Cardiac Embryology Essentials

Rapid Review·Cardiovascular

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Foundations

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For CK, embryology matters only where it explains a congenital lesion: neural crest failure gives the conotruncal defects, septation failure gives ASD/VSD/AV canal, and the derivative table tells you what a persistent structure is.

Key takeaways

Looping and the outflow tract

Looping sets left-right polarity

the primary heart tube loops in week 4.

A defect in left-right dynein causes dextrocardia.
With situs inversus it is Kartagener syndrome (primary ciliary dyskinesia): recurrent sinusitis, bronchiectasis, infertility.

What this shows

Formation of the aorta, pulmonary trunk and interventricular septum. (1) Truncal ridges appear at opposite sides of the truncal lumen. (2) The ridges grow toward each other. (3) Cells at the ventricle base grow toward the endocardial cushions and form the muscular interventricular septum. (4) The aorticopulmonary septum spirals, which is what leaves the pulmonary trunk anterior to the aorta; failure to spiral gives transposition of the great arteries.

Neural crest cells build the aorticopulmonary septum

they drive the truncal and bulbar ridges to spiral and fuse.

Follow the four stages on the aorta and pulmonary trunk formation figure.
The spiral is why the pulmonary trunk ends up anterior to the aorta.

Conotruncal defects cluster with DiGeorge syndrome (22q11.2)

because neural crest also builds the pharyngeal pouches.

Look for hypocalcemia, thymic hypoplasia and cleft palate.

Conotruncal (neural crest) defects
Defect	Mechanism	Page
Transposition of the great arteries	Aorticopulmonary septum fails to spiral	Transposition of the Great Arteries
Tetralogy of Fallot	Anterosuperior displacement of the infundibular septum	Tetralogy of Fallot
Persistent truncus arteriosus	Aorticopulmonary septum never forms	Truncus Arteriosus

Atrial septation

What this shows

Atrial septation in five steps. (1) Septum primum grows down toward the dorsal endocardial cushion, narrowing the ostium primum. (2) Ostium secundum opens in the septum primum by cell death as the ostium primum closes. (3) Septum secundum develops to the right of septum primum. (4) Septum secundum covers most of the ostium secundum, and the residual opening is the foramen ovale. (5) The remaining septum primum degenerates into the one-way valve that closes the foramen at birth.

Follow the five panels

on the atrial septation diagram above, and the same sequence inside the heart on the atrial septation anatomy.

Septum primum

grows toward the endocardial cushions, narrowing the ostium primum.

Ostium secundum

opens within the septum primum.

Septum secundum

grows to its right and overlaps the opening.

Foramen ovale

the residual channel, valved by the remaining septum primum.

At birth the flap closes

left atrial (LA) pressure exceeds right atrial (RA) pressure, so the flap closes and later fuses.

Failure to fuse leaves a Patent Foramen Ovale (~25% of adults).
It is a flap, not a hole, but still a route for paradoxical embolism.
A true tissue defect is an Atrial Septal Defect (ASD).
Secundum ASD is the most common; the primum type belongs to atrioventricular (AV) canal defects and Down syndrome.

Ventricular septation and the AV canal

What this shows

Ventricular septation in three steps. (1) The muscular interventricular septum grows up from the apex, leaving the interventricular foramen open beside the atrioventricular canals. (2) The aorticopulmonary septum descends. (3) It fuses with the muscular septum to form the membranous interventricular septum, closing the foramen; failure here is the membranous VSD.

The membranous septum forms last

the muscular septum grows up from the apex, leaving the interventricular foramen.

The aorticopulmonary septum descends and fuses with it to form the membranous septum (three panels on the ventricular septation diagram).

Membranous ventricular septal defect (VSD)

the most common congenital heart defect overall.

Small muscular VSDs usually close on their own (Ventricular Septal Defect).

Endocardial cushions build three things

the atrial septum, the membranous ventricular septum and both AV valves.

That is why a cushion (AV canal) defect gives combined lesions.
Complete: common AV valve + primum ASD + VSD.
Partial: primum ASD + cleft mitral valve (mitral regurgitation, MR) with no VSD.
Strong Down syndrome association.

Valve development

Two sets of cushions

aortic and pulmonary valves come from the outflow-tract cushions; mitral and tricuspid from the fused AV-canal cushions.

Four kinds of anomaly

stenotic, regurgitant, atretic (Tricuspid Atresia) or displaced (Ebstein Anomaly, lithium exposure).

What each embryonic structure becomes

The organizing rule

smooth walls come from absorbed venous inflow (sinus venosus, pulmonary vein) and the bulbus cordis; trabeculated walls are the original primitive chambers.

The crista terminalis and pectinate muscles mark the boundary.

Embryonic structure to adult derivative
Embryonic structure	Gives rise to
Truncus arteriosus	Ascending aorta and pulmonary trunk
Bulbus cordis	Smooth outflow tracts of both ventricles
Primitive ventricle	Trabeculated parts of left ventricle (LV) and right ventricle (RV)
Primitive atrium	Trabeculated parts of LA and RA
Primitive pulmonary vein	Smooth part of LA
Right horn of sinus venosus	Smooth part of RA (sinus venarum)
Left horn of sinus venosus	Coronary sinus
Right common / anterior cardinal veins	Superior vena cava
Endocardial cushions	Atrial septum, membranous ventricular septum, AV and semilunar valves

Which embryonic structure fails in the combined heart defect of Down syndrome, and what three lesions make up its complete form?

The endocardial cushions, which build the atrial septum, the membranous ventricular septum and both AV valves. The complete AV canal defect is a common AV valve + primum ASD + VSD.

How it's tested

A newborn with hypocalcemic seizures, absent thymic shadow and a conotruncal defect: neural crest failure — DiGeorge (22q11.2 deletion); the cardiac lesion is truncus, tetralogy or an interrupted arch.

Dextrocardia on a chest film in a young adult with chronic sinusitis and infertility: Kartagener syndrome, a dynein defect that also derailed cardiac looping.

A child with Down syndrome has a fixed split S2 and a holosystolic murmur: an endocardial cushion (AV canal) defect — primum ASD plus VSD plus a common AV valve.

Cryptogenic stroke in a 35-year-old with a DVT: paradoxical embolism through a patent foramen ovale — a failure of fusion, present in a quarter of adults.

Go deeper

Related Step 2 pages: Fetal Circulation & Transition at Birth, Approach to Congenital Heart Disease, Atrial Septal Defect, Ventricular Septal Defect, Tetralogy of Fallot, Transposition of the Great Arteries, Truncus Arteriosus

High-yield images4

Formation of the aorta, pulmonary trunk and interventricular septum. (1) Truncal ridges appear at opposite sides of the truncal lumen. (2) The ridges grow toward each other. (3) Cells at the ventricle base grow toward the endocardial cushions and form the muscular interventricular septum. (4) The aorticopulmonary septum spirals, which is what leaves the pulmonary trunk anterior to the aorta; failure to spiral gives transposition of the great arteries.

Atrial septation in five steps. (1) Septum primum grows down toward the dorsal endocardial cushion, narrowing the ostium primum. (2) Ostium secundum opens in the septum primum by cell death as the ostium primum closes. (3) Septum secundum develops to the right of septum primum. (4) Septum secundum covers most of the ostium secundum, and the residual opening is the foramen ovale. (5) The remaining septum primum degenerates into the one-way valve that closes the foramen at birth.

Ventricular septation in three steps. (1) The muscular interventricular septum grows up from the apex, leaving the interventricular foramen open beside the atrioventricular canals. (2) The aorticopulmonary septum descends. (3) It fuses with the muscular septum to form the membranous interventricular septum, closing the foramen; failure here is the membranous VSD.

Atrial septation seen in the heart itself, four panels. Septum primum (green) descends toward the endocardial cushions leaving the ostium primum; the ostium secundum then perforates it; septum secundum (blue) grows down to its right and overlaps the opening, leaving the foramen ovale as the residual channel between the two septa.

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