Origin and development of the epicardium in the mouse embryo.

The formation of the epicardium was investigated in the mouse embryo using scanning electron microscopy (SEM) in order to establish a three-dimensional perspective concerning epicardial development in mammals. The epicardium first appears as aggregates of cells scattered on the caudal surface of the ventricle and atria where these regions face the septum transversum in a 9-day-old embryo. These aggregated cells seem to have originated from the mesothelial projections extending from the surface of the septum transversum. Then, the cells of each aggregate flatten, subsequently fusing with each other to form a continuous sheet of epicardium. The fusion of aggregates proceeds in a cranial direction. Finally, the bulbus cordis and truncus arteriosus become invested by migrating cells at the cranial end of the epicardial sheet about 11 days after fertilization. The present observations are discussed in comparison with those made previously in avian embryos.