Atrial septation is a highly coordinated developmental process that separates the primitive common atrium into right and left atrial chambers while preserving a controlled interatrial communication during fetal life. The mature atrial septum is not a simple flat partition. Rather, it is formed by the overlapping development of the septum primum, septum secundum, and the foramen ovale valve mechanism [1].
This layered embryologic architecture is essential for understanding three clinically important structures:
From the surgeon’s perspective, the atrial septum should therefore be understood not only as an anatomic wall, but also as the remnant of a dynamic fetal valve system.
The first major step in atrial septation is the formation of the septum primum. This thin, crescent-shaped membrane grows downward from the superior aspect of the primitive atrium toward the developing endocardial cushions.
Initially, the inferior edge of the septum primum does not reach the cushions. This creates a temporary communication between the right and left atria, known as the foramen primum.
The foramen primum is not a defect at this stage. It is a normal transient communication that allows blood to pass between the primitive atrial chambers during early septation.
As the septum primum continues to grow inferiorly, it approaches and eventually fuses with the endocardial cushions. This process progressively narrows and closes the foramen primum [2].
Failure of septum primum fusion with the endocardial cushions is the embryologic basis of a primum atrial septal defect. Because this region is closely related to the atrioventricular junction, primum ASD is often associated with abnormalities of the atrioventricular valves and belongs to the broader atrioventricular septal defect spectrum [3, 4].
Before the foramen primum fully closes, programmed perforations appear in the superior portion of the septum primum. These small openings then coalesce to form a new interatrial communication: the foramen secundum.