A molecular approach towards the understanding of early heart development: an emerging synthesis

In the past decade we have made an inventory of the changing three-dimensional patterns of expression of a number of key proteins involved in contraction, energy metabolism and conduction in developing and adult chicken, rat, bovine and human hearts. These integrated morphological and immunohistochemical studies were complemented with electrophysiological studies in developing chicken hearts and have resulted in a preliminary model of heart development, that explains how the embryonic heart can function without valves and without an atrioventricular conduction system that is indispensable for the adult heart. Cardiomyocyte-specific proteins are first expressed in the cardiogenic plate when 6 somites have developed, while electrical activity becomes detectable only slightly later. Development proceeds as follows: 1. Upon its formation 'primary' myocardium is characterised by anteroposterior gradients in gene expression. Therefore cardiogenesis resembles many other developmental processes in the embryo. It serves as source for endocardial cells and cells specialized in mechanical contraction and in impulse generation/conduction supporting the view that a single population of cells (the 'primary' myocardium) serves as a precursor for these distinct cell types. 2. 'Primary' myocardium is characterized by the expression of alpha and beta myosin, acetylcholinesterase and the absence of fast sodium channels and of connexin 43. It has a peristaltoid contraction form due to a relatively slow propagation of the impulse. 3. In the looping stage, two cardiac segments appear due to the development of atrial and ventricular working myocardium, that is characterized by the expression of either alpha or beta myosin, connexin 43, fast sodium channels, the disappearance of acetylcholinesterase and by a relatively fast conduction.2+ sinuatrial and atrioventricular nodes.