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Individual compartments of cardiac conduction system (CCS) became functional in order which correlates with cardiac morphogenesis. Ventricular CCS compartments mature with ventricular septation and it is accompanied by shift of activation pattern from primitive base to apex, which follows blood flow, to advanced apex to base. There are some important differences between avian and murine CCS. From the functional point of view, the main contrast is the appearance of mature activation sequence well before time of ventricular septation completion (embryonic day [ED] 13.5 in mouse) and also that there was never reported activation originating from the base of heart.Function of CCS was studied by optical mapping and monitored parameters were speed of electrical impulse propagation, location first activation site on ventricular epicardial surface together with direction of action potential spread [evaluated as activation patterns, namely activation utilizing primary ring, left and right apical breakthroughs, only right or only left apical breakthrough - corresponding from ED14.5 with left and right bundle branches, respectively].By measurement of time necessary for activation of the left ventricle from ED9.5 to ED18.5 we observed remarkable acceleration between ED9.5 and ED11.5 where activation time dropped to a half. This was due to decrease in frequency of primary ring activation pattern with a slow speed of action potential propagation compared to activation from apex to base. Primary ring is a primitive, temporary preferential activation pathway located in future interventricular septum and was the most common activation pattern for ED9.5 and 10.5; at ED11.5 one third of hearts was activated through primary ring and at ED12.5 it was recorded only sporadically. Activation from apex to base appeared from ED9.5, where it first originated from the primitive left ventricle; for mature activation originating from apex in the later stages was the most typical activation from left and right or only right apical breakthrough (since ED11.5). Appearance of these apical breakthrough sites correlated with expression of Cx40 in ventricular trabeculae. This analysis of normal development of CCS was useful for interpretation of changes observed in Cx40 deficient mice as well as in other transgenic strains with CCS phenotype.