In cardiovascular embryology, the septum primum grows from the roof of the common atrium toward the endocardial cushions. Before it fuses with the cushions, the gap between its free edge and the cushions is the foramen primum (ostium primum). Which event creates the foramen secundum and maintains right-to-left shunting?

• A The septum primum tears when cardiac pressure rises
• B The septum secundum fuses with the septum primum, obliterating the ostium primum
• C Growth of the endocardial cushions creates a new opening in the centre of the septum primum
• D Apoptotic perforations in the upper part of septum primum create the foramen secundum before the ostium primum closes ✓

Explanation

As the foramen primum (ostium primum) is closing by fusion of septum primum with the endocardial cushions, apoptotic cell death in the cranial portion of septum primum creates multiple perforations that coalesce to form the foramen secundum (ostium secundum), maintaining right-to-left blood flow. Subsequently, septum secundum grows from the roof of the right atrium, covering (but not fusing with) the foramen secundum, forming the oval fossa. The persisting flap of septum primum acts as a one-way valve (foramen ovale) allowing right-to-left shunting in fetal life, which normally closes at birth when left atrial pressure exceeds right atrial pressure.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.