Downregulation of atrial markers during cardiac chamber morphogenesis is irreversible in murine embryos

Vertebrate cardiogenesis is a complex process involving multiple, distinct tissue types which interact to form a four-chambered heart. Molecules have been identified whose expression patterns co-segregate with the maturation of the atrial and ventricular muscle cell lineages. It is not currently kno wn what role intrinsic events versus external influences play in cardiac ch amber morphogenesis, We developed novel, fluorescent-based, myocardial, cel lular transplantation systems in order to study these questions in murine e mbryos and report the irreversible nature of chamber specification with res pect to the downregulation of atrial myosin light chain 2 (MLC-2a) and alph a myosin heavy chain (alpha-MHC). Grafting ventricular cells into the atria l chamber does not result in upregulation of MLC-2a expression in ventricul ar cells. Additionally, wild-type ventricular muscle cells grafted into the wild-type background appropriately downregulate MLC-2a and alpha-MHC. Fina lly, grafting of RXR alpha gene-deficient ventricular muscle cells into the ventricular chambers of wild-type embryos does not rescue the persistent e xpression of MLC-2a, providing further evidence that ventricular chamber ma turation is an early event. These studies provide a new approach for the me chanistic dissection of critical signaling events during cardiac chamber gr owth, maturation and morphogenesis in the mouse, and should find utility wi th other approaches of cellular transplantation in murine embryos. These ex periments document the irreversible nature of the downregulation of atrial markers after the onset of cardiogenesis during ventricular chamber morphog enesis and temporally define the response of cardiac muscle cells to signal s regulating chamber specification.