The epicardium as a source of mesenchyme for the developing heart

Abstract

The primitive epicardium of the vertebrate embryo has traditionally been regarded as a rather passive mesothelium, lining the embryonic myocardium and forming the adult visceral pericardium. However, in recent years, there is an increasing evidence that the primitive epicardium is a highly dynamic element which supplies cells to the developing heart through a process of epithelial-mesenchymal transition. This process seems to be more active at the atrioventricular canal and outflow tract, i.e. the cardiac segments where the endothelium transforms into mesenchyme. In this paper we review the current evidence which supports such epicardial-mesenchymal transition, namely: 1) morphological features, 2) colocalization of cytokeratin and vimentin in the epicardial and subepicardial mesenchymal cells, 3) presence of common antigens in the transforming epicardium and endocardial cushions (fibrillin-2/JB3, ES/130, Ets-1). Recendy, we have immunolocated the transcription factor Slug in the developing avian heart. Slug is a zinc-finger protein involved in the formation of the neural crest, a developmental event which implies an epithelial-mesenchymal transition. All cells of the primitive epicardium are Slug+ from their differentiation until the stage HH24. However, only a fraction of the endothelial cells from the endocardial cushions are Slug+. We speculate that the expression of Slug marks competence of the epicardial cells to transform into mesenchyme, although this transformation is only achieved where an inducing signal is produced. Regarding the developmental fate of the epicardial-derived cell population, there is strong evidence of its differentiation in fibroblasts and vascular smooth muscle cells, although a contribution to the coronary endothelium cannot be discarded.