Heart Tube, Looping & Outflow Tract Formation

Rapid Review·Cardiovascular

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EMBRYOLOGY

T1Must know

FA P284-285

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The first organ to function, the loop that sets left from right, and the neural crest cells that divide the outflow tract.

Key takeaways

The heart tube and its loop

The heart is the FIRST functional organ in vertebrate embryos

beating spontaneously by weeks 4 to 5, and detectable on transvaginal ultrasound by week 6.

What this shows

The heart tube from day 22 to birth. At day 22 it is a straight tube, arterial outflow at the top through the bulbus cordis, then the primitive ventricle, the primitive atrium, and the sinus venosus receiving the venous return at the bottom. Over days 24 to 35 the tube loops, swinging the atrium up and behind the ventricle, which is how a single in-line tube becomes a four-chambered pump with the atria sitting above and posterior to the ventricles.

Cardiac looping establishes LEFT-RIGHT polarity, beginning in week 4

the primary heart tube bends so that the structures destined to be right-sided and left-sided end up on the correct side of the midline.

A defect in left-right dynein, the motor protein that drives the ciliary movement setting left-right asymmetry, causes dextrocardia, as in Kartagener syndrome.

Outflow tract formation

Outflow tract division is driven by NEURAL CREST cell migration

the migrating cells build the truncal and bulbar ridges, which spiral and fuse into the aorticopulmonary septum, dividing the outflow tract into the ascending aorta and the pulmonary trunk.

The spiral is the whole reason the pulmonary trunk ends up ANTERIOR to the aorta

trace the four stages and each failure mode becomes obvious.

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.

Conotruncal abnormalities all trace back to failed neural crest migration

which is why they cluster together in question stems and why they share the 22q11 deletion association.

Conotruncal (neural crest migration failure) abnormalities
Defect	Why neural crest matters
Transposition of the great arteries	Aorticopulmonary septum fails to spiral
Tetralogy of Fallot	Anterosuperior displacement of the infundibular septum
Persistent truncus arteriosus	Aorticopulmonary septum fails to form at all

Valve development

Semilunar valves from the outflow tract cushions, AV valves from the AV canal cushions

Aortic and pulmonary valves from the endocardial cushions of the outflow tract.
Mitral and tricuspid valves from the fused endocardial cushions of the AV canal.
Anomalies may be stenotic, regurgitant, atretic (tricuspid atresia), or displaced (Ebstein anomaly).

Which structure builds which valve
Valve	Derived from
Aortic / Pulmonary	Endocardial cushions of the outflow tract
Mitral / Tricuspid	Fused endocardial cushions of the AV canal

One cell population, one spiral, three diseases

Almost every question from this page is really asking about neural crest cells, and the three conotruncal lesions are three different amounts of the same failure. If the septum never forms, aorta and pulmonary trunk stay as one vessel, which is persistent truncus arteriosus. If it forms but does not spiral, the two vessels separate in parallel rather than crossing, so the aorta arises from the right ventricle and the pulmonary trunk from the left, which is transposition of the great arteries. If it forms and spirals but sits too far anterosuperiorly, it overrides the septum and narrows the right outflow, which is tetralogy of Fallot. The same cell population builds the thymus, the parathyroids and the facial structures, which is why these hearts arrive with 22q11 deletion syndromes, and why a stem mentioning hypocalcemia, absent thymic shadow or a cleft palate alongside a murmur has already told you which cells failed.

A newborn is cyanotic from the first minutes of life. Echocardiography shows the aorta arising anteriorly from the right ventricle and the pulmonary trunk arising posteriorly from the left ventricle, running parallel rather than crossing. What embryologic step failed, what keeps this infant alive, and how would the picture differ if the same cells had failed completely?

This is transposition of the great arteries, and the failed step is the spiraling of the aorticopulmonary septum. Normally the septum, built by migrating neural crest cells forming the truncal and bulbar ridges, twists as it divides the outflow tract, which is what leaves the pulmonary trunk anterior to the aorta and makes each vessel connect to the correct ventricle. Without the spiral the septum still divides the tract but straight down, so the two circulations run in parallel instead of in series, and that parallel arrangement is fatal unless the blood can mix. What keeps the infant alive is a shunt: a patent foramen ovale, a VSD, or the ductus arteriosus, which is why prostaglandin E1 is given immediately to keep the duct open until surgery; see Fetal Circulation & Postnatal Derivatives. Had the same neural crest cells failed completely, no septum would form at all and the outflow tract would remain a single vessel, which is persistent truncus arteriosus. Note that both lesions, plus tetralogy of Fallot, carry the association with 22q11 deletion, because the same cells build the thymus and parathyroids.

How it's tested

Dextrocardia in a patient with recurrent sinusitis and infertility is Kartagener syndrome. The defect is in left-right dynein, which is exactly what fails during cardiac looping in week 4, so the embryology and the clinical triad are the same question. The outflow tract is tested through its neural crest dependence: if a stem describes the great vessels arising in parallel, the failure was the spiral of the aorticopulmonary septum, giving transposition, and if the septum never forms at all the answer is persistent truncus arteriosus.

Go deeper

First Aid 2026 — CV/Embryology (p.284-285) · B&B — Heart embryology; outflow tract & neural crest · Mehlman — HY Cardio (conotruncal anomalies, transposition)

High-yield images2

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 ao

Heart Tube, Looping & Outflow Tract Formation

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What is the structure forming in between the ventricles in the embryonic heart shown in the image?

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NextAtrial & Ventricular Septation

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