Epithelial to mesenchymal transition (EMT) is an essential process during embryogenesis for the development of organ systems, including the heart and its vasculature. The development of both coronary vessels and heart valves depends on EMT. In this dissertation, we first present data demonstrating that increasedoligosaccharide hyaluronan (o-HA) levels after EMT induction within atrioventricular (AV) valves leads to a decrease in EMT due to the induction of VEGF expression. Regulated EMT inhibition prevents the formation of hyperplastic valves. Next, we show that the proepicardium, which provides the precursor cells required for epicardial and coronary vessel development, migrates to the developing heart via direct contact of multicellular proepicardial villi to the developing myocardium. This shifts the paradigm from a migration consisting of floating cysts to one of direct contact and differential adhesion forces to form the initial epicardium. A subset of epicardial cells undergoes EMT, migrates into the developing heart, and differentiates into cardiac fibroblast, vascular endothelial, and smooth muscle cells. In order to more effectively study epicardial EMT in vitro, we developed several new methods for the in vitro study of coronary vessel development. We developed an improved protocol for isolating embryonic myocyte cells, for use in co-cultures with epicardial cells. This co-culture system allows investigation into the effects of myocyte derived soluble factors uponepicardial EMT and mesenchymal cell differentiation. We also present a protocol for isolating epicardial clonal colonies from an epicardial cell line derived from the ImmortoMouse. These clones provided direct evidence that the epicardium is a heterogeneous population of cells. These unique clones allow for to study into specific epicardial cell lineages and phenotypes. Finally, we provide data defining the expression of Wnts within the developing heart and the role may play during epicardial EMT. We conclude that canonical Wnts are both necessary and sufficient to inhibit epicardial EMT. These results provide the first direct evidence for a role of Wnt proteins during coronary vessel development. Collectively our results provide significant advancements in our understanding of EMT regulation during cardiac development.

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