In vivo formation of complex microvessels lined by human endothelial cells in an immunodeficient mouse

Breathing new life into tissue engineering: exploring cutting-edge vascularization strategies for skin substitutes

Tissue-engineered skin substitutes (TESS) emerged as a new therapeutic option to improve skin transplantation. However, establishing an adequate and rapid vascularization in TESS is a critical factor for their clinical application and successful engraftment in patients. Therefore, several methods have been applied to improve the vascularization of skin substitutes including (i) modifying the structural and physicochemical properties of dermal scaffolds; (ii) activating biological scaffolds with growth factor-releasing systems or gene vectors; and (iii) developing prevascularized skin substitutes by loading scaffolds with capillary-forming cells. This review provides a detailed overview of the most recent and important developments in the vascularization strategies for skin substitutes. On the one hand, we present cell-based approaches using stem cells, microvascular fragments, adipose tissue derived stromal vascular fraction, endothelial cells derived from blood and skin as well as other pro-angiogenic stimulation methods. On the other hand, we discuss how distinct 3D bioprinting techniques and microfluidics, miRNA manipulation, cell sheet engineering and photosynthetic scaffolds like GelMA, can enhance skin vascularization for clinical applications. Finally, we summarize and discuss the challenges and prospects of the currently available vascularization techniques that may serve as a steppingstone to a mainstream application of skin tissue engineering.

Biofabrication of engineered blood vessels for biomedical applications

To successfully engineer large-sized tissues, establishing vascular structures is essential for providing oxygen, nutrients, growth factors and cells to prevent necrosis at the core of the tissue. The diameter scale of the biofabricated vasculatures should range from 100 to 1,000 µm to support the mm-size tissue while being controllably aligned and spaced within the diffusion limit of oxygen. In this review, insights regarding biofabrication considerations and techniques for engineered blood vessels will be presented. Initially, polymers of natural and synthetic origins can be selected, modified, and combined with each other to support maturation of vascular tissue while also being biocompatible. After they are shaped into scaffold structures by different fabrication techniques, surface properties such as physical topography, stiffness, and surface chemistry play a major role in the endothelialization process after transplantation. Furthermore, biological cues such as growth factors (GFs) and endothelial cells (ECs) can be incorporated into the fabricated structures. As variously reported, fabrication techniques, especially 3D printing by extrusion and 3D printing by photopolymerization, allow the construction of vessels at a high resolution with diameters in the desired range. Strategies to fabricate of stable tubular structures with defined channels will also be discussed. This paper provides an overview of the many advances in blood vessel engineering and combinations of different fabrication techniques up to the present time.

Highly Parallel Tissue Grafting for Combinatorial In Vivo Screening

Material- and cell-based technologies such as engineered tissues hold great promise as human therapies. Yet, the development of many of these technologies becomes stalled at the stage of pre-clinical animal studies due to the tedious and low-throughput nature of in vivo implantation experiments. We introduce a 'plug and play' in vivo screening array platform called Highly Parallel Tissue Grafting (HPTG). HPTG enables parallelized in vivo screening of 43 three-dimensional microtissues within a single 3D printed device. Using HPTG, we screen microtissue formations with varying cellular and material components and identify formulations that support vascular self-assembly, integration and tissue function. Our studies highlight the importance of combinatorial studies that vary cellular and material formulation variables concomitantly, by revealing that inclusion of stromal cells can "rescue" vascular self-assembly in manner that is material-dependent. HPTG provides a route for accelerating pre-clinical progress for diverse medical applications including tissue therapy, cancer biomedicine, and regenerative medicine.

Realizations of vascularized tissues: From in vitro platforms to in vivo grafts

Vascularization is essential for realizing thick and functional tissue constructs that can be utilized for in vitro study platforms and in vivo grafts. The vasculature enables the transport of nutrients, oxygen, and wastes and is also indispensable to organ functional units such as the nephron filtration unit, the blood-air barrier, and the blood-brain barrier. This review aims to discuss the latest progress of organ-like vascularized constructs with specific functionalities and realizations even though they are not yet ready to be used as organ substitutes. First, the human vascular system is briefly introduced and related design considerations for engineering vascularized tissues are discussed. Second, up-to-date creation technologies for vascularized tissues are summarized and classified into the engineering and cellular self-assembly approaches. Third, recent applications ranging from in vitro tissue models, including generic vessel models, tumor models, and different human organ models such as heart, kidneys, liver, lungs, and brain, to prevascularized in vivo grafts for implantation and anastomosis are discussed in detail. The specific design considerations for the aforementioned applications are summarized and future perspectives regarding future clinical applications and commercialization are provided.

Engineered tissue vascularization and engraftment depends on host model

Developing vascular networks that integrate with the host circulation and support cells engrafted within engineered tissues remains a key challenge in tissue engineering. Most previous work in this field has focused on developing new methods to build human vascular networks within engineered tissues prior to their implant in vivo, with substantively less attention paid to the role of the host in tissue vascularization and engraftment. Here, we assessed the role that different host animal models and anatomic implant locations play in vascularization and cardiomyocyte survival within engineered tissues. We found major differences in the formation of graft-derived blood vessels and survival of cardiomyocytes after implantation of identical tissues in immunodeficient athymic nude mice versus rats. Athymic mice supported robust guided vascularization of human microvessels carrying host blood but relatively sparse cardiac grafts within engineered tissues, regardless of implant site. Conversely, athymic rats produced substantive inflammatory changes that degraded grafts (abdomen) or disrupted vascular patterning (heart). Despite disrupted vascular patterning, athymic rats supported > 3-fold larger human cardiomyocyte grafts compared to athymic mice. This work demonstrates the critical importance of the host for vascularization and engraftment of engineered tissues, which has broad translational implications across regenerative medicine.

Membrane attack complexes, endothelial cell activation, and direct allorecognition

Endothelial cells (ECs) form a critical immune interface regulating both the activation and trafficking of alloreactive T cells. In the setting of solid organ transplantation, donor-derived ECs represent sites where alloreactive T cells encounter major and minor tissue-derived alloantigens. During this initial encounter, ECs may formatively modulate effector responses of these T cells through expression of inflammatory mediators. Direct allorecognition is a process whereby recipient T cells recognize alloantigen in the context of donor EC-derived HLA molecules. Direct alloresponses are strongly modulated by human ECs and are galvanized by EC-derived inflammatory mediators.

Complement are immune proteins that mark damaged or foreign surfaces for immune cell activation. Following labeling by natural IgM during ischemia reperfusion injury (IRI) or IgG during antibody-mediated rejection (ABMR), the complement cascade is terminally activated in the vicinity of donor-derived ECs to locally generate the solid-phase inflammatory mediator, the membrane attack complex (MAC). Via upregulation of leukocyte adhesion molecules, costimulatory molecules, and cytokine trans-presentation, MAC strengthen EC:T cell direct alloresponses and qualitatively shape the alloimmune T cell response. These processes together promote T cell-mediated inflammation during solid organ transplant rejection.

In this review we describe molecular pathways downstream of IgM- and IgG-mediated MAC assembly on ECs in the setting of IRI and ABMR of tissue allografts, respectively. We describe work demonstrating that MAC deposition on ECs generates ‘signaling endosomes’ that sequester and post-translationally enhance the stability of inflammatory signaling molecules to promote EC activation, a process potentiating EC-mediated direct allorecognition. Additionally, with consideration to first-in-human xenotransplantation procedures, we describe clinical therapeutics based on inhibition of the complement pathway. The complement cascade critically mediates EC activation and improved understanding of relevant effector pathways will uncover druggable targets to obviate dysregulated alloimmune T cell infiltration into tissue allografts.

Efficient co-isolation of microvascular endothelial cells and satellite cell-derived myoblasts from human skeletal muscle

Vascularization of tissue-engineered constructs remains a key challenge in the field of skeletal muscle tissue engineering. One strategy for vascularizing organoids is in vitro pre-vascularization, relying on de novo assembly of undifferentiated endothelial cells into capillaries, a process termed vasculogenesis. In most endothelial cell research to date, human umbilical vein endothelial cells have been used primarily because of their availability. Nevertheless, this endothelial cell type is naturally not occurring in skeletal muscle tissue. Since endothelial cells display a tissue-specific phenotype, it is of interest to use muscle-specific microvascular endothelial cells to study pre-vascularization in skeletal muscle tissue engineering research. Thus far, tissue biopsies had to be processed in two separate protocols to obtain cells from the myogenic and the endothelial compartment. Here, we describe a novel, detailed protocol for the co-isolation of human skeletal muscle microvascular endothelial cells and satellite cell-derived myoblasts. It incorporates an automated mechanical and enzymatic tissue dissociation followed by magnetically activated cell sorting based on a combination of endothelial and skeletal muscle cell markers. Qualitative, quantitative, and functional characterization of the obtained cells is described and demonstrated by representative results. The simultaneous isolation of both cell types from the same donor is advantageous in terms of time efficiency. In addition, it may be the only possible method to isolate both cell types as the amount of tissue biopsy is often limited. The isolation of the two cell types is crucial for further studies to elucidate cell crosstalk in health and disease. Furthermore, the use of muscle-specific microvascular endothelial cells allows a shift towards engineering more physiologically relevant functional tissue, with downstream applications including drug screening and regenerative medicine.

Modeling human HSV infection via a vascularized immune-competent skin-on-chip platform

Herpes simplex virus (HSV) naturally infects skin and mucosal surfaces, causing lifelong recurrent disease worldwide, with no cure or vaccine. Biomimetic human tissue and organ platforms provide attractive alternatives over animal models to recapitulate human diseases. Combining prevascularization and microfluidic approaches, we present a vascularized, three-dimensional skin-on-chip that mimics human skin architecture and is competent to immune-cell and drug perfusion. The endothelialized microvasculature embedded in a fibroblast-containing dermis responds to biological stimulation, while the cornified epidermis functions as a protective barrier. HSV infection of the skin-on-chip displays tissue-level key morphological and pathophysiological features typical of genital herpes infection in humans, including the production of proinflammatory cytokine IL-8, which triggers rapid neutrophil trans-endothelial extravasation and directional migration. Importantly, perfusion with the antiviral drug acyclovir inhibits HSV infection in a dose-dependent and time-sensitive manner. Thus, our vascularized skin-on-chip represents a promising platform for human HSV disease modeling and preclinical therapeutic evaluation.

Advances of Engineered Hydrogel Organoids within the Stem Cell Field: A Systematic Review

Organoids are novel in vitro cell culture models that enable stem cells (including pluripotent stem cells and adult stem cells) to grow and undergo self-organization within a three-dimensional microenvironment during the process of differentiation into target tissues. Such miniature structures not only recapitulate the histological and genetic characteristics of organs in vivo, but also form tissues with the capacity for self-renewal and further differentiation. Recent advances in biomaterial technology, particularly hydrogels, have provided opportunities to improve organoid cultures; by closely integrating the mechanical and chemical properties of the extracellular matrix microenvironment, with novel synthetic materials and stem cell biology. This systematic review critically examines recent advances in various strategies and techniques utilized for stem-cell-derived organoid culture, with particular emphasis on the application potential of hydrogel technology in organoid culture. We hope this will give a better understanding of organoid cultures for modelling diseases and tissue engineering applications.

Methods for vascularization and perfusion of tissue organoids

Tissue organoids or "mini organs" can be invaluable tools for understanding health and disease biology, modeling tissue dynamics, or screening potential drug candidates. Effective vascularization of these models is critical for truly representing the in vivo tissue environment. Not only is the formation of a vascular network, and ultimately a microcirculation, essential for proper distribution and exchange of oxygen and nutrients throughout larger organoids, but vascular cells dynamically communicate with other cells to modulate overall tissue behavior. Additionally, interstitial fluid flow, mediated by a perfused microvasculature, can have profound influences on tissue biology. Thus, a truly functionally and biologically relevant organoid requires a vasculature. Here, we review existing strategies for fabricating and incorporating vascular elements and perfusion within tissue organoids.

Cell-Seeded Biomaterial Scaffolds: The Urgent Need for Unanswered Accelerated Angiogenesis

One of the most arduous challenges in tissue engineering is neovascularization, without which there is a lack of nutrients delivered to a target tissue. Angiogenesis should be completed at an optimal density and within an appropriate period of time to prevent cell necrosis. Failure to meet this challenge brings about poor functionality for the tissue in comparison with the native tissue, extensively reducing cell viability. Prior studies devoted to angiogenesis have provided researchers with some biomaterial scaffolds and cell choices for angiogenesis. For example, while most current angiogenesis approaches require a variety of stimulatory factors ranging from biomechanical to biomolecular to cellular, some other promising stimulatory factors have been underdeveloped (such as electrical, topographical, and magnetic). When it comes to choosing biomaterial scaffolds in tissue engineering for angiogenesis, key traits rush to mind including biocompatibility, appropriate physical and mechanical properties (adhesion strength, shear stress, and malleability), as well as identifying the appropriate biomaterial in terms of stability and degradation profile, all of which may leave essential trace materials behind adversely influencing angiogenesis. Nevertheless, the selection of the best biomaterial and cells still remains an area of hot dispute as such previous studies have not sufficiently classified, integrated, or compared approaches. To address the aforementioned need, this review article summarizes a variety of natural and synthetic scaffolds including hydrogels that support angiogenesis. Furthermore, we review a variety of cell sources utilized for cell seeding and influential factors used for angiogenesis with a concentrated focus on biomechanical factors, with unique stimulatory factors. Lastly, we provide a bottom-to-up overview of angiogenic biomaterials and cell selection, highlighting parameters that need to be addressed in future studies.

Engineering the multiscale complexity of vascular networks

The survival of vertebrate organisms depends on highly regulated delivery of oxygen and nutrients through vascular networks that pervade nearly all tissues in the body. Dysregulation of these vascular networks is implicated in many common human diseases such as hypertension, coronary artery disease, diabetes and cancer. Therefore, engineers have sought to create vascular networks within engineered tissues for applications such as regenerative therapies, human disease modelling and pharmacological testing. Yet engineering vascular networks has historically remained difficult, owing to both incomplete understanding of vascular structure and technical limitations for vascular fabrication. This Review highlights the materials advances that have enabled transformative progress in vascular engineering by ushering in new tools for both visualizing and building vasculature. New methods such as bioprinting, organoids and microfluidic systems are discussed, which have enabled the fabrication of 3D vascular topologies at a cellular scale with lumen perfusion. These approaches to vascular engineering are categorized into technology-driven and nature-driven approaches. Finally, the remaining knowledge gaps, emerging frontiers and opportunities for this field are highlighted, including the steps required to replicate the multiscale complexity of vascular networks found in nature.

Regenerative Engineering: Current Applications and Future Perspectives

Many pathologies, congenital defects, and traumatic injuries are untreatable by conventional pharmacologic or surgical interventions. Regenerative engineering represents an ever-growing interdisciplinary field aimed at creating biological replacements for injured tissues and dysfunctional organs. The need for bioengineered replacement parts is ubiquitous among all surgical disciplines. However, to date, clinical translation has been limited to thin, small, and/or acellular structures. Development of thicker tissues continues to be limited by vascularization and other impediments. Nevertheless, currently available materials, methods, and technologies serve as robust platforms for more complex tissue fabrication in the future. This review article highlights the current methodologies, clinical achievements, tenacious barriers, and future perspectives of regenerative engineering.

Selection of different endothelialization modes and different seed cells for tissue-engineered vascular graft

Tissue-engineered vascular grafts (TEVGs) have enormous potential for vascular replacement therapy. However, thrombosis and intimal hyperplasia are important problems associated with TEVGs especially small diameter TEVGs (<6 mm) after transplantation. Endothelialization of TEVGs is a key point to prevent thrombosis. Here, we discuss different types of endothelialization and different seed cells of tissue-engineered vascular grafts. Meanwhile, endothelial heterogeneity is also discussed. Based on it, we provide a new perspective for selecting suitable types of endothelialization and suitable seed cells to improve the long-term patency rate of tissue-engineered vascular grafts with different diameters and lengths.

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The material, diameter and length of tissue-engineered vascular graft are all key factors affecting its long-term patency.

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Endothelialization strategies should consider the different diameters and lengths of tissue-engineered vascular grafts.

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Cell heterogeneity and tissue heterogeneity should be considered in the application of seed cells.

Microvascular maturation by mesenchymal stem cells in vitro improves blood perfusion in implanted tissue constructs

Blood perfusion of grafted tissue constructs is a hindrance to the success of stem cell-based therapies by limiting cell survival and tissue regeneration. Implantation of a pre-vascularized network engineered in vitro has thus emerged as a promising strategy for promoting blood supply deep into the construct, relying on inosculation with the host vasculature. We aimed to fabricate in vitro tissue constructs with mature microvascular networks, displaying perivascular recruitment and basement membrane, taking advantage of the angiogenic properties of dental pulp stem cells and self-assembly of endothelial cells into capillaries. Using digital scanned light-sheet microscopy, we characterized the generation of dense microvascular networks in collagen hydrogels and established parameters for quantification of perivascular recruitment. We also performed original time-lapse analysis of stem cell recruitment. These experiments demonstrated that perivascular recruitment of dental pulp stem cells is driven by PDGF-BB. Recruited stem cells participated in deposition of vascular basement membrane and vessel maturation. Mature microvascular networks thus generated were then compared to those lacking perivascular coverage generated using stem cell conditioned medium. Implantation in athymic nude mice demonstrated that in vitro maturation of microvascular networks improved blood perfusion and cell survival within the construct. Taken together, these data demonstrate the strong potential of in vitro production of mature microvasculature for improving cell-based therapies.

Vasculogenesis from Human Dental Pulp Stem Cells Grown in Matrigel with Fully Defined Serum-Free Culture Media

The generation of vasculature is one of the most important challenges in tissue engineering and regeneration. Human dental pulp stem cells (hDPSCs) are some of the most promising stem cell types to induce vasculogenesis and angiogenesis as they not only secrete vascular endothelial growth factor (VEGF) but can also differentiate in vitro into both endotheliocytes and pericytes in serum-free culture media. Moreover, hDPSCs can generate complete blood vessels containing both endothelial and mural layers in vivo, upon transplantation into the adult brain. However, many of the serum free media employed for the growth of hDPSCs contain supplements of an undisclosed composition. This generates uncertainty as to which of its precise components are necessary and which are dispensable for the vascular differentiation of hDPSCs, and also hinders the transfer of basic research findings to clinical cell therapy. In this work, we designed and tested new endothelial differentiation media with a fully defined composition using standard basal culture media supplemented with a mixture of B27, heparin and growth factors, including VEGF-A165 at different concentrations. We also optimized an in vitro Matrigel assay to characterize both the ability of hDPSCs to differentiate to vascular cells and their capacity to generate vascular tubules in 3D cultures. The description of a fully defined serum-free culture medium for the induction of vasculogenesis using human adult stem cells highlights its potential as a relevant innovation for tissue engineering applications. In conclusion, we achieved efficient vasculogenesis starting from hDPSCs using serum-free culture media with a fully defined composition, which is applicable for human cell therapy purposes.

From Grafts to Human Bioengineered Vascularized Skin Substitutes

The skin plays an important role in the maintenance of the human's body physiological homeostasis. It acts as a coverage that protects against infective microorganism or biomechanical impacts. Skin is also implied in thermal regulation and fluid balance. However, skin can suffer several damages that impede normal wound-healing responses and lead to chronic wounds. Since the use of autografts, allografts, and xenografts present source limitations and intense rejection associated problems, bioengineered artificial skin substitutes (BASS) have emerged as a promising solution to address these problems. Despite this, currently available skin substitutes have many drawbacks, and an ideal skin substitute has not been developed yet. The advances that have been produced on tissue engineering techniques have enabled improving and developing new arising skin substitutes. The aim of this review is to outline these advances, including commercially available skin substitutes, to finally focus on future tissue engineering perspectives leading to the creation of autologous prevascularized skin equivalents with a hypodermal-like layer to achieve an exemplary skin substitute that fulfills all the biological characteristics of native skin and contributes to wound healing.

Synthetic peptide hydrogels as 3D scaffolds for tissue engineering

The regeneration of tissues and organs poses an immense challenge due to the extreme complexity in the research work involved. Despite the tissue engineering approach being considered as a promising strategy for more than two decades, a key issue impeding its progress is the lack of ideal scaffold materials. Nature-inspired synthetic peptide hydrogels are inherently biocompatible, and its high resemblance to extracellular matrix makes peptide hydrogels suitable 3D scaffold materials. This review covers the important aspects of peptide hydrogels as 3D scaffolds, including mechanical properties, biodegradability and bioactivity, and the current approaches in creating matrices with optimized features. Many of these scaffolds contain peptide sequences that are widely reported for tissue repair and regeneration and these peptide sequences will also be discussed. Furthermore, 3D biofabrication strategies of synthetic peptide hydrogels and the recent advances of peptide hydrogels in tissue engineering will also be described to reflect the current trend in the field. In the final section, we will present the future outlook in the design and development of peptide-based hydrogels for translational tissue engineering applications.

Biomaterials for Bioprinting Microvasculature

Microvasculature functions at the tissue and cell level, regulating local mass exchange of oxygen and nutrient-rich blood. While there has been considerable success in the biofabrication of large- and small-vessel replacements, functional microvasculature has been particularly challenging to engineer due to its size and complexity. Recently, three-dimensional bioprinting has expanded the possibilities of fabricating sophisticated microvascular systems by enabling precise spatiotemporal placement of cells and biomaterials based on computer-aided design. However, there are still significant challenges facing the development of printable biomaterials that promote robust formation and controlled 3D organization of microvascular networks. This review provides a thorough examination and critical evaluation of contemporary biomaterials and their specific roles in bioprinting microvasculature. We first provide an overview of bioprinting methods and techniques that enable the fabrication of microvessels. We then offer an in-depth critical analysis on the use of hydrogel bioinks for printing microvascularized constructs within the framework of current bioprinting modalities. We end with a review of recent applications of bioprinted microvasculature for disease modeling, drug testing, and tissue engineering, and conclude with an outlook on the challenges facing the evolution of biomaterials design for bioprinting microvasculature with physiological complexity.

Short-term in vitro glutamine restriction differentially impacts the chromosomal stability of transformed and non-transformed cells

Glutamine (Gln) is a non-essential amino acid central for generating building blocks and cellular energy in tumours and rapidly proliferating non-transformed cells. However, the influence of Gln on regulating chromosomal stability of transformed and non-transformed cells remain poorly understand. We hypothesised that Gln is required for maintaining a homeostatic level of chromosomal stability. To this end, transformed cells HeLa and A375 and non-transformed cells NCM460 and HUVEC cells were intervened with varying concentrations of Gln (10, 1, 0.1 and 0.01 mM), with or without cisplatin (0.1 µg/ml), for 24 h. The cytokinesis-block micronucleus (MN) assay was used to determine chromosomal instability (CIN), the extent of which is reflected by the frequency of MN, nucleoplasmic bridge (NPB) and nuclear bud (NB). We demonstrated an unexpected decrease in the spontaneous rate of MN, but not NPB and NB, after Gln restriction in HeLa and A375 cells. Gln restriction reduced cisplatin-induced MN, but not NPB and NB, in HeLa and A375 cells. We further revealed that Gln restriction suppressed the proliferation of HeLa cells with high CIN induced by nocodazole, partially explaining why Gln restriction decreased the frequency of spontaneous and cisplatin-induced MN in transformed cells. In contrast, Gln restriction increased MN and NB, but not NPB, in NCM460 cells. In HUVEC cells, Gln restriction increased MN, NPB and NB. Meanwhile, Gln restriction sensitised NCM460 cells to cisplatin-induced genotoxicity. A similar but more pronounced pattern was observed in HUVEC cells. Collectively, these results suggest that the in vitro influences of Gln metabolism on CIN depend on cellular contexts: Transformed cells require high Gln to fine tune their CIN in an optimal rate to maximise genomic heterogeneity and fitness, whereas non-transformed cells need high Gln to prevent CIN.

An Alginate-Based Hydrogel with a High Angiogenic Capacity and a High Osteogenic Potential

In bone tissue engineering, autologous cells are combined with osteoconductive scaffolds and implanted into bone defects. The major challenge is the lack of post-implantation vascular growth into biomaterial. The objective of the present study was to develop a new alginate-based hydrogel that enhances the regeneration of bone defects after surgery. The viability of human bone marrow-derived mesenchymal stem cells (BM-MSCs) or human endothelial cells (ECs) cultured alone or together on the hydrogel was analyzed for 24 and 96 h. After seeding, the cells self-assembled and aggregated to form clusters. For functional validation, empty or cellularized hydrogel matrices were implanted ectopically at subcutaneous sites in nude mice. After 2 months, the matrices were explanted. Transplanted human cells were present, and we observed vessels expressing human von Willebrand factor (resulting from the incorporation of transplanted ECs into neovessels and/or the differentiation of BM-MSCs into ECs). The addition of BM-MSCs improved host vascularization and neovessel formation from human cells, relative to ECs alone. Although we did not observe bone formation, the transplanted BM-MSCs were able to differentiate into osteoblasts. This new biomaterial provided an appropriate three-dimensional environment for transplanted cells and has a high angiogenic capacity and an osteogenic potential.

Tissue Engineering Using Vascular Organoids From Human Pluripotent Stem Cell Derived Mural Cell Phenotypes

Diffusion is a limiting factor in regenerating large tissues (100–200 μm) due to reduced nutrient supply and waste removal leading to low viability of the regenerating cells as neovascularization of the implant by the host is a slow process. Thus, generating prevascularized tissue engineered constructs, in which endothelial (ECs) and mural (MCs) cells, such as smooth muscle cells (SMCs), and pericytes (PCs), are preassembled into functional in vitro vessels capable of rapidly connecting to the host vasculature could overcome this obstacle. Toward this purpose, using feeder-free and low serum conditions, we developed a simple, efficient and rapid in vitro approach to induce the differentiation of human pluripotent stem cells-hPSCs (human embryonic stem cells and human induced pluripotent stem cells) to defined SMC populations (contractile and synthetic hPSC-SMCs) by extensively characterizing the cellular phenotype (expression of CD44, CD73, CD105, NG2, PDGFRβ, and contractile proteins) and function of hPSC-SMCs. The latter were phenotypically and functionally stable for at least 8 passages, and could stabilize vessel formation and inhibit vessel network regression, when co-cultured with ECs in vitro. Subsequently, using a methylcellulose-based hydrogel system, we generated spheroids consisting of EC/hPSC-SMC (vascular organoids), which were extensively phenotypically characterized. Moreover, the vascular organoids served as focal starting points for the sprouting of capillary-like structures in vitro, whereas their delivery in vivo led to rapid generation of a complex functional vascular network. Finally, we investigated the vascularization potential of these vascular organoids, when embedded in hydrogels composed of defined extracellular components (collagen/fibrinogen/fibronectin) that can be used as scaffolds in tissue engineering applications. In summary, we developed a robust method for the generation of defined SMC phenotypes from hPSCs. Fabrication of vascularized tissue constructs using hPSC-SMC/EC vascular organoids embedded in chemically defined matrices is a significant step forward in tissue engineering and regenerative medicine.

Developing a xenograft model of human vasculature in the mouse ear pinna

Humanised xenograft models allow for the analysis of human tissue within a physiological environment in vivo. However, current models often rely on the angiogenesis and ingrowth of recipient vasculature to perfuse tissues, preventing analysis of biological processes and diseases involving human blood vessels. This limits the effectiveness of xenografts in replicating human physiology and may lead to issues with translating findings into human research. We have designed a xenograft model of human vasculature to address this issue. Human subcutaneous fat was cultured in vitro to promote blood vessel outgrowth prior to implantation into immunocompromised mice. We demonstrate that implants survived, retained human vasculature and anastomosed with the circulatory system of the recipient mouse. Significantly, by performing transplants into the ear pinna, this system enabled intravital observation of xenografts by multiphoton microscopy, allowing us to visualise the steps leading to vascular cytoadherence of erythrocytes infected with the human parasite Plasmodium falciparum. This model represents a useful tool for imaging the interactions that occur within human tissues in vivo and permits visualization of blood flow and cellular recruitment in a system which is amenable to intervention for various studies in basic biology together with drug evaluation and mechanism of action studies.

Vascularization strategies for skin tissue engineering

Lack of proper vascularization after skin trauma causes delayed wound healing. This has sparked the development of various tissue engineering strategies to improve vascularization.

Bone Tissue Regeneration in the Oral and Maxillofacial Region: A Review on the Application of Stem Cells and New Strategies to Improve Vascularization

Bone tissue engineering techniques are a promising alternative for the use of autologous bone grafts to reconstruct bone defects in the oral and maxillofacial region. However, for successful bone regeneration, adequate vascularization is a prerequisite. This review presents and discusses the application of stem cells and new strategies to improve vascularization, which may lead to feasible clinical applications. Multiple sources of stem cells have been investigated for bone tissue engineering. The stromal vascular fraction (SVF) of human adipose tissue is considered a promising single source for a heterogeneous population of essential cells with, amongst others, osteogenic and angiogenic potential. Enhanced vascularization of tissue-engineered grafts can be achieved by different mechanisms: vascular ingrowth directed from the surrounding host tissue to the implanted graft, vice versa, or concomitantly. Vascular ingrowth into the implanted graft can be enhanced by (i) optimizing the material properties of scaffolds and (ii) their bioactivation by incorporation of growth factors or cell seeding. Vascular ingrowth directed from the implanted graft towards the host tissue can be achieved by incorporating the graft with either (i) preformed microvascular networks or (ii) microvascular fragments (MF). The latter may have stimulating actions on both vascular ingrowth and outgrowth, since they contain angiogenic stem cells like SVF, as well as vascularized matrix fragments. Both adipose tissue-derived SVF and MF are cell sources with clinical feasibility due to their large quantities that can be harvested and applied in a one-step surgical procedure. During the past years, important advancements of stem cell application and vascularization in bone tissue regeneration have been made. The development of engineered in vitro 3D models mimicking the bone defect environment would facilitate new strategies in bone tissue engineering. Successful clinical application requires innovative future investigations enhancing vascularization.

Progenitor-derived human endothelial cells evade alloimmunity by CRISPR/Cas9-mediated complete ablation of MHC expression

Tissue engineering may address organ shortages currently limiting clinical transplantation. Off-the-shelf engineered vascularized organs will likely use allogeneic endothelial cells (ECs) to construct microvessels required for graft perfusion. Vasculogenic ECs can be differentiated from committed progenitors (human endothelial colony-forming cells or HECFCs) without risk of mutation or teratoma formation associated with reprogrammed stem cells. Like other ECs, these cells can express both class I and class II major histocompatibility complex (MHC) molecules, bind donor-specific antibody (DSA), activate alloreactive T effector memory cells, and initiate rejection in the absence of donor leukocytes. CRISPR/Cas9-mediated dual ablation of β2-microglobulin and class II transactivator (CIITA) in HECFC-derived ECs eliminates both class I and II MHC expression while retaining EC functions and vasculogenic potential. Importantly, dually ablated ECs no longer bind human DSA or activate allogeneic CD4+ effector memory T cells and are resistant to killing by CD8+ alloreactive cytotoxic T lymphocytes in vitro and in vivo. Despite absent class I MHC molecules, these ECs do not activate or elicit cytotoxic activity from allogeneic natural killer cells. These data suggest that HECFC-derived ECs lacking MHC molecule expression can be utilized for engineering vascularized grafts that evade allorejection.

Engineering tissue-specific blood vessels

Vascular diversity among organs has recently become widely recognized. Several studies using mouse and human fetal tissues revealed distinct characteristics of organ-specific vasculature in molecular and functional levels. Thorough understanding of vascular heterogeneities in human adult tissues is significant for developing novel strategies for targeted drug delivery and tissue regeneration. Recent advancements in microfabrication techniques, biomaterials, and differentiation protocols allowed for incorporation of microvasculature into engineered organs. Such vascularized organ models represent physiologically relevant platforms that may offer innovative tools for dissecting the effects of the organ microenvironment on vascular development and expand our present knowledge on organ-specific human vasculature. In this article, we provide an overview of the current structural and molecular evidence on microvascular diversity, bioengineering methods used to recapitulate the microenvironmental cues, and recent vascularized three-dimensional organ models from the perspective of tissue-specific vasculature.

Emergence of Three Dimensional Printed Cardiac Tissue: Opportunities and Challenges in Cardiovascular Diseases

Three-dimensional (3D) printing, also known as additive manufacturing, was developed originally for engineering applications. Since its early advancements, there has been a relentless de-velopment in enthusiasm for this innovation in biomedical research. It allows for the fabrication of structures with both complex geometries and heterogeneous material properties. Tissue engineering using 3D bio-printers can overcome the limitations of traditional tissue engineering methods. It can match the complexity and cellular microenvironment of human organs and tissues, which drives much of the interest in this technique. However, most of the preliminary evaluations of 3D-printed tissues and organ engineering, including cardiac tissue, relies extensively on the lessons learned from tradi-tional tissue engineering. In many early examples, the final printed structures were found to be no bet-ter than tissues developed using traditional tissue engineering methods. This highlights the fact that 3D bio-printing of human tissue is still very much in its infancy and more work needs to be done to realise its full potential. This can be achieved through interdisciplinary collaboration between engi-neers, biomaterial scientists and molecular cell biologists. This review highlights current advance-ments and future prospects for 3D bio-printing in engineering ex vivo cardiac tissue and associated vasculature, such as coronary arteries. In this context, the role of biomaterials for hydrogel matrices and choice of cells are discussed. 3D bio-printing has the potential to advance current research signif-icantly and support the development of novel therapeutics which can improve the therapeutic out-comes of patients suffering fatal cardiovascular pathologies.

Hypoxia-Inducible Factor Drives Vascularization of Modularly Assembled Engineered Tissue

IMPACT STATEMENT

Using two inhibitory methods, we demonstrated that hypoxia-inducible factor (HIF) plays an important role in vascularizing and oxygenating modularly-assembled engineered tissues. Each inhibitory technique elucidated a different mechanism by which this occurred. Whereas systemic inhibition negatively impacted early recruitment of host-derived cells, genetic inhibition in grafted endothelial cells was detrimental to their survival. Taken together, our study suggests that methods of HIF-mediated mechanisms could be harnessed to tune the extent and rate of vascularization in engineered tissue constructs.

Tissue Engineering of the Microvasculature

The ability to generate new microvessels in desired numbers and at desired locations has been a long-sought goal in vascular medicine, engineering, and biology. Historically, the need to revascularize ischemic tissues nonsurgically (so-called therapeutic vascularization) served as the main driving force for the development of new methods of vascular growth. More recently, vascularization of engineered tissues and the generation of vascularized microphysiological systems have provided additional targets for these methods, and have required adaptation of therapeutic vascularization to biomaterial scaffolds and to microscale devices. Three complementary strategies have been investigated to engineer microvasculature: angiogenesis (the sprouting of existing vessels), vasculogenesis (the coalescence of adult or progenitor cells into vessels), and microfluidics (the vascularization of scaffolds that possess the open geometry of microvascular networks). Over the past several decades, vascularization techniques have grown tremendously in sophistication, from the crude implantation of arteries into myocardial tunnels by Vineberg in the 1940s, to the current use of micropatterning techniques to control the exact shape and placement of vessels within a scaffold. This review provides a broad historical view of methods to engineer the microvasculature, and offers a common framework for organizing and analyzing the numerous studies in this area of tissue engineering and regenerative medicine. © 2019 American Physiological Society. Compr Physiol 9:1155-1212, 2019.

ZFYVE21 is a complement-induced Rab5 effector that activates non-canonical NF-κB via phosphoinosotide remodeling of endosomes

Complement promotes vascular inflammation in transplant organ rejection and connective tissue diseases. Here we identify ZFYVE21 as a complement-induced Rab5 effector that induces non-canonical NF-κB in endothelial cells (EC). In response to membrane attack complexes (MAC), ZFYVE21 is post-translationally stabilized on MAC+Rab5+ endosomes in a Rab5- and PI(3)P-dependent manner. ZFYVE21 promotes SMURF2-mediated polyubiquitinylation and proteasome-dependent degradation of endosome-associated PTEN to induce vesicular enrichment of PI(3,4,5)P3 and sequential recruitment of activated Akt and NF-κB-inducing kinase (NIK). Pharmacologic alteration of cellular phosphoinositide content with miltefosine reduces ZFYVE21 induction, EC activation, and allograft vasculopathy in a humanized mouse model. ZFYVE21 induction distinctly occurs in response to MAC and is detected in human renal and synovial tissues. Our data identifies ZFYVE21 as a Rab5 effector, defines a Rab5-ZFYVE21-SMURF2-pAkt axis by which it mediates EC activation, and demonstrates a role for this pathway in complement-mediated conditions.

Complement activation contributes to vascular inflammation in the contexts of allograft rejection and connective tissue disease. Here Fang et al. identify ZFYVE21 as a novel effector of Rab5 and find it regulates pro-inflammatory NF-κB signaling in endothelial cells in response to complement activation.

Co-culture of human umbilical vein endothelial cells and human bone marrow stromal cells into a micro-cavitary gelatin-methacrylate hydrogel system to enhance angiogenesis

Vascular tissue engineering seeks to develop functional blood vessels that comprise of both endothelial cells and pericytes for translational medicine and is often faced with numerous challenges such as nutrients and wastes diffusion problem in the centre of the scaffolds. Various strategies have been adopted to solve the diffusion problem in thick engineered scaffolds. Typically, microchannels or dissolvable microspheres are introduced into three-dimensional (3D) scaffolds as an alternative way to improve the infiltration of scaffolds and endothelial cells are usually incorporated into the biomaterials. While some research groups now focus on finding supporting cells to build further vascularized structures in the scaffolds. In this study, a bioinspired 3D gelatin-methacrylate (Gel-MA) hydrogel with dissolvable microspheres was created to encapsulate human bone marrow stromal cells (HMSCs) and human umbilical vein endothelial cells (HUVECs) which was used to investigate whether HMSCs could play a pericytes-like role and enhance vascularization within the engineered scaffolds. The results showed co-culture of HMSCs and HUVECs demonstrated significantly improved vascularization when compared to either HUVECs or HMSCs monoculture. Angiogenic genes were expressed significantly higher in co-culture group. Moreover, when implanting the pre-vascularized scaffolds in vivo, co-culture system integrated more successfully with host tissue and showed higher host tissue invasion than any other groups. More importantly, both the qPCR and immunofluorescence results indicated MSCs differentiated towards pericytes to enhance vascularization in this study. This paper highlights the enhanced capability of 3D micro-cavitary Gel-MA hydrogel for co-culturing HUVECs and HMSCs to promote vascularization which presents a potential strategy for future tissue repair and regeneration.

Bioengineering human vascular networks: trends and directions in endothelial and perivascular cell sources

Tissue engineering holds great promise in regenerative medicine. However, the field of tissue engineering faces a myriad of difficulties. A major challenge is the necessity to integrate vascular networks into bioengineered constructs to enable physiological functions including adequate oxygenation, nutrient delivery, and removal of waste products. The last two decades have seen remarkable progress in our collective effort to bioengineer human-specific vascular networks. Studies have included both in vitro and in vivo investigations, and multiple methodologies have found varying degrees of success. What most approaches to bioengineer human vascular networks have in common, however, is the synergistic use of both (1) endothelial cells (ECs)-the cells used to line the lumen of the vascular structures and (2) perivascular cells-usually used to support EC function and provide perivascular stability to the networks. Here, we have highlighted trends in the use of various cellular sources over the last two decades of vascular network bioengineering research. To this end, we comprehensively reviewed all life science and biomedical publications available at the MEDLINE database up to 2018. Emphasis was put on selective studies that definitively used human ECs and were specifically related to bioengineering vascular networks. To facilitate this analysis, all papers were stratified by publication year and then analyzed according to their use of EC and perivascular cell types. This study provides an illustrating discussion on how each alternative source of cells has come to be used in the field. Our intention was to reveal trends and to provide new insights into the trajectory of vascular network bioengineering with regard to cellular sources.

Subendothelial matrix components influence endothelial cell apoptosis in vitro

The programmed form of cell death (apoptosis) is essential for normal development of multicellular organisms. Dysregulation of apoptosis has been linked with embryonal death and is involved in the pathophysiology of various diseases. Specifically, endothelial apoptosis plays pivotal roles in atherosclerosis whereas prevention of endothelial apoptosis is a prerequisite for neovascularization in tumors and metastasis. Endothelial biology is intertwined with the composition of subendothelial basement membrane proteins. Apoptosis was induced by addition of tumor necrosis factor-α to cycloheximide-sensitized endothelial cells. Cells were either grown on polystyrene culture plates or on plates precoated with healthy basement membrane proteins (collagen IV, fibronectin, or laminin) or collagen I. Our results reveal that proteins of healthy basement membrane alleviate cytokine-induced apoptosis whereas precoating with collagen type I had no significant effect on apoptosis by addition of tumor necrosis factor-α to cycloheximide-sensitized endothelial cells compared with cells cultured on uncoated plates. Yet, treatment with transforming growth factor-β1 significantly reduced the rate of apoptosis endothelial cells grown on collagen I. Detailed analysis reveals differences in intracellular signaling pathways for each of the basement membrane proteins studied. We provide additional insights into the importance of basement membrane proteins and the respective cytokine milieu on endothelial biology. Exploring outside-in signaling by basement membrane proteins may constitute an interesting target to restore vascular function and prevent complications in the atherosclerotic cascade.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

3D bioprinting of skin tissue: From pre-processing to final product evaluation

Bioprinted skin tissue has the potential for aiding drug screening, formulation development, clinical transplantation, chemical and cosmetic testing, as well as basic research. Limitations of conventional skin tissue engineering approaches have driven the development of biomimetic skin equivalent via 3D bioprinting. A key hope for bioprinting skin is the improved tissue authenticity over conventional skin equivalent construction, enabling the precise localization of multiple cell types and appendages within a construct. The printing of skin faces challenges broadly associated with general 3D bioprinting, including the selection of cell types and biomaterials, and additionally requires in vitro culture formats that allow for growth at an air-liquid interface. This paper provides a thorough review of current 3D bioprinting technologies used to engineer human skin constructs and presents the overall pipelines of designing a biomimetic artificial skin via 3D bioprinting from the design phase (i.e. pre-processing phase) through the tissue maturation phase (i.e. post-processing) and into final product evaluation for drug screening, development, and drug delivery applications.

An important step towards a prevascularized islet macroencapsulation device-effect of micropatterned membranes on development of endothelial cell network

The development of immune protective islet encapsulation devices could allow for islet transplantation in the absence of immunosuppression. However, the immune protective membrane / barrier introduced there could also impose limitations in transport of oxygen and nutrients to the encapsulated cells resulting to limited islet viability. In the last years, it is well understood that achieving prevascularization of the device in vitro could facilitate its connection to the host vasculature after implantation, and therefore could provide sufficient blood supply and oxygenation to the encapsulated islets. However, the microvascular networks created in vitro need to mimic well the highly organized vasculature of the native tissue. In earlier study, we developed a functional macroencapsulation device consisting of two polyethersulfone/polyvinylpyrrolidone (PES/PVP) membranes, where a bottom microwell membrane provides good separation of encapsulated islets and the top flat membrane acts as a lid. In this work, we investigate the possibility of creating early microvascular networks on the lid of this device by combining novel membrane microfabrication with co-culture of human umbilical vein endothelial cell (HUVEC) and fibroblasts. We create thin porous microstructured PES/PVP membranes with solid and intermittent line-patterns and investigate the effect of cell alignment and cell interconnectivity as a first step towards the development of a stable prevascularized layer in vitro. Our results show that, in contrast to non-patterned membranes where HUVECs form unorganized HUVEC branch-like structures, for the micropatterned membranes, we can achieve cell alignment and the co-culture of HUVECs on a monolayer of fibroblasts attached on the membranes with intermittent line-pattern allows for the creation of HUVEC branch-like structures over the membrane surface. This important step towards creating early microvascular networks was achieved without the addition of hydrogels, often used in angiogenesis assays, as gels could block the pores of the membrane and limit the transport properties of the islet encapsulation device.

Modeling the human bone marrow niche in mice: From host bone marrow engraftment to bioengineering approaches

Xenotransplantation of patient-derived samples in mouse models has been instrumental in depicting the role of hematopoietic stem and progenitor cells in the establishment as well as progression of hematological malignancies. The foundations for this field of research have been based on the development of immunodeficient mouse models, which provide normal and malignant human hematopoietic cells with a supportive microenvironment. Immunosuppressed and genetically modified mice expressing human growth factors were key milestones in patient-derived xenograft (PDX) models, highlighting the importance of developing humanized microenvironments. The latest major improvement has been the use of human bone marrow (BM) niche-forming cells to generate human-mouse chimeric BM tissues in PDXs, which can shed light on the interactions between human stroma and hematopoietic cells. Here, we summarize the methods used for human hematopoietic cell xenotransplantation and their milestones and review the latest approaches in generating humanized BM tissues in mice to study human normal and malignant hematopoiesis.

Endothelial and beta cell composite aggregates for improved function of a bioartificial pancreas encapsulation device

INTRODUCTION

Encapsulation of pancreatic islets or beta cells is a promising strategy for treatment of type 1 diabetes by providing an immune isolated environment and allowing for transplantation in a different location than the liver. However, islets used for encapsulation often show lower functionality due to the damaging of islet endothelial cells during the isolation procedure. Factors produced by endothelial cells have great impact on beta cell insulin secretion. Therefore, mutual signaling between endothelial cells and beta cells should be considered for the development of encapsulation systems to achieve high insulin secretion and maintain beta cell viability. Here, we investigate whether co-culture of beta cells with endothelial cells could improve beta cell function within encapsulation devices.

MATERIALS AND METHODS

Mouse insulinoma MIN6 cells and human umbilical vein endothelial cells were used for creating composite aggregates on agarose microwell platform. The composite aggregates were encapsulated within flat poly(ether sulfone)/polyvinylpyrrolidone device. Their functionality was assessed by glucose-induced insulin secretion test and compared to non-encapsulated free-floating aggregates.

RESULTS

We created composite aggregates of 80-100 µm in diameter, closely mimicking pancreatic islets. Upon glucose stimulation, their insulin secretion is improved in comparison to aggregates consisting of only MIN6 cells. Moreover, the composite aggregates encapsulated within a device secrete more insulin than aggregates consisting of only MIN6 cells.

CONCLUSION

Composite aggregates of MIN6 cells with human umbilical vein endothelial cells have improved insulin secretion in comparison to MIN6 aggregates showing that the interaction of beta cell and endothelial cell is crucial for a functional encapsulation system.

3D matrix-embedding inhibits cycloheximide-mediated sensitization to TNF-alpha-induced apoptosis of human endothelial cells

The programmed form of cell death (apoptosis) is essential for normal development of multicellular organisms. Dysregulation of apoptosis has been linked with embryonal death and is involved in the pathophysiology of various diseases. Others and we previously demonstrated endothelial biology being intertwined with biochemical and structural composition of the subendothelial basement membrane. We now demonstrate that a three-dimensional growing environment significantly shields endothelial cells from cytokine-induced apoptosis. Detailed analysis reveals differences in intracellular signaling pathways in naive endothelial cells and cytokine-stimulated endothelial cells when cells are grown within a three-dimensional collagen-based matrix compared to cells grown on two-dimensional tissue culture plates. Main findings are significantly reduced p53 expression and level of p38-phosphorylation in three-dimensional grown endothelial cells. Despite similar concentrations of focal adhesion kinase, three-dimensional matrix-embedded endothelial cells express significantly less tyrosine-phosphorylated focal adhesion kinase. Pretreatment with antibodies against integrin α_v β₃ partially reversed the protective effect of three-dimensional matrix-embedding on endothelial apoptosis. Our findings provide detailed insights into the mechanisms of endothelial apoptosis with respect to the spatial matrix environment. These results enhance our understanding of endothelial biology and may otherwise help in the design of tissue-engineered materials. Furthermore, findings on focal adhesion kinase phosphorylation might enhance our understanding of clinical studies with tyrosine kinase inhibitors.

Assessment of hydrogels for bioprinting of endothelial cells

In tissue engineering applications, vascularization can be accomplished by coimplantation of tissue forming cells and endothelial cells (ECs), whereby the latter are able to form functional blood vessels. The use of three-dimensional (3D) bioprinting technologies has the potential to improve the classical tissue engineering approach because these will allow the generation of scaffolds with high spatial control of endothelial cell allocation. This study focuses on a side by side comparison of popular commercially available bioprinting hydrogels (Matrigel, fibrin, collagen, gelatin, agarose, Pluronic F-127, alginate, and alginate/gelatin) in the context of their physicochemical parameters, their swelling/degradation characteristics, their biological effects on vasculogenesis-related EC parameters and their printability. The aim of this study was to identify the most suitable hydrogel or hydrogel combination for inkjet printing of ECs to build prevascularized tissue constructs. Most tested hydrogels displayed physicochemical characteristics suitable for inkjet printing. However, Pluronic F-127 and the alginate/gelatin blend were rapidly degraded when incubated in cell culture medium. Agarose, Pluronic F-127, alginate and alginate/gelatin hydrogels turned out to be unsuitable for bioprinting of ECs because of their non-adherent properties and/or their incapability to support EC proliferation. Gelatin was able to support EC proliferation and viability but was unable to support endothelial cell sprouting. Our experiments revealed fibrin and collagen to be most suitable for bioprinting of ECs, because these hydrogels showed acceptable swelling/degradation characteristics, supported vasculogenesis-related EC parameters and showed good printability. Moreover, ECs in constructs of preformed spheroids survived the printing process and formed capillary-like cords. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 935-947, 2018.

Engineering of an angiogenic niche by perfusion culture of adipose-derived stromal vascular fraction cells

In vitro recapitulation of an organotypic stromal environment, enabling efficient angiogenesis, is crucial to investigate and possibly improve vascularization in regenerative medicine. Our study aims at engineering the complexity of a vascular milieu including multiple cell-types, a stromal extracellular matrix (ECM), and molecular signals. For this purpose, the human adipose stromal vascular fraction (SVF), composed of a heterogeneous mix of pericytes, endothelial/stromal progenitor cells, was cultured under direct perfusion flow on three-dimensional (3D) collagen scaffolds. Perfusion culture of SVF-cells reproducibly promoted in vitro the early formation of a capillary-like network, embedded within an ECM backbone, and the release of numerous pro-angiogenic factors. Compared to static cultures, perfusion-based engineered constructs were more rapidly vascularized and supported a superior survival of delivered cells upon in vivo ectopic implantation. This was likely mediated by pericytes, whose number was significantly higher (4.5-fold) under perfusion and whose targeted depletion resulted in lower efficiency of vascularization, with an increased host foreign body reaction. 3D-perfusion culture of SVF-cells leads to the engineering of a specialized milieu, here defined as an angiogenic niche. This system could serve as a model to investigate multi-cellular interactions in angiogenesis, and as a module supporting increased grafted cell survival in regenerative medicine.

Endothelial Progenitor Cells for the Vascularization of Engineered Tissues

Self-assembled microvasculature from cocultures of endothelial cells (ECs) and stromal cells has significantly advanced efforts to vascularize engineered tissues by enhancing perfusion rates in vivo and producing investigative platforms for microvascular morphogenesis in vitro. However, to clinically translate prevascularized constructs, the issue of EC source must be resolved. Endothelial progenitor cells (EPCs) can be noninvasively supplied from the recipient through adult peripheral and umbilical cord blood, as well as derived from induced pluripotent stem cells, alleviating antigenicity issues. EPCs can also differentiate into all tissue endothelium, and have demonstrated potential for therapeutic vascularization. Yet, EPCs are not the standard EC choice to vascularize tissue constructs in vitro. Possible reasons include unresolved issues with EPC identity and characterization, as well as uncertainty in the selection of coculture, scaffold, and culture media combinations that promote EPC microvessel formation. This review addresses these issues through a summary of EPC vascular biology and the effects of tissue engineering design parameters upon EPC microvessel formation. Also included are perspectives to integrate EPCs with emerging technologies to produce functional, organotypic vascularized tissues.

Host non-inflammatory neutrophils mediate the engraftment of bioengineered vascular networks

Notwithstanding remarkable progress in vascular network engineering, implanted bioengineered microvessels largely fail to form anastomoses with the host vasculature. Here, we demonstrate that implants containing assembled human vascular networks (A-Grafts) fail to engraft due to their inability to engage non-inflammatory host neutrophils upon implantation into mice. In contrast, unassembled vascular cells (U-Grafts) readily engage alternatively polarized neutrophils, which in turn serve as indispensable mediators of vascular assembly and anastomosis. The depletion of host neutrophils abrogated vascularization in U-Grafts, whereas an adoptive transfer of neutrophils fully restored vascularization in myeloid-depleted mice. Neutrophil engagement was regulated by secreted factors and was progressively silenced as the vasculature matured. Exogenous addition of factors from U-Grafts reengaged neutrophils and enhanced revascularization in A-Grafts, a process that was recapitulated by blocking Notch signaling. Our data suggest that the pro-vascularization potential of neutrophils can be harnessed to improve the engraftment of bioengineered tissues.

Next generation human skin constructs as advanced tools for drug development

Many diseases, as well as side effects of drugs, manifest themselves through skin symptoms. Skin is a complex tissue that hosts various specialized cell types and performs many roles including physical barrier, immune and sensory functions. Therefore, modeling skin in vitro presents technical challenges for tissue engineering. Since the first attempts at engineering human epidermis in 1970s, there has been a growing interest in generating full-thickness skin constructs mimicking physiological functions by incorporating various skin components, such as vasculature and melanocytes for pigmentation. Development of biomimetic in vitro human skin models with these physiological functions provides a new tool for drug discovery, disease modeling, regenerative medicine and basic research for skin biology. This goal, however, has long been delayed by the limited availability of different cell types, the challenges in establishing co-culture conditions, and the ability to recapitulate the 3D anatomy of the skin. Recent breakthroughs in induced pluripotent stem cell (iPSC) technology and microfabrication techniques such as 3D-printing have allowed for building more reliable and complex in vitro skin models for pharmaceutical screening. In this review, we focus on the current developments and prevailing challenges in generating skin constructs with vasculature, skin appendages such as hair follicles, pigmentation, immune response, innervation, and hypodermis. Furthermore, we discuss the promising advances that iPSC technology offers in order to generate in vitro models of genetic skin diseases, such as epidermolysis bullosa and psoriasis. We also discuss how future integration of the next generation human skin constructs onto microfluidic platforms along with other tissues could revolutionize the early stages of drug development by creating reliable evaluation of patient-specific effects of pharmaceutical agents. Impact statement Skin is a complex tissue that hosts various specialized cell types and performs many roles including barrier, immune, and sensory functions. For human-relevant drug testing, there has been a growing interest in building more physiological skin constructs by incorporating different skin components, such as vasculature, appendages, pigment, innervation, and adipose tissue. This paper provides an overview of the strategies to build complex human skin constructs that can faithfully recapitulate human skin and thus can be used in drug development targeting skin diseases. In particular, we discuss recent developments and remaining challenges in incorporating various skin components, availability of iPSC-derived skin cell types and in vitro skin disease models. In addition, we provide insights on the future integration of these complex skin models with other organs on microfluidic platforms as well as potential readout technologies for high-throughput drug screening.

Developing a Rapid Algorithm to Enable Rapid Characterization of Alginate Microcapsules

The islets of Langerhans are endocrine tissue clusters that secrete hormones that regulate the body's glucose, carbohydrate, and fat metabolism, the most important of which is insulin, a hormone secreted by β-cells within the islets. In certain instances, a person's own immune system attacks and destroys them, leading to the development of type 1 diabetes (T1D), a life-long condition that needs daily insulin administration to maintain health and prolong survival. Islet transplantation is a surgical procedure that has demonstrated the ability to normalize blood sugar levels for up to a few years, but the need for chronic immunosuppression relegates it to a last resort that is often only used sparingly and in seriously ill patients. Islet microencapsulation is a biomedical innovation designed to protect islets from the immune system by coating them with a biocompatible polymer, and this new technology has demonstrated various degrees of success in small- and large-animal studies. This success is significantly impacted by microcapsule morphology and encapsulation efficiency. Since hundreds of thousands of microcapsules are generated during the process, characterization of encapsulated islets without the help of some degree of automation would be difficult, time-consuming, and error prone due to inherent observer bias. We have developed an image analysis algorithm that can analyze hundreds of microencapsulated islets and characterize their size, shape, circularity, and distortion with minimal observer bias. This algorithm can be easily adapted to similar nano- or microencapsulation technologies to implement stricter quality control and improve biomaterial device design and success.

Ectopic tissue engineered ligament with silk collagen scaffold for ACL regeneration: A preliminary study

Anterior cruciate ligament (ACL) reconstruction remains a formidable clinical challenge because of the lack of vascularization and adequate cell numbers in the joint cavity. In this study, we developed a novel strategy to mimic the early stage of repair in vivo, which recapitulated extra-articular inflammatory response to facilitate the early ingrowth of blood vessels and cells. A vascularized ectopic tissue engineered ligament (ETEL) with silk collagen scaffold was developed and then transferred to reconstruct the ACL in rabbits without interruption of perfusion. At 2weeks after ACL reconstruction, more well-perfused cells and vessels were found in the regenerated ACL with ETEL, which decreased dramatically at the 4 and 12week time points with collagen deposition and maturation. ACL treated with ETEL exhibited more mature ligament structure and enhanced ligament-bone healing post-reconstructive surgery at 4 and 12weeks, as compared with the control group. In addition, the ETEL group was demonstrated to have higher modulus and stiffness than the control group significantly at 12weeks post-reconstructive surgery. In conclusion, our results demonstrated that the ETEL can provide sufficient vascularity and cellularity during the early stages of healing, and subsequently promote ACL regeneration and ligament-bone healing, suggesting its clinic use as a promising therapeutic modality.

STATEMENT OF SIGNIFICANCE

Early inflammatory cell infiltration, tissue and vessels ingrowth were significantly higher in the extra-articular implanted scaffolds than theses in the joint cavity. By mimicking the early stages of wound repair, which provided extra-articular inflammatory stimulation to facilitate the early ingrowth of blood vessels and cells, a vascularized ectopic tissue engineered ligament (ETEL) with silk collagen scaffold was constructed by subcutaneous implantation for 2weeks. The fully vascularized TE ligament was then transferred to rebuild ACL without blood perfusion interruption, and was demonstrated to exhibit improved ACL regeneration, bone tunnel healing and mechanical properties.

A short discourse on vascular tissue engineering

Vascular tissue engineering has significant potential to make a major impact on a wide array of clinical problems. Continued progress in understanding basic vascular biology will be invaluable in making further advancements. Past and current achievements in tissue engineering of microvasculature to perfuse organ specific constructs, small vessels for dialysis grafts, and modified synthetic and pediatric large caliber-vessel grafts will be discussed. An emphasis will be placed on clinical trial results with small and large-caliber vessel grafts. Challenges to achieving engineered constructs that satisfy the physiologic, immunologic, and manufacturing demands of engineered vasculature will be explored.