Development of the Cardiac Conduction System

  • Moorman A
  • de Jong F
  • Denyn M
 et al. 
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Abstract

In the formed heart, it is convention to distinguish working myocardium (the primary function of which is contraction) from the conduction system (the primary function of which is the generation and conduction of the electrical impulse). The conduction system comprises separate components with distinct functions. The SAN, which contains the leading pacemaker, generates the impulse. The impulse is subsequently conducted, via the atrial myocardium, which in this sense is part of the conduction pathway as well, toward the AVN. With a delay, the impulse is then rapidly transmitted from the AVN via the bundle branches and PPN to ensure a coordinated activation of the ventricular myocardium from apex to base. Classic reports cover the anatomy,1 pathology,1 and histology2 of the adult and developing conduction system. The myocytes of the conduction system share with those of the ordinary working myocardium four basic elements: (1) contraction, (2) autorhythmicity, (3) intercellular conduction, and (4) electromechanical coupling. In the early embryonic heart tube, an ECG, similar to an adult ECG, can be recorded, indicating the presence of sequentially activated chambers.3 Given this observation, it is as confusing to accept the presence of a conduction system because it is functionally present as it is to deny its existence because it is not morphologically recognizable. Rather, it is of paramount importance to appreciate that the arrangement of myocyte populations, with distinct contractile, conductive, and pacemaking properties, establishes the coordinated activation of the heart. Departures from these tenets have led to a confusing and fruitless search for so-called “cardiac specialized tissues” during development. The obvious key question is how this arrangement is being achieved. Early cardiac development starts with the formation of a primary heart tube from the cardiogenic mesoderm (Fig 1); this topic has been reviewed recently.4 The primary heart tube is a peristaltic pump that moves blood ahead as a result of a unidirectional wave of contractions along the tube. Within this slow-conducting heart tube, fast-conducting and synchronously contracting atrial and ventricular chambers develop; these chambers remain flanked by the slow-conducting primary myocardium of the IFT, AVC, and OFT (Fig 2).5 The configuration of alternating slow- and fast-conducting segments guarantees that the downstream ventricular segment does not contract before the termination of the contraction of the upstream atrial segment and is responsible for the embryonic ECG. This configuration ensures also that relaxation of the atrial or ventricular segment does not occur before contraction of a downstream flanking segment, by which regurgitation of the blood is prevented. The sphincter-like prolonged peristaltic contraction form of the slow-conducting flanking segments substitutes for the adult type of one-way valves; this phenomenon is essential in a heart in which atrioventricular and semilunar valves have not yet developed. 6 The slow-conducting SAN and AVN will take origin from the slow-conducting myocardium of IFT and AVC, sometimes referred to as “cardiac specialized tissues.” The fast-conducting AVB, bundle branches, and their ramifications will develop from the ventricular segment. This building plan of early cardiac development in modules provides the framework to structure the anecdotal facts on the developing conduction system. Our aim here is to make evident that a consequent morphological, functional, and molecular description of the development of the cardiac tube and its constituting segments encompasses, of necessity, the “conduction system.”

Author-supplied keywords

  • cardiac development
  • conduction system
  • embryonic heart

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Authors

  • Antoon F. M. Moorman

  • Frits de Jong

  • Marylene M. F. J. Denyn

  • Wouter H. Lamers

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