Heart failure (HF) following myocardial infarction (MI) is the leading cause of mortality and morbidity worldwide. Existing medical and interventional therapies can only reduce the cardiomyocytes (CMs) lost during MI. They are unable to replenish the permanent loss of CMs and this contributes to progressive pathological left ventricular (LV) remodeling and HF. Cell-based therapies using adult stem cells or embryonic stem cells (ESCs) and their cardiac derivatives have frequently been explored as a potential therapeutic approach to restore cardiac function in HF. The objectives of this thesis are to evaluate the efficacy and safety of different approaches of stem cell based therapy to improve cardiac function using small and large animal MI models. In Chapter 3, we studied the functional consequences of direct intramyocardial transplantation of ESCs and ESC-derived cardiomyocytes (ESC-CMs) in a murine model of acute MI. LV ejection fraction (LVEF) and maximal positive or negative pressure derivative (dP/dt) improved 4 weeks after transplantation of either ESCs or ESC-CMs. Nevertheless there was a higher incidence of inducible ventricular tachyarrhythmia (VT) and higher mortality in animals transplanted with ESC-CMs than those with ESCs. At a single cell level, ESC-CMs exhibited immature electrophysiological properties such as depolarized resting membrane potential (RMP), longer action potential duration (APD) and automaticity. In Chapter 4, we tested the hypothesis that genetic modification of these immature electrophysiological properties of ESC-CMs by overexpression of Kir2.1 gene encoding the ion channels for IK1, may alleviate the pro-arrhythmic risk. In this study, Kir2.1 channels expression could be controlled with the administration of doxycycline (DOX). The DOX-treated ESC-CMs were more mature with hyperpolarized RMP and shorter APD than their counterparts without DOX treatment. A similar improvement in LV systolic function was observed 4 weeks after both DOX treated and untreated ESCCMs transplantation, although those animals transplanted with DOX-treated ESC-CMs had a significantly lower incidence of spontaneous and inducible VT. Histological analysis in both studies suggested that the major mechanisms of improvement in cardiac function were related to angiogenesis and low apoptosis rate of native cardiomyocytes mediated via paracrine effects. Importantly, very limited retention of ESC-CMs was observed 4 weeks after transplantation. Cell-based patches that use different bioengineering techniques have been proposed to improve cell retention and survival following transplantation. In Chapter 5, the efficacy of a passive epicardial patch was tested in a chronic large animal MI model with HF created with catheter-based coronary embolization. The implantation of an epicardical patch over the infarcted LV region was performed 8 weeks after MI in pigs with impaired LVEF. At week 20, pigs implanted with epicardical patches had significantly thicker LV wall thickness at the infarction sites, smaller LV dilation and better LV systolic function compared with control animals. The expression of MMP-9 was significant lower in the epicardical patch group at the peri-infarct zones. These findings suggested that a passive epicardial patch can improve LV function in HF and provides important proof-of-principle data to support its use as a platform for delivery of cell-based therapies after MI. / published_or_final_version / Medicine / Doctoral / Doctor of Philosophy