DurgicalDurgical
EN
Open the app

Embryology · Heart and vessels

Heart tube: formation and looping !

  • Process

The heart starts as a simple tube that beats from week 4, then folds on itself to put its chambers in place.

Updated 2026-11-01

See it in 3D: Right atrium · Left atrium · Right ventricle · Left ventricle · Pericardium · Sinoatrial (SA) node

What it is

The heart arises from splanchnic mesoderm in the cardiogenic field, cranial to the neural plate. The paired endocardial tubes fuse into one tube that starts beating around day 22, and rightward (D) looping brings the atria and ventricles into position. It is the embryo's first working organ and the starting point of every congenital heart defect.

Key facts

  • Week 3 (day 18–19): the horseshoe-shaped cardiogenic field forms in splanchnic mesoderm.
  • Day 21–22: the endocardial tubes fuse; the single tube starts to beat.
  • Days 23–28: rightward looping; the bulbus moves down and to the right, the atrium up and behind.
  • The second heart field lengthens both poles: arterial (right ventricle and outflow tract) and venous (atria and sinus venosus).
  • By the end of week 4 blood is circulating, though no septa exist yet.

Structure

  • Endocardium: the inner endothelial lining.
  • Cardiac jelly: acellular matrix between endocardium and myocardium; in the cushions it gives rise to valves and septa.
  • Myocardium: the muscle layer, differentiated from splanchnic mesoderm.
  • Epicardium: comes from the proepicardium near the sinus venosus and also forms the coronary arteries.

Step by step

  1. Cardiac progenitor cells migrate through the primitive streak and gather in cranial splanchnic mesoderm.
  2. They form two endocardial tubes wrapped in myoblasts.
  3. Lateral folding brings the two tubes together in the midline: a single heart tube.
  4. Cranial folding swings the heart into the chest, ventral to the foregut.
  5. The tube beats and lengthens with cells from the second heart field.
  6. Rightward looping brings atria and ventricles together and sets the stage for septation.

Markers

  • NKX2-5
  • GATA4
  • TBX5
  • HAND1/HAND2
  • ISL1 (second heart field)
  • Nodal / PITX2 (laterality)

Clinical notes

  • Reversed looping or a failed laterality signal gives dextrocardia and heterotaxy, often with complex heart defects.
  • Congenital heart defects are the most common birth defect; many begin in these early weeks, before the pregnancy is known.

Test yourself

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

Around which day does the embryonic heart begin to beat?

Around day 22, in week 4: the heart is the first functioning organ.

Which germ layer gives rise to the myocardium?

Splanchnic (lateral plate) mesoderm of the cardiogenic field.

What results if the heart tube loops to the left instead of the right?

A mirror-image arrangement: dextrocardia, often with heterotaxy and other heart defects.

Add to my cards (3)

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); About Congenital Heart Defects | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.