Cardiac development is a complex developmental process that results in the formation of the four-chambered organ from a single linear heart tube. In the early stages of development, the linear heart tube is composed of only two tissue layers, an external myocardium that is internally lined by the endocardium. Subsequently, the proepicardium emerges at the septum transversum and soon thereafter proepicardial cells will migrate into the naked myocardium, leading to the formation of the embryonic epicardium. As cardiac development proceeds, epicardial-derived cells migrate into the subepicardial space and subsequently invade the developing ventricular chambers leading to the contribution of distinct cardiovascular cell types such as the cardiac fibroskeleton and different components of coronary vasculature. At present, the molecular mechanisms that regulate the transitional process from the proepicardium to the embryonic myocardium are largely unexplored. In this study we have implemented an ex vivo proepicardium/septum transversum (PE/ST)-embryonic myocardium explant model and we demonstrated that miR-223, but not miR-195, is capable of modulating PE/ST migration, a process that seems to be mediated by Slug expression. Thus, our study demonstrates for the first time the implication of distinct microRNAs in the PE/ST to embryonic myocardium transition in chicken embryonic hearts.