A A PILLAI, A HANDA, V BALASUBRAMANIAN, SARANYA GOUSY In the embryonic heart, the normal atrial septum, along with the surrounding atrial structures, is formed from several embryological tissue components that develop, remodel and fuse in the correct sequence. The development of the atrial septum occurs following the initial looping of the heart. As the initial step in septation, a ridge of tissue develops from the superior aspect of the primary atrial component of the heart tube. This ridge is the primary septum (septum primum), and its leading edge is covered by cushion-like mesenchymal tissue that is continuous over the dorsal mesocardium. This dividing crest of tissue is part of the atrial chamber expressing genes demonstrating morphologically leftness.1 As the atrial septum grows into the cavity, it extends down towards the endocardial cushions which are developing concomitantly within the atrioventricular (AV) canal. Normal septal development also involves incorporation of another mass of tissue derived from the dorsal mesocardium, known as the vestibular spine (spina vestibuli), and it, too, carries on its leading edge a mesenchymal cap. As the primary septum approaches the atrioventricular endocardial cushions, the various mesenchymal structures fuse together.2 The spina vestibuli then muscularises, eventually forming the prominent infero-anterior border of the oval foramen. During the process of development the ventricular septum also ‘moves’ up towards the endocardial cushions, resulting in the septation of the ventricular chambers.3 Subsequent to the fusion between the primary septum and the endocardial cushions of the atrioventricular canal, the upper part of the primary septum disintegrates to form the ostium secundum. The remaining part of the primary septum becomes the flap valve of the fossa ovalis. The flap valve of the fossa ovalis, along with the muscularized antero-inferior rim, forms the true septum that separates the cavities of the atrial chambers. Only after integration of the pulmonary veins into the left atrium, the superior walls of the two atriums ‘infold’, creating the septum secundum in the superior portion of the atriums.2 The flap valve overlaps, but is not completely adherent to, the rims of this superior atrial fold, known as the Waterston’s or Sondergaard’s groove, providing a passage for blood to pass from the right to the left atrium during foetal life (Figure 3.1). In postnatal life, this deep superior interatrial fold becomes filled with extracardiac fibro-fatty tissue2 (Figure 3.2). Figure 3.1 Diagrammatic representation viewing the oval fossa from the right atrial aspect. The oval fossa flap valve and the immediate rim is the true extent of the atrial septum. (Illustration by Saranya Gousy.) Figure 3.2 Diagrammatic representation of the heart in four-chamber section. The flap valve overlaps the superiorly infolded walls of the atriums, partitioning the two chambers. (Illustration by Saranya Gousy.) The atrial septum is best defined as the tissue which directly separates the atrial cavities, and which can be removed without excising walls or valves of the heart. When viewed in the light of this definition, the septum is confined to the thin flap of fibromuscular valvar tissue which forms the floor of the oval fossa, along with the immediate infero-anterior muscular rims of the fossa derived from the vestibular spine (Figure 3.3). Figure 3.3 Four-chamber view of the RA and RV, showing the muscular superior and inferior rims and the flap valve of the oval fossa dividing the two atriums. (Illustration by Saranya Gousy.) Thus, the septal area is only a small part of the wall dividing the atriums. The aortic mound, in contrast, which is to the right of the oval fossa when observed from the right atrial aspect, and which seems to represent an apparently solid muscular structure, is part of the external wall of the heart. Passage of an instrument through this area does not take one into the left atrium but rather into the transverse sinus of the pericardial cavity, in front of the bulging right coronary sinus at the base of the aortic root (Figure 3.4). Similarly, passing a needle from the right to the left atrial chambers superiorly to the oval fossa also passes through extracardiac tissue, as this part of the dividing wall is in reality an infolding of the right atrial wall and hence is not septal (Figure 3.5). In adult life, this fold is filled with fatty extracardiac tissue. It is within this area that the sinus nodal artery usually takes its course (Figure 3.6
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Embryology and anatomy
THE NORMAL POSTNATAL HEART
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