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Embryology · Heart and vessels

Outflow tract: the spiral aorticopulmonary septum and neural crest !

  • Process

A septum that twists like a spiral staircase splits the single outflow vessel into the aorta and the pulmonary trunk.

Updated 2026-11-01

See it in 3D: Pulmonary trunk · Ascending aorta · Right ventricle · Left ventricle · Interventricular septum · Aortic right coronary cusp · Aortic left coronary cusp · Aortic non-coronary cusp · Pulmonary anterior cusp · Pulmonary left cusp · Pulmonary right cusp

What it is

In week 5 ridges appear in the truncus arteriosus and conus, built largely from cardiac neural crest cells. They grow in a spiral and fuse into the aorticopulmonary septum, which separates the aorta from the pulmonary trunk and explains why the two vessels twist around each other. Errors in this septum (absent, straight or displaced) explain persistent truncus arteriosus, transposition and tetralogy of Fallot.

Key facts

  • Week 5: truncal (right superior and left inferior) and conal ridges appear.
  • Weeks 5–7: the ridges twist about 180 degrees and fuse.
  • The conal septum joins the muscular septum and the cushions to close the interventricular foramen.
  • Tissue swellings in the truncus form the three semilunar cusps of each valve, aortic and pulmonary.

Structure

  • Truncus arteriosus: distal part of the tube; gives the roots of the ascending aorta and pulmonary trunk.
  • Conus cordis: proximal part; gives the right ventricular infundibulum and the left ventricular aortic vestibule.
  • Aorticopulmonary septum: neural crest mesenchyme plus second heart field tissue.

Step by step

  1. Neural crest cells leave the hindbrain and travel through pharyngeal arches 3, 4 and 6.
  2. They reach the outflow tract and form ridges in the walls of the truncus and conus.
  3. The ridges grow in a spiral, like a spiral staircase.
  4. They fuse in the midline: the aorticopulmonary septum.
  5. The pulmonary trunk ends up on the right ventricle and the aorta on the left, twisted around each other.
  6. The conal septum joins the ventricular septum and completes the membranous part.

Markers

  • Cardiac neural crest
  • TBX1 (22q11.2)
  • ISL1 (second heart field)
  • PAX3

Clinical notes

  • Tetralogy of Fallot: infundibular septum displaced anteriorly and superiorly; cyanotic.
  • Transposition of the great arteries: the septum fails to spiral; the aorta leaves the right ventricle; cyanotic from birth.
  • Persistent truncus arteriosus: the septum fails to form; a single vessel overrides a VSD; cyanotic.
  • All three are seen in 22q11.2 deletion (DiGeorge) syndrome.

Test yourself

Think of the answer, then tap the question to see it.

Which cell population builds much of the aorticopulmonary septum?

Cardiac neural crest cells, which migrate from the hindbrain through pharyngeal arches 3, 4 and 6.

Which defect results if the aorticopulmonary septum grows straight instead of spiraling?

Transposition of the great arteries: the aorta arises from the right ventricle and the pulmonary trunk from the left.

Which part of the outflow tract gives the right ventricular infundibulum?

The conus cordis (proximal outflow tract).

Why does 22q11.2 deletion combine conotruncal defects with hypocalcemia?

It affects neural crest and the pharyngeal pouches: the outflow tract, parathyroids and thymus share that origin.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 13: Cardiovascular System (Wolters Kluwer, 2015); Tetralogy of Fallot | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2026); Dextro-Transposition of the Great Arteries (d-TGA) | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2024); Truncus Arteriosus | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2024).

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Representative model, for educational purposes only.