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EMBRYOLOGY

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Heart Embryology Derivatives

FA P286

Focus on

What each embryonic structure becomes, and why half of each atrium is smooth and half is trabeculated.

Key takeaways

The organizing idea: smooth versus trabeculated

Smooth walls are absorbed VENOUS inflow; trabeculated walls are the ORIGINAL chamber

deriving this once beats memorizing the table, because the pectinate muscles and the crista terminalis mark exactly where the two meet.
  • Sinus venosus and pulmonary vein origins give smooth walls.
  • Primitive atrium and primitive ventricle origins give trabeculated walls.
  • Bulbus cordis gives the smooth outflow tracts.
The same heart at 22, 24, 35 and 50 days, with each embryonic structure keeping its colour as it becomes its adult derivative. At 22 days the tube runs from the first aortic arch and bulbus cordis down through the ventricle and atrium to the sinus horns receiving the cardinal veins. By 24 days the loop has begun, the truncus arteriosus and pericardium are labelled, and the sinus venosus sits behind. By 35 days the primitive left atrium has swung upward and the left ventricle sits below. By 50 days the ascending aorta and pulmonary trunk have separated out of the truncus, the aortic arch is formed, the SVC enters the right atrium, and the two ventricles are defined. Following one colour across the four panels is the fastest way to learn the derivative table.
What this shows
The same heart at 22, 24, 35 and 50 days, with each embryonic structure keeping its colour as it becomes its adult derivative. At 22 days the tube runs from the first aortic arch and bulbus cordis down through the ventricle and atrium to the sinus horns receiving the cardinal veins. By 24 days the loop has begun, the truncus arteriosus and pericardium are labelled, and the sinus venosus sits behind. By 35 days the primitive left atrium has swung upward and the left ventricle sits below. By 50 days the ascending aorta and pulmonary trunk have separated out of the truncus, the aortic arch is formed, the SVC enters the right atrium, and the two ventricles are defined. Following one colour across the four panels is the fastest way to learn the derivative table.

The clinical consequence of a failed truncus division

persistent truncus arteriosus, in which the truncus arteriosus fails to divide into the aorta and pulmonary artery before birth. See Heart Tube, Looping & Outflow Tract Formation.
Embryonic structure to adult derivative
Embryonic structureGives rise to
Right common / anterior cardinal veinsSuperior vena cava
Posterior, subcardinal, supracardinal veinsInferior vena cava
Right horn of sinus venosusSmooth part of RA (sinus venarum)
Left horn of sinus venosusCoronary sinus
Primitive pulmonary veinSmooth part of LA
Primitive atriumTrabeculated parts of LA and RA
Endocardial cushionAtrial septum, membranous IV septum, AV and semilunar valves
Primitive ventricleTrabeculated parts of LV and RV
Bulbus cordisSmooth outflow tracts of LV and RV
Truncus arteriosusAscending aorta and pulmonary trunk

Where the great veins come from

The embryonic circulation, colour-coded red for oxygen-rich, blue for oxygen-poor and purple for mixed. Three venous systems drain into the horns of the sinus venosus: the CARDINAL veins (anterior from the head, posterior from the body, joining as the common cardinal vein) which give the venae cavae, the VITELLINE veins from the yolk sac which give the portal system, and the UMBILICAL veins from the placenta which carry the oxygenated blood. The ventral aorta leaves the truncus arteriosus and the dorsal aorta supplies the body and sends the umbilical arteries back to the placenta. Knowing which of the three systems a vein belongs to is what makes the derivative table derivable rather than memorized.
What this shows
The embryonic circulation, colour-coded red for oxygen-rich, blue for oxygen-poor and purple for mixed. Three venous systems drain into the horns of the sinus venosus: the CARDINAL veins (anterior from the head, posterior from the body, joining as the common cardinal vein) which give the venae cavae, the VITELLINE veins from the yolk sac which give the portal system, and the UMBILICAL veins from the placenta which carry the oxygenated blood. The ventral aorta leaves the truncus arteriosus and the dorsal aorta supplies the body and sends the umbilical arteries back to the placenta. Knowing which of the three systems a vein belongs to is what makes the derivative table derivable rather than memorized.

Three venous systems drain into the sinus venosus, and each has its own fate

the cardinal veins become the systemic venae cavae, the vitelline veins become the portal system, and the umbilical veins carry placental blood and leave the ligamentum teres.

The coronary sinus derives from the LEFT horn of the sinus venosus

it runs in the left AV groove and drains into the right atrium. The right horn is absorbed instead into the smooth-walled sinus venarum of the right atrium.

The aortic arches

The surviving aortic arches colour-coded, 3rd in blue, 4th in purple, 6th in green, with the carotid system arising above them. The recurrent laryngeal nerves are the reason the arches are worth learning: each nerve hooks under the last arch derivative on its side, so the RIGHT nerve loops around the product of the 4th arch (the right subclavian artery) and the LEFT nerve loops around the product of the 6th arch (the ductus arteriosus, later the ligamentum arteriosum). The left nerve therefore descends much further into the thorax, which is why it is the one injured by an aortic arch aneurysm or an enlarged left atrium.
What this shows
The surviving aortic arches colour-coded, 3rd in blue, 4th in purple, 6th in green, with the carotid system arising above them. The recurrent laryngeal nerves are the reason the arches are worth learning: each nerve hooks under the last arch derivative on its side, so the RIGHT nerve loops around the product of the 4th arch (the right subclavian artery) and the LEFT nerve loops around the product of the 6th arch (the ductus arteriosus, later the ligamentum arteriosum). The left nerve therefore descends much further into the thorax, which is why it is the one injured by an aortic arch aneurysm or an enlarged left atrium.

The arches are learned by mnemonic, one line each

  • 1st arch: part of the maxillary artery, a branch of the external carotid. The 1st arch is maximal.
  • 2nd arch: stapedial artery and hyoid artery. Second = Stapedial.
  • 3rd arch: common carotid and the proximal internal carotid. C is the 3rd letter.
  • 4th arch: left forms the aortic arch, right forms the proximal right subclavian artery. The 4th arch has 4 limbs, so it is systemic.
  • 6th arch: right forms the right pulmonary artery, left forms the left pulmonary artery and the ductus arteriosus. The 6th arch is pulmonary, plus the pulmonary-to-systemic shunt.
An echocardiogram in a 60-year-old with long-standing mitral stenosis shows a markedly dilated left atrium. He has become hoarse and has difficulty swallowing solids. Trace both symptoms back to embryology, and say which structure the coronary sinus came from while you are there.

Both symptoms come from the left atrium being the most posterior chamber, pressing on two structures behind it. The dysphagia is direct compression of the esophagus. The hoarseness is compression of the left recurrent laryngeal nerve, and this is Ortner syndrome. The embryology explains why it is the left nerve: each recurrent laryngeal nerve hooks under the last persisting aortic arch derivative on its side, and because the left 6th arch persists as the ductus arteriosus, the left nerve is pulled deep into the thorax to loop under the ligamentum arteriosum, right beside the enlarging atrium. On the right the 6th arch regresses, so that nerve only reaches the 4th arch derivative, the right subclavian artery, and stays safely in the neck. As for the coronary sinus, it derives from the left horn of the sinus venosus, and it too runs posteriorly, in the left atrioventricular groove, which is why a dilated coronary sinus appears on the same echocardiogram whenever right atrial pressure is raised. Contrast that with the right horn of the sinus venosus, which is absorbed to become the smooth-walled sinus venarum of the right atrium, the part separated from the trabeculated original atrium by the crista terminalis.

How it's tested

The derivative table is asked directly, most often for the coronary sinus, which comes from the left horn of the sinus venosus, and for the smooth part of the right atrium, which comes from the right horn. The organizing rule is worth deriving rather than memorising: the smooth-walled portion is always the absorbed venous inflow and the trabeculated portion is always the original primitive chamber, which is why the pectinate muscles and the crista terminalis mark the boundary between them.

Go deeper

First Aid 2026 — CV/Embryology (p.286) · B&B — Cardiac derivatives · Mehlman — HY Cardio (fetal remnants)

High-yield images3
The same heart at 22, 24, 35 and 50 days, with each embryonic structure keeping its colour as it becomes its adult derivative. At 22 days the tube runs from the first aortic arch and bulbus cordis down through the ventricle and atrium to the sinus horns receiving the cardinal veins. By 24 days the loop has begun, the truncus arteriosus and pericardium are labelled, and the sinus venosus sits behind. By 35 days the primitive left atrium has swung upward and the left ventricle sits below. By 50 days the ascending aorta and pulmonary trunk have separated out of the truncus, the aortic arch is formed, the SVC enters the right atrium, and the two ventricles are defined. Following one colour across the four panels is the fastest way to learn the derivative table.
The embryonic circulation, colour-coded red for oxygen-rich, blue for oxygen-poor and purple for mixed. Three venous systems drain into the horns of the sinus venosus: the CARDINAL veins (anterior from the head, posterior from the body, joining as the common cardinal vein) which give the venae cavae, the VITELLINE veins from the yolk sac which give the portal system, and the UMBILICAL veins from the placenta which carry the oxygenated blood. The ventral aorta leaves the truncus arteriosus and the dorsal aorta supplies the body and sends the umbilical arteries back to the placenta. Knowing which of the three systems a vein belongs to is what makes the derivative table derivable rather than memorized.

Flashcards for this page

Card 1 of 5 · try-out only, nothing is saved

Which embryonic structure gives rise to the coronary sinus?

(...) horn of (...)
PreviousAtrial & Ventricular SeptationNextFetal Circulation & Postnatal Derivatives

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