U01.07.002 Septation of the chambers

Understanding the formation of heart chambers and valves is critical for medical students preparing for the USMLE Step 1. This guide presents the septation of the heart chambers and the development of valves with diagrams and clinical correlations.

Atrial Septation

Sequential Development

Formation of Septum Primum
- The septum primum grows downward toward the endocardial cushions, narrowing the foramen primum.
Development of Foramen Secundum
- Foramen secundum forms in the septum primum as the foramen primum regresses.
Formation of Septum Secundum
- The septum secundum develops on the right side of the septum primum, maintaining a right-to-left shunt through the foramen secundum.
Creation of Foramen Ovale
- The expanding septum secundum covers most of the foramen secundum, leaving a residual opening known as the foramen ovale.
Valve Formation
- The remaining portion of the septum primum forms a one-way valve of the foramen ovale.
Closure at Birth
- After birth, increased left atrial pressure and decreased right atrial pressure force the septum primum to close against the septum secundum, sealing the foramen ovale.
Final Fusion
- During infancy or early childhood, the septum primum and septum secundum fuse, forming the complete atrial septum.

Clinical Correlation: Patent Foramen Ovale (PFO)

Cause: Failure of the septum primum and septum secundum to fuse after birth.
Significance: Often asymptomatic but can lead to paradoxical emboli (e.g., venous thromboembolic entering systemic circulation).

Structure	Function
Septum Primum	Initiates separation of atria
Foramen Secundum	Maintains right-to-left shunt
Septum Secundum	Expands and forms the foramen ovale
Foramen Ovale	One-way valve enabling fetal blood flow

Ventricular Septation

Sequential Development

Formation of the Muscular Septum
- The muscular interventricular septum grows upward, leaving an opening called the interventricular foramen.
Closure of Interventricular Foramen
- The aorticopulmonary septum rotates and fuses with the muscular septum, forming the membranous interventricular septum and closing the foramen.
Role of Endocardial Cushions
- Growth of endocardial cushions separates the atria and ventricles, contributing to both atrial and ventricular septation.

Clinical Correlation: Ventricular Septal Defect (VSD)

Most Common Congenital Cardiac Anomaly: Typically occurs in the membranous septum.

Structure	Function
Muscular Septum	Primary structure separating ventricles
Membranous Septum	Completes separation with input from cushions
Endocardial Cushions	Key in forming septa and AV valves

Outflow Tract Formation

Neural Crest Cell Migration
- Neural crest and endocardial cells form truncal and bulbar ridges, which spiral and fuse to form the aorticopulmonary septum, dividing the ascending aorta and pulmonary trunk.
Clinical Correlations:
- Transposition of Great Vessels
- Tetralogy of Fallot
- Persistent Truncus Arteriosus

Valve Development

Aortic/Pulmonary Valves
- Derived from endocardial cushions of the outflow tract.
Mitral/Tricuspid Valves
- Derived from fused endocardial cushions of the atrioventricular (AV) canal.
Valvular Anomalies:
- Stenosis: Narrowing of valve orifice.
- Regurgitation: Incomplete closure causing backflow.
- Atresia: Failure to form a functional opening (e.g., tricuspid atresia).
- Displacement: Example includes Ebstein anomaly.

Points to Remember

Foramen Ovale Closure: Dependent on pressure changes at birth.
Endocardial Cushions: Crucial for both septation and valve formation.
Clinical Relevance: Most defects (e.g., PFO, VSD) are linked to improper fusion or migration.

Bibliography

Moore, K.L., Dalley, A.F., & Agur, A.M. Clinically Oriented Anatomy.
Sadler, T.W. Langman’s Medical Embryology.
USMLE Step 1 Study Resources – Kaplan and First Aid.

USMLE Step 1 Review Series

Curriculum

U01.07.002 Septation of the chambers

Atrial Septation

Sequential Development

Clinical Correlation: Patent Foramen Ovale (PFO)

Ventricular Septation

Sequential Development

Clinical Correlation: Ventricular Septal Defect (VSD)

Outflow Tract Formation

Valve Development

Points to Remember

Bibliography

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