Vasculogenesis of the Embryonic Heart: Contribution of Nucleated Red Blood Cells to Early Vascular Structures

Abstract

During embryogenesis, coronary vessels develop via vasculogenesis and angiogenesis. Vasculogenesis is formation in situ of primary vessels from angioblasts - endothelial cell progenitors, and angiogenesis is formation of vessels from the existing ones. In the embryonic heart vasculogenesis precedes and overlaps angiogenesis and lasts till the end of the fetal life. What is unique about heart vasculogenesis is the fact that nucleated blood cells accompany early angioblasts in a spatiotemporal way. Morphologically these structures resemble yolk sac blood islands, thus, they have been called blood-island-like structures. In addition, these early vascular structures (blood-island-like) are found in the heart before coronary vessel system connects with the systemic circulation. We present the recent data regarding endothelial cell properties and derivation during coronary vessel formation and hypotheses concerning a source of blood cells in early vascular structures of the heart; the latter has received little attention in the literature. This review summarizes current knowledge on the endothelial cell origination from epicardial mesothelium or liver primordium. This review also focuses on blood cell contribution to coronary vessel vasculogenesis. The role of proepicardium in the epicardial cover formation and the epicardium as a source of cellular components of coronary vasculature and interstitial fibroblasts is presented. It seems that blood cells and angioblasts, which form the early vascular structures do not derive from the same hemangioblastic precursor.