Septation of atrium (embryology)

The common atrium of the heart divides into two during the fourth week of gestation. Initially, a semi-circular fold grows inferiorly from the roof of the common atrium. It is termed the septum primum. The sheet begins to merge ventrally and dorsally with the endocardial cushions within the atrioventricular canal; before complete fusion a gap exists at the inferior margin of the septum - the ostium primum. Normally, growth of the endocardial cushions seals this defect at a time when cell death within the superior part of the septum primum produces a new conduit between the primitive left and right atria - the ostium secundum.

Then, to the right of the septum primum a new fold grows inferiorly - the septum secundum. Again, it is semi-circular in shape but it does not extend as far centrally as the septum primum. Dorsally, it receives contributions from the septum spurium and the left venous valve at the junction of the sinus venosus and the right atrium.

Before birth, the anterior and posterior parts of the septum secundum fuse with the endocardial cushions; although its central portion descends below the area of the ostium secundum in the septum primum to its left, it is deficient centrally and inferior to this - the aperture is termed the foramen ovale. As part of the normal fetal circulation, blood is able to pass through the foramen ovale then obliquely and superiorly through the ostium secundum in the septum primum: it provides a route for blood to flow from the right to the left atrium. After birth, the pressure in the left atrium increases. This results in fusion of the ostium primum and secundum to form the interatrial septum. There is closure of the foramen ovale in all but 20% of the population. The latter group do not tend to have shunting of blood between the atria. The fossa ovalis of the mature right atrium is the region where the ostium secundum was deficient; its limbus is formed by the inferior margin of the septum secundum.