Learning Objective

By the end of this topic, the learner should be able to describe the key stages of heart morphogenesis, explain the mechanisms of cardiac septation and outflow tract formation, and correlate developmental defects with common congenital heart diseases.

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Heart Morphogenesis

• The heart is the first functional organ in vertebrate embryos.
• It begins spontaneous beating by week 4 of development.

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Cardiac Looping

• The primary heart tube loops to establish normal left–right polarity.
• This process begins in week 4.
• Defects in left–right dynein, which regulates laterality, can result in dextrocardia, as seen in Kartagener syndrome.

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Septation of the Chambers

Atrial Septation

• Septum primum grows toward the endocardial cushions, narrowing the ostium primum.
• Cell death in the septum primum forms the ostium secundum as the ostium primum regresses.
• Septum secundum develops to the right of septum primum, maintaining a right-to-left shunt.
• Septum secundum expands and partially covers the ostium secundum, leaving the foramen ovale.
• The remaining septum primum forms the one-way valve of the foramen ovale.
• After birth, increased left atrial pressure and decreased right atrial pressure cause the septum primum to close against the septum secundum.
• Fusion of both septa during infancy or early childhood forms the definitive atrial septum.

Patent Foramen Ovale (PFO)

• Failure of the septum primum and septum secundum to fuse after birth.
• Present in ~25% of the population.
• Usually asymptomatic.
• Can cause paradoxical emboli due to right-to-left shunting.

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Ventricular Septation

• The muscular interventricular septum grows upward, leaving the interventricular foramen.
• The aorticopulmonary septum rotates and fuses with the muscular septum to form the membranous interventricular septum, closing the foramen.

Endocardial cushions:

• Separate atria from ventricles
• Contribute to atrial septation
• From the membranous portion of the ventricular septum

Ventricular Septal Defect (VSD)

• Most common congenital heart defect
• Typically involves the membranous septum

Atrioventricular Septal Defect (AVSD)

• Also called an endocardial cushion defect or AV canal defect
• Features:
  • Single common AV valve
  • Partial AVSD: ASD only
  • Complete AVSD: ASD + VSD
• Associated with:
  • Down syndrome
  • Maternal diabetes
  • Maternal obesity
• Typically acyanotic

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Outflow Tract Formation

• Neural crest cell migration forms the truncal and bulbar ridges.
• These ridges spiral and fuse to form the aorticopulmonary septum, separating:
  • Ascending aorta
  • Pulmonary trunk

Conotruncal Abnormalities (Neural Crest Cell Failure)

• Transposition of the great arteries (TGA)
• Tetralogy of Fallot
• Persistent truncus arteriosus

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Valve Development

• Aortic and pulmonary valves: derived from endocardial cushions of the outflow tract
• Mitral and tricuspid valves: derived from fused endocardial cushions of the AV canal

Valvular Anomalies

• Stenosis
• Regurgitation
• Atresia (e.g., tricuspid atresia)
• Displacement (e.g., Ebstein anomaly)

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Activity