Development of the atrial septum in relation to postnatal anatomy and interatrial communications

Abstract

The atrial septum is probe patent in some 30% of the population, and is prone to have overt defects. Atrial septation is the coming together of several myocardial structures and mesenchymal tissues of intracardiac and extracardiac origin that must change identity to myocardium. We propose that the propensity for malformation of the atrial septum reflects this complicated morphogenesis. The morphogenesis of the atrial septum initiates from a ridge of mesenchyme, only a few hundred micrometres long, in the roof of the undivided atrial cavity. By growth of the myocardial primary septum, the mesenchymal ridge will be approximated to, and ultimately fuse, with the mesenchyme of the atrioventricular cushions. This fusion also takes in the so-called vestibular spine, and serves to close the primary atrial foramen. Interatrial communication is maintained by the development of perforations in the myocardial septum that will coalesce to produce the secondary foramen. Late in gestation, an infolding of the right atrial roof, previously identified as the secondary septum, will come to form the roof of the secondary foramen. Muscularisation of the mesenchymal ridge and vestibular spine serves to reinforce the attachment of the primary muscular septum to the atrioventricular insulating plane, with the muscularised components, and the cranial infolding, then producing the rims of the oval fossa as seen in the postnatal heart. We show that other lesions that produce the potential for interatrial shunting are outside the confines of the atrial septum, and hence are best considered as interatrial communications, rather than 'atrial septal defects'.

Sequential segmental analysis of the crocodilian heart

“…Formation of the oval foramen, and its closure by an infolding of the atrial roof, commonly referred to by the misnamed ‘second atrial septum’, is therefore unique to placental mammals (Table ), (Jensen et al. ).…”

Demonstration of living anatomy clarifies the morphology of interatrial communications

“…Even when fused, it has recently been emphasised that the so-called ‘pouches’ can be produced between the flap valve and the margins of the fossa 30 31. Absence of fusion produces the so-called probe patency of the oval fossa 18. In this setting, when viewed from the left atrial side, the opening of the persistent foramen is seen as crescent-shaped slit, located between the two horns of the flap valve 18 32.…”

“…In this setting, when viewed from the left atrial side, the opening of the persistent foramen is seen as crescent-shaped slit, located between the two horns of the flap valve 18 32. When viewed from the right atrial side, the opening is at the anterosuperior margin of the oval fossa18 (figures 3 and 4A). The persistently patent foramen, therefore, is a slit-like tunnel rather than a hole, as it is often illustrated.…”

“…Not all the lesions producing the potential for interatrial communication are confined to these areas 18. The most frequent true defect of the atrial septum, present within the oval fossa, is the consequence of deficiencies of the primary atrial septum (figure 4A,B).…”

Heartbeat:Blood urea nitrogen to creatinine ratio predicts outcome in acute heart failure

“…Also in this issue is an excellent review article entitled: ‘Development of the atrial septum in relation to anatomy and interatrial communications’5 by Professor…”