Development of the Cardiovascular System in Embryoid Bodies Derived from Embryonic Stem Cells

Abstract

The vertebrate heart and circulatory system is the first embryonic organ to develop from cells in the embryoid body. Understanding how this happens has been examined through the use of reporter genes by expression of green fluorescent protein under the control of cardiac- and endothelial cell—specific promoters. The cardiac primordial may be recognized as bilaterally symmetric strands of mesenchymal cells derived from the lateral plate mesoderm, which forms separate layers of myocardium and endocardium separated by extracellular matrix (cardiac jelly). The endoderm derives from ingressing cells during gastrulation and provides signals (factors such as bone morphogenetic protein-2 (BMP-2), fibroblast growth factor-1,2,4 (FGF-1,2,4)) that influence the adjacent mesoderm. The endocardium is in turn influenced by transforming growth factor-β and vascular endothelial growth factor (VEGF). Other factors such as cardiotrophin-1 and leukemia inhibitory factor also control cardiac development. First there is a simple linear heart tube, which goes on to form modular elements (atria, ventricles, septa, and valves). By folding on itself and fusing, the four-chambered heart is formed. These structural events are triggered by specific signaling molecules and involve reactive oxygen species and characteristic action potentials correlated with specialized types of ion channels, as well as a developmentally controlled expression pattern of the cardiac-specific genes encoding atrial natriuretic factor and sarcomeric proteins (i.e., α- and β-cardiac myosin heavy chain, myosin light chain isoform 2V, titin [Z-disk], titin [M-band], a-actinin, myomesin, sarcomeric a-actin, and troponin T), followed by M-protein. Whereas in early stage embryonic stem (ES) cell—derived cardiomyocytes cell contraction is triggered by Ca2+ transients arising from intracellular Ca2+ stores, contraction of terminally differentiated cardiac cells is dependent on an evoked action potential leading to opening of L-type voltage-dependent Ca2+ channels that appear several days before “beating” of cardiomyocytes. The vascular structures of the embryo develop from angioblasts in the paraaxial and lateral plate mesoderm as well as in the yolk sac extraembryonic mesoderm, where they form the outer layer of blood islands. Vasculogenesis and angiogenesis are strictly regulated by the pericellular oxygen tension of the tissue through hypoxia inducible factor-1. Many of the genes involved in cardiovascular differentiation are directly or indirectly regulated by hypoxia, with VEGF being a principle factor required for vasculogenesis. Therapeutic use of ES cells would be greatly enhanced if we knew how to direct ES cells to specific cell lineages in the cardiovascular or other organ systems.