Immunolocalization of the vascular endothelial growth factor receptor-2 in the subepicardial mesenchyme of hamster embryos: identification of the coronary vessel precursors

The earliest evidence of the development of the cardiac vessels in mammals is the emergence of subepicardial blood islands, which are thought to originate from mesenchymal progenitors. In order to identify these progenitor cells, we have studied the immunohistochemical localization in the heart of Syrian hamster embryos of the type 2 vascular endothelial growth factor receptor, the earliest molecule known to be expressed in the vasculogenic cell lineage. Only a few immunoreactive subepicardial mesenchymal cells were present by 10 days post coitum. By 11 days post coitum, the subepicardial mesenchymal cells became abundant at the dorsal part of the ventricle, the atrioventricular and the conoventricular grooves. About 20% of cells were labelled with the antibody. Immunoreactive cells were isolated or formed pairs, short cords, rounded clusters or ring-like structures at the subepicardium or, occasionally, within the ventricular myocardium. Other labelled cells were simultaneously cytokeratin immunoreactive. By 12 days post coitum, most immunoreactive mesenchymal cells have been replaced by a capillary network. We propose that an active process of vascular differentiation occurs between 10 and 12 days post coitum in the subepicardium of this species, and it might be a suitable model for the study of vasculogenetic mechanisms.