Abstract
Two extracardiac sources, the cardiac neural crest and proepicardial organ (PEO), provide cells that are essential for cardiac morphogenesis. Cardiac neural crest cells generate a range of cell types, including both neuronal and non-neuronal adventitial cells and smooth muscle. Endothelin (ET) signalling plays an important role in the development of cardiac neural crest cell lineages, yet the mechanism(s) by which ET may act to control this development remain largely unclear. Analysis of the expression of ET signalling components revealed dynamic patterns of expression, suggesting that only a subpopulation of neural crest cells in particular locations are responsive to ET signaling at specific developmental time-points. Constitutive expression of preproET-1, a precursor of mature ET-1 ligand, in the cardiac neural crest resulted in a selective expansion of the adventitial cell population in the great vessels of the heart, at least partially due to increased cell survival. This suggests that localized ET production acts as a spatial regulator to control the development of a specific subset of cardiac neural crest cells. To address the issue of the spatial and temporal segregation of cardiac neural crest lineages, we have constructed an alkaline-phosphatase-expressing retroviral vector for future cell tracing studies. We also examined the role of the PEO-derived epicardium in development and patterning of the myocardium. Animal models lacking an epicardium show cardiac abnormalities, most notably an abnormally thin myocardial wall. We generated epicardium-deficient hearts to characterize the cell biological effects on the compact ventricular myocardium. Analysis of myocyte proliferation revealed that the transmural gradient of myocyte proliferation across the myocardial wall was maintained in the absence of the epicardium. However, a moderate decrease in the rate of myocyte proliferation in the compact myocardium appears sufficient to generate the thin myocardial wall. This data indicates that myocardial proliferation can occur independently of signals from the epicardium. However, the presence of the epicardium is essential for normal myocardial development and epicardial signaling acts primarily to exert subtle yet essential influences over myocyte proliferation.