Evolution of the vertebrate heart

Knowledge of the evolution of the vertebrate heart is unequivocally of importance. The recent review by Stephenson et al. (2017), therefore, could have been of great significance to those investigating the anatomy of not only the normal, but also the abnormal heart. This is because, with increasing knowledge, some of the features of cardiac anatomy as seen in some phylums, such as reptiles and crocodilians, are now being encountered in humans with congenitally malformed hearts. An example is the arrangement of three arterial roots as seen in crocodilians, which is a rare and unexpected finding in the setting of human congenital cardiac disease (Katewa et al. 2017). If such reviews are to achieve their desired effect, however, then surely they should be written with their translational significance in mind? This, sadly, is not the case with the offering from Stephenson et al. (2017). When describing the arrangement of the intrapericardial arterial trunks in the various phylums, for example, they allude to ‘left and right aortic arches’. One of the malformations found in humans with congenital cardiac disease is persistence bilaterally of right and left aortic arches, with both arising from a solitary intrapericardial aortic trunk. And a more frequent finding is for the aortic root, arising from the cardiac base, then to cross to the right of the trachea, rather than the left, as it extends to become the descending aorta. It is more accurate, therefore, when accounting for the arrangement of the arterial roots, to describe their ventricular origin, while not ignoring the relationship of the aortic arch relative to the trachea (Cook et al. 2017). This potential shortcoming, however, pales into insignificance when considered relative to the conjectures offered by the authors in regard to the ‘unique myocardial band’. Their review has been published in a journal devoted to anatomy. Should not, therefore, the assessment made by the authors regarding the existence of this purported arrangement of the ventricular cardiomyocytes have been based on anatomical criteria? It remains a fact that those proposing the existence of the ‘unique band’ have yet to validate their dissections by histological investigations. As Stephenson and colleagues discuss, my colleagues and I have published extensive studies showing the counter-evidence regarding the existence of the purported band. We have emphasised the fact that dissectors can uniformly show the arrangement of features such as the muscles of the limbs, demonstrating with clarity their origins and insertions. This is because each muscle is an individual anatomical entity, and is compartmentalised by enclosing fibrous sheaths. This is not the case for the ventricular myocardial mass. We accept that it is possible to produce an entity as described by Torrent-Guasp et al. (2001) but its demonstration is not achieved by following pre-existing anatomical boundaries. More recently, we have reviewed not only the anatomic evidence, but also the evidence from related disciplines, such as those discussed by the authors of the current review (Maciver et al. 2018a,b). I appreciate that these recent publications will not have been available to the authors when writing their review. None of this additional evidence, however, supports the arrangement as described initially by Torrent-Guasp and as then championed by Buckberg et al. (2015). In essence, these investigators had likened the ventricular myocardium to skeletal musculature. In this light, we have already assembled the anatomic evidence showing that such comparisons are inappropriate (Stephenson et al. 2016). Stephenson et al. (2017) make reference to the echocardiographic investigation of Hayabuchi and associates (Hayabuchi et al. 2015). They fail to mention our own study, conducted in response to these findings, which showed that the ‘midventricular line’ emphasised by the Japanese investigators was produced by cardiomyocytes aligned in circumferential fashion, rather than representing the alleged ‘cross-over’ of components of the purported ‘unique myocardial band’ (Agger et al. 2016). Our recent studies, furthermore, have also included a discussion of evolutionary aspects of ventricular mural architecture (Maciver et al. 2018a,b), although I again recognise that this information would not have been available to Stephenson et al. (2017) when assembling their review. In summary, nonetheless, although I fully support the importance of anatomical investigations of cardiac evolution as proposed by Stephenson et al. (2017), I would suggest that such studies will be misleading unless based on anatomic facts, rather than conjecture, as is the case concerning the so-called ‘unique myocardial band’.