The cardiovascular system is the first organ system to form and function in the developing embryo. In the early stages the heart is a slow conducting, single tubular structure generating an even peristaltic contraction. During embryogenesis it then develops into a four chambered heart with synchronous contraction for a dual circulation. These synchronous contractions of the atrial and ventricular chambers are dependent on the development of high conduction velocities resulting in alternating slow and fast conducting segments. In the early embryonic heart tube, an ECG similar to that of an adult has been recorded. This indicates the electrical activity of sequentially activated heart chambers,¹ so demonstrating the functioning of the cardiac conduction system (CCS), before any morphological development can be distinguished. Throughout the developmental process the heart must maintain its rhythmical contractions through coordinated activation of the myocardium.

The components of this system include the sinoatrial (SA) node, the atrioventricular (AV) node, and the ventricular conduction pathway consisting of the bundle of His, the right bundle branch, the left bundle branch, and the Purkinje fibres.

THE SINOATRIAL NODE

In the adult the SA node is found at the junction of the right common cardinal vein and the wall of the right atrium, within the terminal groove. These cells have the most rapid inherent rhythm so setting the rate for the rest of the myocardium, and thereby functioning as the dominant pacemaker. This pacemaker activity has been illustrated in chick embryos at an age approximately equivalent to 20 days in the human embryo.² The pacemaker activity develops in the inflow …