Fig. 2

Evolution of atrial septation in afrontal view (a) and lateral view (b). The fifth drawing of panel b is also a frontal view. The septum primum (SP, drawn in red) grows towards the developing endocardial cushions (asterisk) at the level of the atrioventricular (AV) canal, dividing the primitive atrium into what will become the left atrium (LA) and right atrium (RA). An opening, called the ostium primum (OP), is left between the edge of the SP and the endocardial cushions at the AV canal. The OP allows oxygenated blood from the placenta to bypass the lungs and flow between the developing atria. During further development, the SP will gradually fuse with the endocardial cushions. However, before closure, a second opening in the SP will appear. This opening, known as the ostium secundum (OS) allows continuing interatrial communication. At the age of 36 days, a second septum, the septum secundum (SS, drawn in blue) appears from the atrial roof immediately to the right of the SP. In its course, it overlaps the OS and eventually fuses with the endocardial cushions. This septum, too, leaves an opening called the foramen ovale (FO). The superior edge of the SP partially regresses. The inferior part acts as a valve, allowing only right-to-left flow through the FO, as prenatal blood passes from the right atrium to the left atrium (dotted arrow) since right atrial pressure exceeds left atrial pressure. After birth, increased pulmonary return will raise the left atrial pressure, which will cause the valve to functionally close, as shown in the fifth drawing of Figure b, and eventually fuse with the septum secundum at the age of 6 months