyperhomocysteinemia and vascular occlusive diseases remain unclear. Homocysteine (Hcy) may disturb calcium (Ca(2+) ) cytosolic regulation in endothelial cells, a process that can directly affect the synthesis of vasoactive substances, such as nitric oxide (NO). We have investigated the effect of acute and chronic incubation with high concentrations of Hcy (100 and 500 μmol/L) on the changes in the intracellular Ca(2+) concentration ([Ca(2+) ]i ) induced by ATP, using primary cultures of human umbilical vein endothelial cells (HUVEC). The changes in [Ca(2+) ]i , expressed as ΔFt /Fb , were measured using the microspectrofluorimetric technique with Fluo-3 as Ca(2+) indicator. HUVEC acutely exposed to Hcy did not produce significant effects on any of the parameters studied. However, chronic exposition (24 h) caused a significant decrease in the speed of store-mediated Ca(2+) entry, expressed as (ΔFt /Fb )/t (s(-1) ). Exposure of HUVEC to 100 and 500 µmol/L Hcy gave significantly lower values (0.019 ± 0.002 s(-1) , n = 5 and 0.021 ± 0.004 s(-1) , n = 6, respectively) compared to the controls (0.046 ± 0.004 s(-1) , n = 8, P < 0.003). This was detected only when the sustained phase of the ATP-induced [Ca(+2) ]i increase was isolated. These results demonstrate that high concentrations of Hcy can affect the mechanisms involved in [Ca(2+) ]i regulation of HUVEC, and that alteration occurs specifically in the sustained phase, which has been directly associated with NO synthesis.

In vitro differentiation of human skin-derived multipotent stromal cells into putative endothelial-like cells

Background

Multipotent stem cells have been successfully isolated from various tissues and are currently utilized for tissue-engineering and cell-based therapies. Among the many sources, skin has recently emerged as an attractive source for multipotent cells because of its abundance. Recent literature showed that skin stromal cells (SSCs) possess mesoderm lineage differentiation potential; however, the endothelial differentiation and angiogenic potential of SSC remains elusive. In our study, SSCs were isolated from human neonatal foreskin (hNFSSCs) and adult dermal skin (hADSSCs) using explants cultures and were compared with bone marrow (hMSC-TERT) and adipose tissue-derived mesenchymal stem cells (hADMSCs) for their potential differentiation into osteoblasts, adipocytes, and endothelial cells.

Results

Concordant with previous studies, both MSCs and SSCs showed similar morphology, surface protein expression, and were able to differentiate into osteoblasts and adipocytes. Using an endothelial induction culture system combined with an in vitro matrigel angiogenesis assay, hNFSSCs and hADSSCs exhibited the highest tube-forming capability, which was similar to those formed by human umbilical vein endothelial cells (HUVEC), with hNFSSCs forming the most tightly packed, longest, and largest diameter tubules among the three cell types. CD146 was highly expressed on hNFSSCs and HUVEC followed by hADSSCs, and hMSC-TERT, while its expression was almost absent on hADMSCs. Similarly, higher vascular density (based on the expression of CD31, CD34, vWF, CD146 and SMA) was observed in neonatal skin, followed by adult dermal skin and adipose tissue. Thus, our preliminary data indicated a plausible relationship between vascular densities, and the expression of CD146 on multipotent cells derived from those tissues.

Conclusions

Our data is the first to demonstrate that human dermal skin stromal cells can be differentiated into endothelial lineage. Hence, SSCs represents a novel source of stem/stromal cells for tissue regeneration and the vascularization of engineered tissues. Moreover, the CD146 investigations suggested that the microenvironmental niche might contribute to direct stromal cells multipotency toward certain lineages, which warrants further investigation.