Figure 2.

Phenotypes of the embryonic and adult heart (A). The embryonic myocytes in the early heart tube possess a phenotype typical for the conduction system with a high automaticity and a low conduction velocity, contractility, and sarcoplasmic reticulum activity. (B). The chambers balloon out from the initial heart tube and immediately initiate a fast conducting working myocardial phenotype. The myocardium at the venous pole, and the region interposed between the developing chambers, the atrioventricular canal, initially retains the conduction system phenotype and will form the cardiac conduction system. ra, right atria; la, left atria; oft, outflow tract; rv, right ventricle; lv, left ventricle.