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.

The embryonic epicardium originates from the proepicardium, an extracardiac primordium constituted by a cluster of mesothelial cells. In early embryos, the embryonic epicardium is characterized by a squamous cell epithelium resting on the myocardium surface. Subsequently, it invades the subepicardial space and thereafter the embryonic myocardium by means of epithelial-mesenchymal transition. Within the myocardium, epicardial-derived cells present multilineage potential, later differentiating into smooth muscle cells and contributing both to coronary vasculature and cardiac fibroblasts in the mature heart. Over the last decades, we have progressively increased our understanding of those cellular and molecular mechanisms driving proepicardial/embryonic epicardium formation. This study provides a state-of-the-art review of the transcriptional and emerging post-transcriptional mechanisms involved in the formation and differentiation of the embryonic epicardium.

Except for a few species, the outer shape of vertebrates normally is characterized by bilateral symmetry. The inner organs, on the other hand, normally are arranged in bilaterally asymmetric patterns, which are of special importance for the normal function of the cardiovascular system of lung-breathing vertebrates. Deviations from the normal organ asymmetry can occur in the form of mirror imagery of the normal arrangement (situs inversus), or in the form of arrangements that have the tendency for development of bilateral symmetry, either in a pattern of bilateral left-sideness (left isomerism) or bilateral right-sidedness (right isomerism). The latter two forms of visceral situs anomalies are called “heterotaxy syndromes”. During the past 30 years, remarkable progress has been made in uncovering of the genetic etiology of heterotaxy syndromes. However, the pathogenetic mechanisms causing the spectrum of cardiovascular defects found in these syndromes remain poorly understood. In the present report, a spontaneous case of left cardiac isomerism found in a HH-stage 23 chick embryo is described. The observations made in this case suggest that hearts with left cardiac isomerism may have the tendency for development of a non-compaction cardiomyopathy caused by defective development of the proepicardium.