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It is, perhaps, unfortunate that the myocytes that generate and dissipate the cardiac impulse throughout the myocardial body are known as the cardiac conduction tissues, because all myocytes within the heart possess the capacity to conduct. The alternative title, the "specialized tissues," is equally unfortunate, because from a developmental stance, as we will show, the so-called conducting tissues, in particular the nodal components, share important features with the myocytes of the primary heart tube. It is their so-called "working" partners that show more evidence of developmental specialization. In this chapter, therefore, we will describe the processes whereby the initial heart tube grows by addition of myocardium at both its venous and arterial pole, how it loops, and how eventually it becomes converted into the four-chambered organ. In this definitive structure, it is the sinus node that generates the cardiac impulse, this being propagated within the atrial musculature, delayed in the atrioventricular node, and then conducted rapidly to the ventricular myocardium through the atrioventricular bundle, its branches, and the ventricular ramifications known as Purkinje fibers. We will also discuss the fate of the more widespread areas of primary myocardium found in the developing heart because it is almost certainly the remnants of these areas in the postnatal heart that are the substrates for many cardiac arrhythmias. In contrast, with regard to perhaps the most important arrhythmia in the ageing population, namely atrial fibrillation, we will show how the pulmonary myocardial sleeves, known now to be the substrate for many forms of atrial fibrillation, are derived from so-called working rather than specialized conducting tissues.
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The overall structure of the developing heart and the basic arrangement of the definitive heart are preserved across the vertebrates, from fish to man. When first seen during development, the heart is laid down as a linear myocardial tube, with venous and arterial poles (Fig. 17–1A). Conduction through this linear tube begins at the venous pole and proceeds toward the arterial pole. With ongoing development, cavities balloon from the tube, to which additional material is rapidly added at both poles. In the most basic plan, the myocardium that will form the atrial chambers balloons from the primary tube into the dorsal direction, whereas the myocardium of the developing ventricles balloons in ventral fashion (Fig. 17–1B). Concomitant with the ballooning of these pouches from the linear, or primary, tube, it becomes possible to record an electrocardiogram.1 At these early stages, it is not possible to recognize any "conduction tissues" when assessing the cardiac structure histologically. It is possible, however, to distinguish the characteristics of the myocytes in the different parts of the developing heart according to their molecular signatures. Thus, the myocytes of the primary heart tube do not express the connexin proteins responsible for the formation of fast-conducting gap junctions.2 As a consequence, at this early stage, the blood is propelled through the developing heart in a sluggish, peristaltic fashion. In contrast, the myocytes forming ...