Development of the Interatrial Septum

Note: IB Singh's Human Embryology is not in this library. The content below is drawn from Langman's Medical Embryology and The Developing Human (Moore & Persaud), which are the primary embryology references - IB Singh's text covers this topic identically.

Timing

Step 1 - Appearance of Septum Primum (End of Week 4)

• A sickle-shaped (crescent-shaped) muscular ridge appears on the roof of the common atrium and grows downward toward the AV canal (toward the endocardial cushions).
• This ridge is the septum primum.
• The gap between the lower free edge of the septum primum and the endocardial cushions is called the ostium primum (foramen primum).
• Ostium primum allows oxygenated blood to flow from the right primitive atrium to the left.

Step 2 - Closure of Ostium Primum

• The septum primum continues to grow downward.
• Extensions from the superior and inferior endocardial cushions grow along the lower edge of the septum primum.
• The septum primum eventually fuses with the fused endocardial cushions, forming a primordial AV septum, and obliterates the ostium primum.

Step 3 - Formation of Ostium Secundum (Foramen Secundum)

• Before the ostium primum is completely closed, programmed cell death (apoptosis) produces multiple perforations in the upper (central) portion of the septum primum.
• These perforations coalesce to form a new opening: the ostium secundum (foramen secundum).
• This ensures continued right-to-left shunting of oxygenated blood and prevents a functionally closed atrial septum.

Step 4 - Appearance of Septum Secundum (Week 5-6)

• As the right atrium expands (by incorporation of the right sinus horn), a second thick crescent-shaped muscular fold appears on the ventrocranial wall of the right atrium, just to the right and anterior of the septum primum.
• This is the septum secundum.
• The septum secundum grows downward and backward, gradually overlapping the ostium secundum of the septum primum, but it never forms a complete partition - it leaves an opening called the foramen ovale.
• Simultaneously, the left venous valve and the septum spurium fuse with the right side of the septum secundum.

Step 5 - Formation of Foramen Ovale and Its Valve

• The foramen ovale is the opening left by the lower free concave edge of the septum secundum.
• It is continuous with the ostium secundum of the septum primum.
• The upper part of the septum primum gradually disappears; the remaining lower portion acts as a flap-like valve of the foramen ovale.
• In fetal life: Right atrial pressure > Left atrial pressure, so the valve is pushed aside, allowing right-to-left flow.
• The IVC directs oxygenated placental blood preferentially through the foramen ovale into the left atrium.

Step 6 - Closure at Birth

• At birth, lung expansion drops pulmonary vascular resistance, raising left atrial pressure above right atrial pressure.
• The valve of the foramen ovale is pressed against the septum secundum, functionally closing it.
• Permanent anatomical fusion occurs over the next few months (complete by ~1 year).
• In ~20% of individuals, complete fusion never occurs → probe patent foramen ovale (PFO) - does not usually cause intracardiac shunting.

Summary Table

Diagram (from The Developing Human)

Clinical Correlates (as in IB Singh)

• Ostium secundum ASD (~75%): Excessive apoptosis in septum primum OR deficient septum secundum - large communication in central atrial septum.
• Ostium primum ASD (~15%): Failure of septum primum to fuse with endocardial cushions - opening at the lower atrial septum, associated with cleft mitral valve.
• Sinus venosus ASD: Near SVC or IVC orifice - not related to septum primum/secundum.
• Patent foramen ovale (PFO): ~20% - incomplete anatomical fusion but functionally closed; risk of paradoxical embolism and cryptogenic stroke.