Development and structure of the atrial septum.

Most cardiologists would probably consider that, during their training, they had received appropriate instruction concerning the mode of development and structure of the atrial septum. This is likely to be founded on the diagrams that exist in most standard textbooks of cardiac embryology. These illustrate the formation of primary and secondary atrial septums as overlapping muscular sheets that grow into the common atrium. This type of illustration implies that similar morphological mechanisms of development lead to the formation of these two “septums”. This is not so. To the best of our knowledge, there is no evidence existing which supports this concept of growth of a second muscular shelf into the developing atriums so as to overlap the primary atrial septum, and to provide the rims of the definitive oval fossa. On the contrary, it has long been established 2 that the superior border of the “septum secundum”, in other words the superior rim of the oval fossa, is an infolding of the atrial roof. In this respect, case reports are to be found that describe the formation of lipomas within the supposed “septum secundum”. Careful study of such lipomas, along with scrutiny of the published images, reveals that the fat accumulates within the deeply infolded superior interatrial groove. All the “classical” accounts of atrial septal development have also ignored totally the contribution to atrial septation made by the “spina vestibuli”, a structure first described by His in the 19th century. Similarly, they take no account of the contributions made by the mesenchymal cap which clothes the leading edge of the muscular primary atrial septum. In reality, therefore, more structures contribute to division of the atriums than the so-called primary and secondary septums. It is appreciation of the roles of all these various components that provides the basis for understanding the proper structure of the muscular walls interposed between the right and left atriums. This information, in this burgeoning era of interventional catheterisation, is now of major practical importance to the cardiologist. In this review, we will describe all the building blocks that contribute to the walls that separate the definitive right and left atrial chambers, showing how knowledge of their position provides the basis for understanding the definitive anatomy.