Molecular anatomy of the developing heart.

Why the embryonic heart can function without one-way valves and without a conduction system, and why the embryonic atrium and ventricle function essentially as they do in the adult, are elementary questions in cardiac embryology. These questions can now be addressed, thanks to the recent development of molecular cell markers, their application by immunohistochemistry and/or in situ hybridization, and the development of advanced computer-aided three-dimensional reconstruction programs.