Right Atrium Anatomy #3: Atrial Septum Development

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:

1. The fossa ovalis
2. The patent foramen ovale
3. The spectrum of atrial septal defects

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.

1. Septum Primum Formation

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.

Key developmental concept

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].

Surgical relevance

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].

2. Formation of the Foramen Secundum

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.

This is an elegant developmental solution. The heart must create an atrial septum, but fetal circulation still requires right-to-left atrial flow. The closing foramen primum is therefore replaced by the more superior foramen secundum.

Developmental logic

Atrial septation must achieve two goals simultaneously:

• Structural separation of the right and left atrial chambers
• Functional preservation of fetal right-to-left interatrial shunting

The foramen secundum maintains this essential fetal pathway while the lower atrial septum continues to mature.

3. Development of the Septum Secundum

The septum secundum develops to the right side of the septum primum. It grows downward from the superior atrial wall and partially overlaps the foramen secundum, but it does not completely close the interatrial communication.

The remaining oblique passage is the foramen ovale.

Unlike the septum primum, the septum secundum is thicker and more muscular. It forms the superior, anterior, and right-sided rim of the future fossa ovalis.

Structural relationship

• Septum primum: thin, mobile, flap-like structure
• Septum secundum: thicker, muscular, right-sided rim
• Foramen ovale: oblique tunnel between the overlapping septa

This is the key point: the foramen ovale is not simply a round hole. It is a valve-like interatrial tunnel created by the overlap between the septum primum and septum secundum.

4. The Septum Primum as the Valve of the Foramen Ovale

During fetal life, pulmonary vascular resistance is high and pulmonary venous return to the left atrium is limited. As a result, right atrial pressure exceeds left atrial pressure.

This pressure relationship allows blood to pass from the right atrium to the left atrium through the foramen ovale. The septum primum functions as the valve of the foramen ovale, opening toward the left atrium and allowing right-to-left shunting [5].

Functional importance in fetal circulation

The foramen ovale permits relatively oxygen-rich blood returning from the inferior vena cava to enter the left atrium, left ventricle, and ascending aorta.

This preferential streaming supports oxygen delivery to:

• Coronary circulation
• Cerebral circulation
• Upper body systemic circulation

Thus, the atrial septum during fetal life is not merely a passive wall. It is an active flow-directing structure.

5. Postnatal Closure of the Foramen Ovale

At birth, lung expansion causes a rapid decrease in pulmonary vascular resistance. Pulmonary blood flow increases, pulmonary venous return rises, and left atrial pressure becomes higher than right atrial pressure.

This reversal of the atrial pressure gradient pushes the septum primum against the septum secundum, producing functional closure of the foramen ovale.

Experimental neonatal studies have shown that postnatal closure is not only a hemodynamic event but also a progressive anatomical process. In neonatal mouse hearts, the septum primum thickens markedly shortly after birth, and anatomical closure progresses through fusion between the septum primum and septum secundum [6].

Sequence of postnatal closure

1. Lung expansion lowers pulmonary vascular resistance.
2. Pulmonary blood flow increases.
3. Pulmonary venous return to the left atrium increases.
4. Left atrial pressure rises above right atrial pressure.
5. The septum primum is pressed against the septum secundum.
6. Functional closure occurs.
7. Anatomical fusion may follow over time.

If anatomical fusion remains incomplete, a patent foramen ovale persists.

6. Fossa Ovalis: The Mature Remnant of Atrial Septation

In the mature right atrium, the fossa ovalis represents the central region of the atrial septum derived mainly from the septum primum. The surrounding muscular rim, or limbus, largely reflects the contribution of the septum secundum.

From the right atrial view, this anatomy is clinically important because the fossa ovalis is the true septal region and is frequently used as a landmark for:

• Surgical ASD closure
• PFO closure
• Transseptal puncture
• Understanding interatrial communications
• Planning approaches in complex congenital heart disease

Surgeon’s view

The fossa ovalis should be understood as a layered embryologic structure:

1. The floor corresponds to the thin septum primum.
2. The limbus corresponds mainly to the thicker septum secundum.
3. A PFO represents a persistent flap-like tunnel between these two components.
4. A true ASD represents deficiency of septal tissue.

This distinction is essential because a PFO and an ASD may both permit interatrial communication, but their anatomical mechanisms are different [7].

7. Clinical Correlations

Patent Foramen Ovale

A patent foramen ovale is not a true absence of atrial septal tissue. Instead, it represents persistence of the potential space between the septum primum and septum secundum.

In most individuals, a PFO is hemodynamically silent. However, it may become clinically relevant when right atrial pressure exceeds left atrial pressure, allowing transient or sustained right-to-left shunting.

Situations that may promote right-to-left shunting include:

• Pulmonary hypertension
• Positive-pressure ventilation
• Valsalva maneuver
• Right ventricular dysfunction
• Glenn or Fontan physiology

In pediatric and congenital heart disease practice, the significance of a PFO depends strongly on the surrounding hemodynamic context [8, 9].

Secundum Atrial Septal Defect

A secundum ASD usually reflects deficiency of tissue in the region of the fossa ovalis. It may result from excessive resorption of the septum primum, inadequate development of the septum secundum, or a combination of both.

Unlike a PFO, a secundum ASD is a true defect in the atrial septal tissue. This distinction is important for surgical and catheter-based closure because rim adequacy, defect size, and the relationship to surrounding structures determine the feasibility and safety of intervention.

Primum Atrial Septal Defect

A primum ASD results from failure of the septum primum to fuse properly with the endocardial cushions. Because the endocardial cushions also contribute to atrioventricular valve development, primum ASD is frequently associated with cleft-like atrioventricular valve morphology and atrioventricular septal defect physiology.

Therefore, primum ASD should not be viewed simply as a low atrial septal hole. It is a defect of the atrioventricular septal complex.

8. Practical Summary for Surgical Anatomy

Atrial septal development can be summarized as a sequence of overlapping structures and changing hemodynamics.

Developmental sequence

1. Septum primum forms from the atrial roof.
2. Foramen primum remains temporarily between the septum primum and endocardial cushions.
3. Foramen primum closes as the septum primum fuses with the cushions.
4. Foramen secundum forms by perforation of the upper septum primum.
5. Septum secundum develops to the right side of the septum primum.
6. Foramen ovale forms as an oblique valve-like channel.
7. Postnatal LA pressure rises, closing the foramen ovale functionally.
8. Anatomical fusion may occur later, forming the mature fossa ovalis.

Core surgical concept

The atrial septum is best understood as an overlapping embryologic valve system, not a single flat wall.

This concept explains why:

• A PFO is a flap-like tunnel.
• A secundum ASD is a true deficiency of septal tissue.
• A primum ASD reflects abnormal atrioventricular septal development.
• The fossa ovalis is the key landmark for safe septal intervention.

Summary

Atrial septation begins with growth of the septum primum toward the endocardial cushions, creating and then closing the foramen primum. Perforations in the upper septum primum form the foramen secundum, while the septum secundum develops to the right side and partially overlaps this opening. The resulting foramen ovale functions as a valve-like fetal communication, with the septum primum acting as its mobile flap.

After birth, increased pulmonary blood flow raises left atrial pressure and presses the septum primum against the septum secundum, producing functional closure. Subsequent anatomical fusion may form the mature fossa ovalis.

For congenital heart surgeons, the essential point is that the atrial septum is a layered embryologic structure. Its development directly explains the morphology of the fossa ovalis, PFO, secundum ASD, and primum ASD.

References

[1] Anderson RH, Brown NA, Webb S. Development and structure of the atrial septum. Heart. 2002;88(1):104-110.

[2] Kachalia P, Bowie JD, Adams DB, Carroll BA. In utero sonographic appearance of the atrial septum primum and septum secundum. J Ultrasound Med. 1991;10(8):423-428.

[3] Edwards FR, Farquhar HG, Hay JD, Rees GJ. Surgical treatment of atrial septal defects. Br Med J. 1955;2(4954):1463-1469.

[4] Patten BM. Persistent interatrial foramen primum. Am J Anat. 1960;107:271-280.

[5] Momma K, Ito T, Ando M. In situ morphology of the foramen ovale in the fetal and neonatal rat. Pediatr Res. 1992;32(6):669-672.

[6] Cole-Jeffrey CT, Terada R, Neth MR, Wessels A, Kasahara H. Progressive anatomical closure of foramen ovale in normal neonatal mouse hearts. Anat Rec (Hoboken). 2012;295(5):764-768.

[7] Naqvi N, McCarthy KP, Ho SY. Anatomy of the atrial septum and interatrial communications. J Thorac Dis. 2018;10(Suppl 24):S2837-S2847.

[8] Webb G, Gatzoulis MA. Atrial septal defects in the adult: recent progress and overview. Circulation. 2006;114(15):1645-1653.

[9] Das BB. Patent foramen ovale in fetal life, infancy and childhood. Med Sci (Basel). 2020;8(3):25.