Tissue-Transplant Fusion and Vascularization of Myocardial Microtissues and Macrotissues Implanted into Chicken Embryos and Rats

Current approaches for the recreation of cardiac ischaemic environment in vitro

Myocardial ischaemia is one of the leading dead causes worldwide. Although animal experiments have historically provided a wealth of information, animal models are time and money consuming, and they usually miss typical human patient's characteristics associated with ischemia prevalence, including aging and comorbidities. Generating reliable in vitro models that recapitulate the human cardiac microenvironment during an ischaemic event can boost the development of new drugs and therapeutic strategies, as well as our understanding of the underlying cellular and molecular events, helping the optimization of therapeutic approaches prior to animal and clinical testing. Although several culture systems have emerged for the recreation of cardiac physiology, mimicking the features of an ischaemic heart tissue in vitro is challenging and certain aspects of the disease process remain poorly addressed. Here, current in vitro cardiac culture systems used for modelling cardiac ischaemia, from self-aggregated organoids to scaffold-based constructs and heart-on-chip platforms are described. The advantages of these models to recreate ischaemic hallmarks such as oxygen gradients, pathological alterations of mechanical strength or fibrotic responses are highlighted. The new models represent a step forward to be considered, but unfortunately, we are far away from recapitulating all complexity of the clinical situations.

Coculture with noncardiac cells promoted maturation of human stem cell-derived cardiomyocyte microtissues

Cardiomyocytes derived from human pluripotent stem cells (hPSC-CM) provided a promising cell source for cell therapy, drug screening, and disease modeling. However, hPSC-CM are immature and phenotypically more similar to fetal rather than adult cardiomyocytes in vitro. We explored the impact of coculture of human embryonic stem cell-derived mesenchymal stem cells (hESC-MSC) and endothelial cells (ECs) with human embryonic stem cells-derived cardiac progenitor cells (hESC-CPC) on the gene expression and electrophysiological properties of hESC-CPC in 3D culture (microtissue spheroid). In this regard, hESC-CPC were cultured either alone (CM microtissue) or in coculture with EC and hESC-MSC (CMEM microtissue) on agar-coated 96-well round-bottomed plates for 1 week. Lumen-like structures were formed in CMEM but not in CM microtissue. Cardiac progenitor markers (TBX5, GATA4) were downregulated and cardiac sarcomeric transcripts (MLC2v and β-MHC) were upregulated in CMEM compared with CM microtissue. Furthermore, beating frequencies, beating cycles, and field potential durations of CMEM resided in the range of adult cardiomyocytes rather than fetal like phenotypes observed in CM microtissue. These findings demonstrated that CPC spheroids in coculture with EC and hESC-MSC may undergo greater maturation toward an adult-like cardiomyocyte.

Human cardiomyocytes undergo enhanced maturation in embryonic stem cell-derived organoid transplants

Human cardiomyocytes (CM) differentiated from pluripotent stem cells (PSCs) are relatively immature when generated in two-dimensional (2D) in vitro cultures, which limits their biomedical applications. Here, we devised a strategy to enhance maturation of human CM in vitro by assembly of three-dimensional (3D) cardiac organoids (CO) containing human embryonic stem cell-derived cardiac progenitor cells (hESC-CPCs), endothelial cells (ECs), and mesenchymal stem cells (MSCs). In contrast to corresponding 2D cultures, 3D CO not only developed into structures containing spontaneously beating CM, but also showed enhanced maturity as indicated by increased expressions of sarcomere and ion channel genes and reduced proliferation. Heterotopic implantation of CO into the peritoneal cavity of immunodeficient mice induced neovascularization, and further stimulated upregulation of genes coding for the contractile apparatus, Ca²⁺ handling and ion channel proteins. In addition, CM in implanted CO were characterized by a more mature ultrastructure compared to CM implanted without CO support. Functional analysis revealed the presence of working cardiomyocytes in both in vivo and ex ovo chorioallantoic membrane implanted CO. Our results demonstrate that cultivation in 3D CO and subsequent heterotopic implantation enhance maturation of CM towards an adult-like phenotype. We reason that CO-derived CM represent an attractive source for drug discovery and other biomedical applications.

Directed fusion of cardiac spheroids into larger heterocellular microtissues enables investigation of cardiac action potential propagation via cardiac fibroblasts

Multicellular spheroids generated through cellular self-assembly provide cytoarchitectural complexities of native tissue including three-dimensionality, extensive cell-cell contacts, and appropriate cell-extracellular matrix interactions. They are increasingly suggested as building blocks for larger engineered tissues to achieve shapes, organization, heterogeneity, and other biomimetic complexities. Application of these tissue culture platforms is of particular importance in cardiac research as the myocardium is comprised of distinct but intermingled cell types. Here, we generated scaffold-free 3D cardiac microtissue spheroids comprised of cardiac myocytes (CMs) and/or cardiac fibroblasts (CFs) and used them as building blocks to form larger microtissues with different spatial distributions of CMs and CFs. Characterization of fusing homotypic and heterotypic spheroid pairs revealed an important influence of CFs on fusion kinetics, but most strikingly showed rapid fusion kinetics between heterotypic pairs consisting of one CF and one CM spheroid, indicating that CMs and CFs self-sort in vitro into the intermixed morphology found in the healthy myocardium. We then examined electrophysiological integration of fused homotypic and heterotypic microtissues by mapping action potential propagation. Heterocellular elongated microtissues which recapitulate the disproportionate CF spatial distribution seen in the infarcted myocardium showed that action potentials propagate through CF volumes albeit with significant delay. Complementary computational modeling revealed an important role of CF sodium currents and the spatial distribution of the CM-CF boundary in action potential conduction through CF volumes. Taken together, this study provides useful insights for the development of complex, heterocellular engineered 3D tissue constructs and their engraftment via tissue fusion and has implications for arrhythmogenesis in cardiac disease and repair.

Inspiration from heart development: Biomimetic development of functional human cardiac organoids

Recent progress in human organoids has provided 3D tissue systems to model human development, diseases, as well as develop cell delivery systems for regenerative therapies. While direct differentiation of human embryoid bodies holds great promise for cardiac organoid production, intramyocardial cell organization during heart development provides biological foundation to fabricate human cardiac organoids with defined cell types. Inspired by the intramyocardial organization events in coronary vasculogenesis, where a diverse, yet defined, mixture of cardiac cell types self-organizes into functional myocardium in the absence of blood flow, we have developed a defined method to produce scaffold-free human cardiac organoids that structurally and functionally resembled the lumenized vascular network in the developing myocardium, supported hiPSC-CM development and possessed fundamental cardiac tissue-level functions. In particular, this development-driven strategy offers a robust, tunable system to examine the contributions of individual cell types, matrix materials and additional factors for developmental insight, biomimetic matrix composition to advance biomaterial design, tissue/organ-level drug screening, and cell therapy for heart repair.

Cell number per spheroid and electrical conductivity of nanowires influence the function of silicon nanowired human cardiac spheroids

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) provide an unlimited cell source to treat cardiovascular diseases, the leading cause of death worldwide. However, current hiPSC-CMs retain an immature phenotype that leads to difficulties for integration with adult myocardium after transplantation. To address this, we recently utilized electrically conductive silicon nanowires (e-SiNWs) to facilitate self-assembly of hiPSC-CMs to form nanowired hiPSC cardiac spheroids. Our previous results showed addition of e-SiNWs effectively enhanced the functions of the cardiac spheroids and improved the cellular maturation of hiPSC-CMs. Here, we examined two important factors that can affect functions of the nanowired hiPSC cardiac spheroids: (1) cell number per spheroid (i.e., size of the spheroids), and (2) the electrical conductivity of the e-SiNWs. To examine the first factor, we prepared hiPSC cardiac spheroids with four different sizes by varying cell number per spheroid (∼0.5k, ∼1k, ∼3k, ∼7k cells/spheroid). Spheroids with ∼3k cells/spheroid was found to maximize the beneficial effects of the 3D spheroid microenvironment. This result was explained with a semi-quantitative theory that considers two competing factors: 1) the improved 3D cell-cell adhesion, and 2) the reduced oxygen supply to the center of spheroids with the increase of cell number. Also, the critical role of electrical conductivity of silicon nanowires has been confirmed in improving tissue function of hiPSC cardiac spheroids. These results lay down a solid foundation to develop suitable nanowired hiPSC cardiac spheroids as an innovative cell delivery system to treat cardiovascular diseases.

STATEMENT OF SIGNIFICANCE

Cardiovascular disease is the leading cause of death and disability worldwide. Due to the limited regenerative capacity of adult human hearts, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have received significant attention because they provide a patient specific cell source to regenerate damaged hearts. Despite the progress, current human hiPSC-CMs retain an immature phenotype that leads to difficulties for integration with adult myocardium after transplantation. To address this, we recently utilized electrically conductive silicon nanowires (e-SiNWs) to facilitate self-assembly of hiPSC-CMs to form nanowired hiPSC cardiac spheroids. Our previous results showed addition of e-SiNWs effectively enhanced the functions of the cardiac spheroids and improved the cellular maturation of hiPSC-CMs. In this manuscript, we examined the effects of two important factors on the functions of nanowired hiPSC cardiac spheroids: (1) cell number per spheroid (i.e., size of the spheroids), and (2) the electrical conductivity of the e-SiNWs. The results from these studies will allow for the development of suitable nanowired hiPSC cardiac spheroids to effectively deliver hiPSC-CMs for heart repair.

Microtissues in Cardiovascular Medicine: Regenerative Potential Based on a 3D Microenvironment

More people die annually from cardiovascular diseases than from any other cause. In particular, patients who suffer from myocardial infarction may be affected by ongoing adverse remodeling processes of the heart that may ultimately lead to heart failure. The introduction of stem and progenitor cell-based applications has raised substantial hope for reversing these processes and inducing cardiac regeneration. However, current stem cell therapies using single-cell suspensions have failed to demonstrate long-lasting efficacy due to the overall low retention rate after cell delivery to the myocardium. To overcome this obstacle, the concept of 3D cell culture techniques has been proposed to enhance therapeutic efficacy and cell engraftment based on the simulation of an in vivo-like microenvironment. Of great interest is the use of so-called microtissues or spheroids, which have evolved from their traditional role as in vitro models to their novel role as therapeutic agents. This review will provide an overview of the therapeutic potential of microtissues by addressing primarily cardiovascular regeneration. It will accentuate their advantages compared to other regenerative approaches and summarize the methods for generating clinically applicable microtissues. In addition, this review will illustrate the unique properties of the microenvironment within microtissues that makes them a promising next-generation therapeutic approach.

Development and Characterization of a Scaffold-Free 3D Spheroid Model of Induced Pluripotent Stem Cell-Derived Human Cardiomyocytes

Cardiomyocytes (CMs) are terminally differentiated cells in the adult heart, and ischemia and cardiotoxic compounds can lead to cell death and irreversible decline of cardiac function. As testing platforms, isolated organs and primary cells from rodents have been the standard in research and toxicology, but there is a need for better models that more faithfully recapitulate native human biology. Hence, a new in vitro model comprising the advantages of 3D cell culture and the availability of induced pluripotent stem cells (iPSCs) of human origin was developed and characterized. Human CMs derived from iPSCs were studied in standard 2D culture and as cardiac microtissues (MTs) formed in hanging drops. Two-dimensional cultures were examined using immunofluorescence microscopy and western blotting, while the cardiac MTs were subjected to immunofluorescence, contractility, and pharmacological investigations. iPSC-derived CMs in 2D culture showed well-formed myofibrils, cell-cell contacts positive for connexin-43, and other typical cardiac proteins. The cells reacted to prohypertrophic growth factors with a substantial increase in myofibrils and sarcomeric proteins. In hanging drop cultures, iPSC-derived CMs formed spheroidal MTs within 4 days, showing a homogeneous tissue structure with well-developed myofibrils extending throughout the whole spheroid without a necrotic core. MTs showed spontaneous contractions for more than 4 weeks that were recorded by optical motion tracking, sensitive to temperature and responsive to electrical pacing. Contractile pharmacology was tested with several agents known to modulate cardiac rate and viability. Calcium transients underlay the contractile activity and were also responsive to electrical stimulation, caffeine-induced Ca(2+) release, and extracellular calcium levels. A three-dimensional culture using iPSC-derived human CMs provides an organoid human-based cellular platform that is free of necrosis and recapitulates vital cardiac functionality, thereby providing a new and promising relevant model for the evaluation and development of new therapies and detection of cardiotoxicity.

In vitro analysis of scaffold-free prevascularized microtissue spheroids containing human dental pulp cells and endothelial cells

INTRODUCTION

Scaffolds often fail to mimic essential functions of the physiologic extracellular matrix (ECM) that regulates cell-cell communication in tissue microenvironments. The development of scaffold-free microtissues containing stem cell-derived ECM may serve as a successful alternative to the use of artificial scaffolds. The current study aimed to fabricate 3-dimensional microtissue spheroids of dental pulp cells (DPCs) prevascularized by human umbilical vein endothelial cells (HUVECs) and to characterize these scaffold-free spheroids for the in vitro formation of pulplike tissue constructs.

METHODS

Three-dimensional microtissue spheroids of DPC alone and DPC-HUVEC co-cultures were fabricated using agarose micro-molds. Cellular organization within the spheroids and cell viability (live/dead assay) were assessed at days 1, 7, and 14. Microtissue spheroids were allowed to self-assemble into macrotissues, induced for odontogenic differentiation (21 days), and examined for expression levels of osteo/odontogenic markers: alkaline phosphatase, bone sialoprotein and RUNX2 (Real-time PCR), mineralization (von-Kossa), and prevascularisation (immunohistochemistry for CD31).

RESULTS

The DPC microtissue microenvironment supported HUVEC survival and capillary network formation in the absence of a scaffolding material and external angiogenic stimulation. Immunohistochemical staining for CD31 showed the capillary network formed by HUVECs did sustain-for a prolonged period-even after the microtissues transformed into a macrotissue. Induced, prevascularized macrotissues showed enhanced differentiation capacity compared with DPC alone macrotissues, as shown by higher osteo/odontogenic gene expression levels and mineralization.

CONCLUSIONS

These findings provide insight into the complex intercellular cross talk occurring between DPCs and HUVECs in the context of angiogenesis and pulp regeneration and highlight the significance of developing a favorable 3-dimensional microenvironment that can, in turn, contribute toward successful pulp regeneration strategies.

The fusion of tissue spheroids attached to pre-stretched electrospun polyurethane scaffolds

Effective cell invasion into thick electrospun biomimetic scaffolds is an unsolved problem. One possible strategy to biofabricate tissue constructs of desirable thickness and material properties without the need for cell invasion is to use thin (<2 µm) porous electrospun meshes and self-assembling (capable of tissue fusion) tissue spheroids as building blocks. Pre-stretched electrospun meshes remained taut in cell culture and were able to support tissue spheroids with minimal deformation. We hypothesize that elastic electrospun scaffolds could be used as temporal support templates for rapid self-assembly of cell spheroids into higher order tissue structures, such as engineered vascular tissue. The aim of this study was to investigate how the attachment of tissue spheroids to pre-stretched polyurethane scaffolds may interfere with the tissue fusion process. Tissue spheroids attached, spread, and fused after being placed on pre-stretched polyurethane electrospun matrices and formed tissue constructs. Efforts to eliminate hole defects with fibrogenic tissue growth factor-β resulted in the increased synthesis of collagen and periostin and a dramatic reduction in hole size and number. In control experiments, tissue spheroids fuse on a non-adhesive hydrogel and form continuous tissue constructs without holes. Our data demonstrate that tissue spheroids attached to thin stretched elastic electrospun scaffolds have an interrupted tissue fusion process. The resulting tissue-engineered construct phenotype is a direct outcome of the delicate balance of the competing physical forces operating during the tissue fusion process at the interface of the pre-stretched elastic scaffold and the attached tissue spheroids. We have shown that with appropriate treatments, this process can be modulated, and thus, a thin pre-stretched elastic polyurethane electrospun scaffold could serve as a supporting template for rapid biofabrication of thick tissue-engineered constructs without the need for cell invasion.

Biomechanical regulation of in vitro cardiogenesis for tissue-engineered heart repair

The heart is a continuously pumping organ with an average lifespan of eight decades. It develops from the onset of embryonic cardiogenesis under biomechanical load, performs optimally within a defined range of hemodynamic load, and fails if acutely or chronically overloaded. Unloading of the heart leads to defective cardiogenesis in utero, but can also lead to a desired therapeutic outcome (for example, in patients with heart failure under left ventricular assist device therapy). In light of the well-documented relevance of mechanical loading for cardiac physiology and pathology, it is plausible that tissue engineers have integrated mechanical stimulation regimens into protocols for heart muscle construction. To achieve optimal results, physiological principles of beat-to-beat myocardial loading and unloading should be simulated. In addition, heart muscle engineering, in particular if based on pluripotent stem cell-derived cardiomyocytes, may benefit from staggered tonic loading protocols to simulate viscoelastic properties of the prenatal and postnatal myocardial stroma. This review will provide an overview of heart muscle mechanics, summarize observations on the role of mechanical loading for heart development and postnatal performance, and discuss how physiological loading regimens can be exploited to advance myocardial tissue engineering towards a therapeutic application.

The role of tissue engineering and biomaterials in cardiac regenerative medicine

In recent years, the development of 3-dimensional engineered heart tissue (EHT) has made large strides forward because of advances in stem cell biology, materials science, prevascularization strategies, and nanotechnology. As a result, the role of tissue engineering in cardiac regenerative medicine has become multifaceted as new applications become feasible. Cardiac tissue engineering has long been established to have the potential to partially or fully restore cardiac function after cardiac injury. However, EHTs may also serve as surrogate human cardiac tissue for drug-related toxicity screening. Cardiotoxicity remains a major cause of drug withdrawal in the pharmaceutical industry. Unsafe drugs reach the market because preclinical evaluation is insufficient to weed out cardiotoxic drugs in all their forms. Bioengineering methods could provide functional and mature human myocardial tissues, ie, physiologically relevant platforms, for screening the cardiotoxic effects of pharmaceutical agents and facilitate the discovery of new therapeutic agents. Finally, advances in induced pluripotent stem cells have made patient-specific EHTs possible, which opens up the possibility of personalized medicine. Herein, we give an overview of the present state of the art in cardiac tissue engineering, the challenges to the field, and future perspectives.

Cardiac Tissue Vascularization: From Angiogenesis to Microfluidic Blood Vessels

Myocardial infarction results from a blockage of a major coronary artery that shuts the delivery of oxygen and nutrients to a region of the myocardium, leading to massive cardiomyocytes death and regression of microvasculature. Growth factor and cell delivery methods have been attempted to revascularize the ischemic myocardium and prevent further cell death. Implantable cardiac tissue patches were engineered to directly revascularize as well as remuscularize the affected muscle. However, inadequate vascularization in vitro and in vivo limits the efficacy of these new treatment options. Breakthroughs in cardiac tissue vascularization will profoundly impact ischemic heart therapies. In this review, we discuss the full spectrum of vascularization approaches ranging from biological angiogenesis to microfluidic blood vessels as related to cardiac tissue engineering.

Cell therapy, 3D culture systems and tissue engineering for cardiac regeneration

Ischemic Heart Disease (IHD) still represents the "Number One Killer" worldwide accounting for the death of numerous patients. However the capacity for self-regeneration of the adult heart is very limited and the loss of cardiomyocytes in the infarcted heart leads to continuous adverse cardiac-remodeling which often leads to heart-failure (HF). The concept of regenerative medicine comprising cell-based therapies, bio-engineering technologies and hybrid solutions has been proposed as a promising next-generation approach to address IHD and HF. Numerous strategies are under investigation evaluating the potential of regenerative medicine on the failing myocardium including classical cell-therapy concepts, three-dimensional culture techniques and tissue-engineering approaches. While most of these regenerative strategies have shown great potential in experimental studies, the translation into a clinical setting has either been limited or too rapid leaving many key questions unanswered. This review summarizes the current state-of-the-art, important challenges and future research directions as to regenerative approaches addressing IHD and resulting HF.

Cardiac Tissue Engineering

The engineering of 3-dimensional (3D) heart muscles has undergone exciting progress for the past decade. Profound advances in human stem cell biology and technology, tissue engineering and material sciences, as well as prevascularization and in vitro assay technologies make the first clinical application of engineered cardiac tissues a realistic option and predict that cardiac tissue engineering techniques will find widespread use in the preclinical research and drug development in the near future. Tasks that need to be solved for this purpose include standardization of human myocyte production protocols, establishment of simple methods for the in vitro vascularization of 3D constructs and better maturation of myocytes, and, finally, thorough definition of the predictive value of these methods for preclinical safety pharmacology. The present article gives an overview of the present state of the art, bottlenecks, and perspectives of cardiac tissue engineering for cardiac repair and in vitro testing.

Rapid formation of multicellular spheroids in double-emulsion droplets with controllable microenvironment

An attractive option for tissue engineering is to use of multicellular spheroids as microtissues, particularly with stem cell spheroids. Conventional approaches of fabricating spheroids suffer from low throughput and polydispersity in size, and fail to supplement cues from extracellular matrix (ECM) for enhanced differentiation. In this study, we report the application of microfluidics-generated water-in-oil-in-water (w/o/w) double-emulsion (DE) droplets as pico-liter sized bioreactor for rapid cell assembly and well-controlled microenvironment for spheroid culture. Cells aggregated to form size-controllable (30–80 μm) spheroids in DE droplets within 150 min and could be retrieved via a droplet-releasing agent. Moreover, precursor hydrogel solution can be adopted as the inner phase to produce spheroid-encapsulated microgels after spheroid formation. As an example, the encapsulation of human mesenchymal stem cells (hMSC) spheroids in alginate and alginate-arginine-glycine-aspartic acid (-RGD) microgel was demonstrated, with enhanced osteogenic differentiation further exhibited in the latter case.

Grafts enriched with subamnion-cord-lining mesenchymal stem cell angiogenic spheroids induce post-ischemic myocardial revascularization and preserve cardiac function in failing rat hearts

A crucial question in post-ischemic cell therapy refers to the ideal method of cell delivery to the heart. We hypothesized that epicardial implantation of subamnion-cord-lining mesenchymal stem cells (CL-MSC) angiogenic spheroids embedded within fibrin grafts (SASG) facilitates donor cell survival and enhances cardiac function in failing rat hearts. Furthermore, we compared the efficacy of this approach applied through two delivery methods. Spheroids made of 1.5×10(4) human CL-MSC coated with 2×10(3) human umbilical vein endothelial cells were self-assembled in hanging drops. SASG were constructed by embedding 150 spheroids in fibrin matrix. Except for untreated rats (MI, n=8), grafts were implanted 2 weeks after myocardial infarction upon confirmation of ensued heart failure through thoracotomy: SASG (n=8) and fibrin graft (FG, n=8); or video-assisted thoracoscopic surgery (VATS): SASG-VATS (n=8) and FG-VATS (n=7). In vivo CL-MSC survival was comparable between both SASG-treated groups throughout the study. SASG and SASG-VATS animals had decreased left ventricular end-diastolic pressure relative to untreated animals, and increased fractional shortening compared to MI and FG controls, 4 weeks after treatment. A 14.1% and 6.2% enhancement in ejection fraction from week 2 to 6 after injury was observed in SASG/SASG-VATS, paralleled by improvement in cardiac output. Treated hearts had smaller scar size, and more blood vessels than MI, while donor CL-MSC contributed to arteriogenesis within the graft and infarct areas. Taken together, our data suggest that SASG treatment has the potential to restore failing hearts by preserving cardiac function and inducing myocardial revascularization, while attenuating cardiac fibrosis. Furthermore, we introduce a method for minimally invasive in situ graft assembly.

Modeling myocardial growth and hypertrophy in engineered heart muscle

The introduction of biomimetic culture paradigms has advanced myocardial tissue engineering fundamentally, enabling today the provision of engineered rodent and human heart muscle with features characteristically found in postnatal myocardium. This is in marked contrasts to "flat" cardiomyocyte cultures with their typically low degree of organotypic maturation. Here, we discuss the collagen hydrogel-based engineered heart muscle (EHM) technology and provide background information on its use in simulations of myocardial growth and disease.

Fate of modular cardiac tissue constructs in a syngeneic rat model

Modular cardiac tissues developed both vascular and cardiac structures in vivo, provided that the host response was attenuated by omitting xenoproteins from the modules. Collagen gel modules (with Matrigel(TM)) containing cardiomyocytes (CMs) alone or CMs with surface-seeded endothelial cells (ECs; CM/EC modules) were injected into the peri-infarct zone of the heart in syngeneic Lewis rats. After 3 weeks, donor ECs developed into blood vessel-like structures that also contained erythrocytes. However, no donor CMs were found within the implant sites, presumably because host cells including macrophages and T cells infiltrated extensively into the injection sites. To lessen the host response, Matrigel was omitted from the matrix and the modules were rinsed with serum-free medium prior to implantation. Host cell infiltration was attenuated, resulting in a higher degree of vascularization with CM/EC modules than with CM modules without ECs. Most importantly, donor CMs matured into striated muscle-like structures in Matrigel-free implants.

Human endothelial colony-forming cells expanded with an improved protocol are a useful endothelial cell source for scaffold-based tissue engineering

One of the major challenges in tissue engineering is to supply larger three-dimensional (3D) bioengineered tissue transplants with sufficient amounts of nutrients and oxygen and to allow metabolite removal. Consequently, artificial vascularization strategies of such transplants are desired. One strategy focuses on endothelial cells capable of initiating new vessel formation, which are settled on scaffolds commonly used in tissue engineering. A bottleneck in this strategy is to obtain sufficient amounts of endothelial cells, as they can be harvested only in small quantities directly from human tissues. Thus, protocols are required to expand appropriate cells in sufficient amounts without interfering with their capability to settle on scaffold materials and to initiate vessel formation. Here, we analysed whether umbilical cord blood (CB)-derived endothelial colony-forming cells (ECFCs) fulfil these requirements. In a first set of experiments, we showed that marginally expanded ECFCs settle and survive on different scaffold biomaterials. Next, we improved ECFC culture conditions and developed a protocol for ECFC expansion compatible with 'Good Manufacturing Practice' (GMP) standards. We replaced animal sera with human platelet lysates and used a novel type of tissue-culture ware. ECFCs cultured under the new conditions revealed significantly lower apoptosis and increased proliferation rates. Simultaneously, their viability was increased. Since extensively expanded ECFCs could still settle on scaffold biomaterials and were able to form tubular structures in Matrigel assays, we conclude that these ex vivo-expanded ECFCs are a novel, very potent cell source for scaffold-based tissue engineering.

Transcatheter based electromechanical mapping guided intramyocardial transplantation and in vivo tracking of human stem cell based three dimensional microtissues in the porcine heart

Stem cells have been repeatedly suggested for cardiac regeneration after myocardial infarction (MI). However, the low retention rate of single cell suspensions limits the efficacy of current therapy concepts so far. Taking advantage of three dimensional (3D) cellular self-assembly prior to transplantation may be beneficial to overcome these limitations. In this pilot study we investigate the principal feasibility of intramyocardial delivery of in-vitro generated stem cell-based 3D microtissues (3D-MTs) in a porcine model. 3D-MTs were generated from iron-oxide (MPIO) labeled human adipose-tissue derived mesenchymal stem cells (ATMSCs) using a modified hanging-drop method. Nine pigs (33 ± 2 kg) comprising seven healthy ones and two with chronic MI in the left ventricle (LV) anterior wall were included. The pigs underwent intramyocardial transplantation of 16 × 10(3) 3D-MTs (1250 cells/MT; accounting for 2 × 10(7) single ATMSCs) into the anterior wall of the healthy pigs (n = 7)/the MI border zone of the infarcted (n = 2) of the LV using a 3D NOGA electromechanical mapping guided, transcatheter based approach. Clinical follow-up (FU) was performed for up to five weeks and in-vivo cell-tracking was performed using serial magnet resonance imaging (MRI). Thereafter, the hearts were harvested and assessed by PCR and immunohistochemistry. Intramyocardial transplantation of human ATMSC based 3D-MTs was successful in eight animals (88.8%) while one pig (without MI) died during the electromechanical mapping due to sudden cardiac-arrest. During FU, no arrhythmogenic, embolic or neurological events occurred in the treated pigs. Serial MRI confirmed the intramyocardial presence of the 3D-MTs by detection of the intracellular iron-oxide MPIOs during FU. Intramyocardial retention of 3D-MTs was confirmed by PCR analysis and was further verified on histology and immunohistochemical analysis. The 3D-MTs appeared to be viable, integrated and showed an intact micro architecture. We demonstrate the principal feasibility and safety of intramyocardial transplantation of in-vitro generated stem cell-based 3D-MTs. Multimodal cell-tracking strategies comprising advanced imaging and in-vitro tools allow for in-vivo monitoring and post-mortem analysis of transplanted 3D-MTs. The concept of 3D cellular self-assembly represents a promising application format as a next generation technology for cell-based myocardial regeneration.

Endothelialization approaches for viable engineered tissues

One of the main limitation in obtaining thick, 3-dimensional viable engineered constructs is the inability to provide a sufficient and functional blood vessel system essential for the in vitro survival and the in vivo integration of the construct. Different strategies have been proposed to simulate the ingrowth of new blood vessels into engineered tissue, such as the use of growth factors, fabrication scaffold technologies, in vivo prevascularization and cell-based strategies, and it has been demonstrated that endothelial cells play a central role in the neovascularization process and in the control of blood vessel function. In particular, different "environmental" settings (origin, presence of supporting cells, biomaterial surface, presence of hemodynamic forces) strongly influence endothelial cell function, angiogenic potential and the in vivo formation of durable vessels. This review provides an overview of the different techniques developed so far for the vascularization of tissue-engineered constructs (with their advantages and pitfalls), focusing the attention on the recent development in the cell-based vascularization strategy and the in vivo applications.

3D microtissue formation of undifferentiated bone marrow mesenchymal stem cells leads to elevated apoptosis

Current implantation formats to deliver bone marrow-derived mesenchymal stem cells (MSCs) to the site of myocardial injury resulted only in limited cell retention and integration. As an alternative concept to single cell transplantation, we investigated the fate of cell tracker-labeled syngenic rat MSC microtissue implants, injected into the scar area in a chronic rat myocardial infarction model. Analysis of the explants after 2 and 7 days revealed substantial amounts of the cell tracker within the infarct region. However, the signal was associated with the extracellular matrix rather than with viable implanted cells. Following these results, we systematically evaluated the behavior of MSCs derived from mouse, rat, and human origin in the microtissue format in vitro. We found that MSC-composed microtissues of all three species displayed highly elevated levels of apoptotic activity and cell death. This effect could be attenuated by initiating osteogenic differentiation during the tissue formation process. We conclude that MSCs used for tissue regeneration undergo apoptosis in their new environment unless they get appropriate signals for differentiation that permit sustained survival. These findings may explain the limited cellular regeneration potential in current MSC-based clinical trials and may change therapeutic strategies away from pure, unmodulated cell delivery concepts.

A modular approach to cardiac tissue engineering

Functional cardiac tissue was prepared using a modular tissue engineering approach with the goal of creating vascularized tissue. Rat aortic endothelial cells (RAEC) were seeded onto submillimeter-sized modules made of type I bovine collagen supplemented with Matrigel™ (25% v/v) embedded with cardiomyocyte (CM)-enriched neonatal rat heart cells and assembled into a contractile, macroporous, sheet-like construct. Modules (without RAEC) cultured in 10% bovine serum (BS) were more contractile and responsive to external stimulus (lower excitation threshold, higher maximum capture rate, and greater en face fractional area changes) than modules cultured in 10% fetal BS. Incorporating 25% Matrigel in the matrix reduced the excitation threshold and increased the fractional area change relative to collagen only modules (without RAEC). A coculture medium, containing 10% BS, low Mg2+ (0.814mM), and normal glucose (5.5mM), was used to maintain RAEC junction morphology (VE-cadherin) and CM contractility, although the responsiveness of CM was attenuated with RAEC on the modules. Macroporous, sheet-like module constructs were assembled by partially immobilizing a layer of modules in alginate gel until day 8, with or without RAEC. RAEC/CM module sheets were electrically responsive; however, like modules with RAEC this responsiveness was attenuated relative to CM-only sheets. Muscle bundles coexpressing cardiac troponin I and connexin-43 were evident near the perimeter of modules and at intermodule junctions. These results suggest the potential of the modular approach as a platform for building vascularized cardiac tissue.

Strategies in cardiac tissue engineering

In heart failure, post-myocardial infarction and some congenital cardiac anomalies, organ transplantation is the only effective cure. Shortage of organ donors and complications of orthotopic heart transplant remain major challenges to the modern field of transplantation. Tissue engineering using cell-based strategies presents itself as a new way of generating functional myocardium. Engineering functional myocardium de novo requires an abundant source of cells that can form cardiomyocytes. These cells may be used with biocompatible scaffold materials to generate a contractile myocardium. Lastly, to sustain the high metabolism of the construct, a functional vasculature needs to be developed with the forming cardiac tissue. This review provides an update on the progress of stem cell research in the context of cardiac tissue development, types of biomaterials used in cardiac tissue engineering (CTE) and currently employed strategies for vascularization in CTE. In addition, a brief overview of strategies utilized in CTE is provided.

Proangiogenic scaffolds as functional templates for cardiac tissue engineering

We demonstrate here a cardiac tissue-engineering strategy addressing multicellular organization, integration into host myocardium, and directional cues to reconstruct the functional architecture of heart muscle. Microtemplating is used to shape poly(2-hydroxyethyl methacrylate-co-methacrylic acid) hydrogel into a tissue-engineering scaffold with architectures driving heart tissue integration. The construct contains parallel channels to organize cardiomyocyte bundles, supported by micrometer-sized, spherical, interconnected pores that enhance angiogenesis while reducing scarring. Surface-modified scaffolds were seeded with human ES cell-derived cardiomyocytes and cultured in vitro. Cardiomyocytes survived and proliferated for 2 wk in scaffolds, reaching adult heart densities. Cardiac implantation of acellular scaffolds with pore diameters of 30-40 microm showed angiogenesis and reduced fibrotic response, coinciding with a shift in macrophage phenotype toward the M2 state. This work establishes a foundation for spatially controlled cardiac tissue engineering by providing discrete compartments for cardiomyocytes and stroma in a scaffold that enhances vascularization and integration while controlling the inflammatory response.

Fate of endothelialized modular constructs implanted in an omental pouch in nude rats

Modular tissue engineering is a novel microscale approach that aims to assemble tissue constructs with inherent vascularization. We transplanted endothelialized modules (sub-millimeter-sized collagen gel cylinders covered with human umbilical vein endothelial cell [HUVEC] on the outside surface) in the omental pouch of nude rats to characterize remodeling of the collagen gels and the fate of the transplanted HUVEC. Endothelialized modules randomly assembled in vivo to form channels among individual modules that persisted for at least 14 days. Transplanted HUVEC migrated and formed primitive vessels in these channels; however, host inflammation limited HUVEC survival beyond 3 days. Temporary depletion of peritoneal macrophages (by treatment with clodronate liposomes) prolonged the survival of HUVEC-derived vessels to 7 days, and some vessels appeared to be perfused with host erythrocytes and invested with host vascular cells (either rat von Willebrand factor or smooth muscle alpha-actin-positive cells). Despite treatment, HUVEC were presumed to be still subject to immune rejection. The presence of primitive HUVEC-derived vessels is encouraging in this first in vivo study of the modular approach, in a partially immune-compromised animal model. It suggests that with appropriate attention to the host response to transplanted endothelial cells and improved vessel survival, cells that would be embedded in modules could be adequately perfused.

Myocardial tissue engineering: the quest for the ideal myocardial substitute

There has been an intense and competitive quest to manufacture bioartificial heart muscle in the last decade. Numerous biocompatible scaffolds and scaffold-free systems, enriched with various cell types, have been used to fabricate 3D grafts for myocardial repair. In spite of the impressive achievements in the myocardial tissue-engineering field, many issues remain to be addressed before clinical application of this strategy becomes feasible. This is largely due to the uniqueness of the heart's structure and function. This review provides a survey upon the reported strategies, and indicates caveats and perspectives in the field of myocardial tissue engineering.

Dissecting coronary angiogenesis: 3D co-culture of cardiomyocytes with endothelial or mesenchymal cells

In embryogenesis, coronary blood vessels are formed by vasculogenesis from epicardium-derived progenitors. Subsequently, growing or regenerating myocardium increases its vasculature by angiogenesis, forming new vessels from the pre-existing ones. Recently, cell therapies for myocardium ischemia that used different protocols have given promising results, using either extra-cardiac blood vessel cell progenitors or stimulating the cardiac angiogenesis. We have questioned whether cardiomyocytes could sustain both vasculogenesis and angiogenesis. We used a 3D culture model of tissue-like spheroids in co-cultures of cardiomyocytes supplemented either with endothelial cells or with bone marrow-derived mesenchymal stroma cells. Murine foetal cardiomyocytes introduced into non-adherent U-wells formed 3D contractile structures. They were coupled by gap junctions. Cardiomyocytes segregated inside the 3D structure into clumps separated by connective tissue septa, rich in fibronectin. Three vascular endothelial growth factor isoforms were produced (VEGF 120, 164 and 188). When co-cultured with human umbilical cord endothelial cells, vascular structures were produced in fibronectin-rich external layer and in radial septa, followed by angiogenic sprouting into the cardiomyocyte microtissue. Presence of vascular structures led to the maintenance of long-term survival and contractile capacity of cardiac microtissues. Conversely, bone marrow mesenchymal cells formed isolated cell aggregates, which progressively expressed the endothelial markers von Willebrand's antigen and CD31. They proceeded to typical vasculogenesis forming new blood vessels organised in radial pattern. Our results indicate that the in vitro 3D model of cardiomyocyte spheroids provides the two basic elements for formation of new blood vessels: fibronectin and VEGF. Within the myocardial environment, endothelial and mesenchymal cells can proceed to formation of new blood vessels either through angiogenesis or vasculogenesis, respectively.

Modulation of cardiomyocyte electrical properties using regulated bone morphogenetic protein-2 expression

Because cardiomyocytes lose their ability to divide after birth, any subsequent cell loss or dysfunction results in pathologic cardiac rhythm initiation or impulse conduction. Strategies to restore and control the electrophysiological activity of the heart may, therefore, greatly affect the regeneration of cardiac tissue functionality. Using lentivirus-derived particles to regulate the bone morphogenetic protein-2 (BMP-2) gene expression in a pristinamycin- or gaseous acetaldehyde-inducible manner, we demonstrated the adjustment of cardiomyocyte electrophysiological characteristics. Complementary metal oxide semiconductor-based high-density microelectrode arrays (HD-MEAs) were used to monitor the electrophysiological activity of neonatal rat cardiomyocytes (NRCs) cultured as monolayers (NRCml) or as microtissues (NRCmt). NRCmt more closely resembled heart tissue physiology than did NRCml and could be conveniently monitored using HD-MEAs because of their ability to detect low-signal events and to sub-select the region of interest, namely, areas where the microtissues were placed. Cardiomyocyte-forming microtissues, transduced using lentiviral vectors encoding BMP-2, were capable of restoring myocardial microtissue electrical activity. We also engineered NRCmt to functionally couple within a cardiomyocyte monolayer, thus showing pacemaker-like activity upon local regulation of transgenic BMP-2 expression. The controlled expression of therapeutic transgenes represents a crucial advance for clinical interventions and gene-function analysis.

Vascularization in tissue engineering

Tissue engineering has been an active field of research for several decades now. However, the amount of clinical applications in the field of tissue engineering is still limited. One of the current limitations of tissue engineering is its inability to provide sufficient blood supply in the initial phase after implantation. Insufficient vascularization can lead to improper cell integration or cell death in tissue-engineered constructs. This review will discuss the advantages and limitations of recent strategies aimed at enhancing the vascularization of tissue-engineered constructs. We will illustrate that combining the efforts of different research lines might be necessary to obtain optimal results in the field.

Adenovirus-mediated transduction of auto- and dual-regulated transgene expression in mammalian cells

Transduction of therapeutic transgenes using multiply attenuated viral vectors is considered an essential technology for gene therapy scenarios. While first-generation viral transduction systems were engineered for constitutive expression of a single therapeutic transgene, most advanced viral gene-transfer technologies enable regulated expression of several transgenes. Efficient transfer of numerous transgenes enables co-expression of therapeutic transgenes along with marker or selection determinants, production of multi-subunit protein complexes, or combinatorial expression of a particular set of genes to treat multigenic disorders. Likewise, adjustable transcription control is fundamental to adapt therapeutic protein production to the changing daily dosing regimes of a patient, to titrate expression of protein pharmaceuticals into the therapeutic window, and to reverse dosing upon completion of the therapy. Also, conditional transcription dosing has been successfully used for production of difficult-to-express protein therapeutics in biopharmaceutical manufacturing and for sophisticated gene-function analysis in basic research programs. By way of example, we provide detailed design (auto-regulated and binary dual-regulated expression configurations), production (generation, purification, and quality control of transgenic adenovirus particles), and handling (transduction) protocols for adenovirus vectors that enable transduction of mammalian cells for regulated expression of several transgenes.

Tissue engineering by self-assembly of cells printed into topologically defined structures

Understanding the principles of biological self-assembly is indispensable for developing efficient strategies to build living tissues and organs. We exploit the self-organizing capacity of cells and tissues to construct functional living structures of prescribed shape. In our technology, multicellular spheroids (bio-ink particles) are placed into biocompatible environment (bio-paper) by the use of a three-dimensional delivery device (bio-printer). Our approach mimics early morphogenesis and is based on the realization that the genetic control of developmental patterning through self-assembly involves physical mechanisms. Three-dimensional tissue structures are formed through the postprinting fusion of the bio-ink particles, in analogy with early structure-forming processes in the embryo that utilize the apparent liquid-like behavior of tissues composed of motile and adhesive cells. We modeled the process of self-assembly by fusion of bio-ink particles, and employed this novel technology to print extended cellular structures of various shapes. Functionality was tested on cardiac constructs built from embryonic cardiac and endothelial cells. The postprinting self-assembly of bio-ink particles resulted in synchronously beating solid tissue blocks, showing signs of early vascularization, with the endothelial cells organized into vessel-like conduits.

Organ printing: promises and challenges

Organ printing or biomedical application of rapid prototyping, also defined as additive layer-by-layer biomanufacturing, is an emerging transforming technology that has potential for surpassing traditional solid scaffold-based tissue engineering. Organ printing has certain advantages: it is an automated approach that offers a pathway for scalable reproducible mass production of tissue engineered products; it allows a precised simultaneous 3D positioning of several cell types; it enables creation tissue with a high level of cell density; it can solve the problem of vascularization in thick tissue constructs; finally, organ printing can be done in situ. The ultimate goal of organ-printing technology is to fabricate 3D vascularized functional living human organs suitable for clinical implantation. The main practical outcomes of organ-printing technology are industrial scalable robotic biofabrication of complex human tissues and organs, automated tissue-based in vitro assays for clinical diagnostics, drug discovery and drug toxicity, and complex in vitro models of human diseases. This article describes conceptual framework and recent developments in organ-printing technology, outlines main technological barriers and challenges, and presents potential future practical applications.

Design and fabrication of sub-mm-sized modules containing encapsulated cells for modular tissue engineering

We have proposed modular tissue engineering as a strategy to construct vascularized tissues containing multiple cell types. To create a modular construct, instead of seeding a preformed scaffold, cells were encapsulated within sub-mm modules, and the outer surface of these modules was covered with a layer of endothelial cells. Modules were then added to a larger structure (here by filling a tube) to form the modular construct. Through a systematic process of materials selection, collagen, human umbilical vein endothelial cells (HUVECs), and HepG2 cells, a human hepatoma cell line, were identified as suitable components for module formation, at least for initial studies. A method, which involved cutting and shaping the modules within a tubular mold, was developed to fabricate sub-mm, cylindrical, collagen modules that contained viable, functioning HepG2 cells and that could be seeded with a surface layer of HUVECs. Module dimensions were reproducible and easily altered in a controlled fashion if desired. The module fabrication process developed here not only generated modules suitable for the assembly of a prototype modular construct, but also could potentially be used more generally for other applications for which the goal is to form submm-diameter cylinders from soft hydrogels.