Development of the epicardium is critical to proper heart formation.
It provides all of the precursor cells that form the coronary system and
supplies signals that stimulate cardiac myocyte proliferation. The
epicardium forms from mesothelial cells associated with the septum
transversum and is referred to as the proepicardium (PE). Two different
methods by which these PE cells colonize the developing heart have been
described. In avians, PE cells form a bridge to the heart over which PE
cells migrate onto the heart. In fish and mammals, PE cells form vesicles
of cells that detach from the mesothelium, float through the pericardial
cavity, and attach to the heart. A previous study of rat PE development
investigated this process at the histological level. Protein markers have
been developed since this study. Thus, we investigated this important
developmental process coupled with these new markers using other
visualization techniques such as scanning electron microscopy (SEM) and
confocal microscopy. Finally, a novel, three-dimensional (3-D) culture
system was used to confirm the identity of the PE cells. In this study, we
found convincing evidence that the rat PE cells directly attach to the
heart in a manner similar to that observed in avians.