Abstract-Transplantation of a tissue-engineered heart muscle represents a novel experimental therapeutic paradigm for myocardial diseases. However, this strategy has been hampered by the lack of sources for human cardiomyocytes and by the scarce vasculature in the ischemic area limiting the engraftment and survival of the transplanted muscle. Beyond the necessity of endothelial capillaries for the delivery of oxygen and nutrients to the grafted muscle tissue, interactions between endothelial and cardiomyocyte cells may also play a key role in promoting cell survival and proliferation. In the present study, we describe the formation of synchronously contracting engineered human cardiac tissue derived from human embryonic stem cells containing endothelial vessel networks. The 3D muscle consisted of cardiomyocytes, endothelial cells (ECs), and embryonic fibroblasts (EmFs). The formed vessels were further stabilized by the presence of mural cells originating from the EmFs. The presence of EmFs decreased EC death and increased EC proliferation. Moreover, the presence of endothelial capillaries augmented cardiomyocyte proliferation and did not hamper cardiomyocyte orientation and alignment. Immunostaining, ultrastructural analysis (using transmission electron microscopy), RT-PCR, pharmacological, and confocal laser calcium imaging studies demonstrated the presence of cardiac-specific molecular, ultrastructural, and functional properties of the generated tissue constructs with synchronous activity mediated by action potential propagation through gap junctions. T he adult mammalian heart has limited regenerative capacity and therefore any significant myocardial cell loss is mostly irreversible and may lead to progressive loss of ventricular function and heart failure development. Despite the improvements in several pharmacological, interventional, and surgical therapeutic measures, the prognosis for heart failure patients remains poor. An attractive experimental solution to this significant medical problem may be to repopulate the damaged heart with new myogenic cells. Consequentially, myocardial cell replacement therapy has emerged as a novel experimental therapeutic paradigm aiming to improve the function of the failing heart. In general, 2 principal strategies were suggested: the first focused on direct transplantation of isolated cells into the dysfunctional myocardial areas, whereas the second attempted to combine ex vivo cells with polymeric scaffolds generating a tissueengineered muscle construct, followed by in vivo engraftment of the engineered muscle.Despite the encouraging results in several animal studies, clinical translation of these approaches have been hampered by the lack of sources for human cardiomyocytes and by the significant cell death following cell transplantation into the hostile ischemic myocardium. 1 The latter problem may be even aggravated following the transplantation of clinically relevant, thick tissue-engineered muscle. Insufficient graft vascularization is considered among the main fac...

Growing interest in using endothelial cells for therapeutic purposes has led to exploring human embryonic stem cells as a potential source for endothelial progenitor cells. Embryonic stem cells are advantageous when compared with other endothelial cell origins, due to their high proliferation capability, pluripotency, and low immunogenity. However, there are many challenges and obstacles to overcome before the vision of using embryonic endothelial progenitor cells in the clinic can be realized. Among these obstacles is the development of a productive method of isolating endothelial cells from human embryonic stem cells and elucidating their differentiation pathway. This review will focus on the endothelial potential of human embryonic stem cells that is de- Tissue vascularization and the clinical importance of endothelial progenitor cellsProgenitor endothelial cells (ECs) are promising key factors for many therapeutic applications. These applications include the following: cell transplantation for the repair of ischemic tissues, formation of blood vessels and heart valves, engineering of artificial vessels, repair of damaged vessels, and inducing the formation of blood vessel networks in engineered tissues. [1][2][3] Vascularization of engineered tissue in vitro before transplantation is essential for building complex and thick tissues because it enhances cell viability during tissue growth, induces structural organization, and promotes integration following implantation.Another area in which embryonic ECs can be beneficial is the study of human embryogenesis. In particular, ECs can serve as a model system for exploring central issues in human vasculogenesis and potentially elucidate vasculogenic and angiogenic mechanisms involved in the pathogenesis of vascular disease. Furthermore, it recently became evident that blood vessels do not just exchange metabolites between blood and tissue but play a more fundamental role in providing developmental cues to organs and differentiating cells. Early development of the pancreas depends on the presence of blood vessels, even in the absence of blood flow. [4][5][6] A similar dependence on ECs for the development of the liver and kidney has also been reported. 7,8 Angioblasts, or progenitor ECs, are associated with emerging buds of the embryonic lung and the nascent glandular portion of the stomach. 7 Studies of neuronal stem cell proliferation and differentiation lend further support to the view that organs must develop in proximity to blood vessels. 9 The potential overlap in signaling that occurs during neurogenesis and angiogenesis may even suggest that neurogenesis is regulated, in part, by an equilibrium between peripherally derived and centrally derived signaling molecules acting on both cell populations. Indeed, it has been shown that dividing cells in the mature hippocampus are immunoreactive for endothelial markers, thereby demonstrating the existence of neurogenesis within an angiogenic niche. 10 All these findings raise the possibility that endothelial signalin...