Development of the Conduction System

Abstract

In the mature heart, the conduction system plays a major role in determining the normal wave patterns of the ECG. Embryos with a four-chambered heart and an immature conduction system produce similar wave patterns. Cardiac electrical activity has been recorded from chick embryos as early as stage 24 (Hoff et al. 1939; Hamburger and Hamilton 1951) and stage 16 (Seidl et al. 1981) (Fig. 85), when no morphologically definable conduction system is present and the outward appearance of the heart is different from the adult. Perhaps the most significant result of electrocardiographic studies is the demonstration of an atrio-ventricular (AV) conduction delay as early as the 20-somite stage in the chick embryo (Hoff et al. 1939). This AV delay may be related to the development of specialised ring tissue (Anderson and Taylor 1972; Wenink 1976; Anderson et al. 1976). Alternatively, conduction delays may be due not to intrinsic properties of cell membranes, but to the effects of tissue geometry and cellular arrangement, which have been demonstrated to affect impulse propagation and action potential configuration in myocardium (Van Capelle and Janse 1976; Spach 1982; Spach and Kootsey 1983). The constriction of the AV canal first appears in the embryo of 19 somites (Hamburger and Hamilton 1951) and may be implicated in providing sufficient local alterations in cell relationships to affect conduction at the 20-somite stage.