Ventricular septation and the endocardial cushions !

Process

A muscular wall rises from the tip of the heart and soft tissue at the center finishes the job; that center also makes the valves.

Updated 2026-11-01

See it in 3D: Interventricular septum · Right ventricle · Left ventricle · Mitral anterior leaflet · Mitral posterior leaflet · Tricuspid septal leaflet · Tricuspid anterior leaflet · Tricuspid posterior leaflet · Left ventricle papillary muscles · Right ventricle papillary muscles · Chordae tendineae

What it is

The endocardial cushions are swellings of cardiac jelly filled by endocardial cells that turn into mesenchyme. They fuse in the center of the heart, split the atrioventricular canal into right and left, and give rise to the mitral and tricuspid valves. The muscular interventricular septum grows up from the floor and the membranous part, closed by about week 7, completes the division; it is the classic site of a ventricular septal defect.

Key facts

Week 5: the dorsal and ventral cushions meet and fuse.
Weeks 4–7: the muscular septum rises as the two ventricles dilate on either side.
End of week 7: the interventricular foramen closes with the membranous part.
The lateral cushions and tissue from the main cushions form the leaflets of the mitral and tricuspid valves.

Structure

Muscular part: most of the septum, thick and trabeculated.
Membranous part: small and fibrous, just below the aortic valve.
AV valves: connective-tissue leaflets tied to papillary muscles by chordae tendineae.

Step by step

Endocardial cushions appear on the dorsal and ventral walls of the AV canal.
Endocardial cells become mesenchyme and thicken the cushions.
The cushions fuse and split the AV canal into right and left orifices.
The ventricular walls grow and raise the muscular septum from the floor.
Conal ridges and cushion tissue close the interventricular foramen: the membranous part.
Myocardium under the leaflets is hollowed out, leaving free leaflets, chordae and papillary muscles.

Markers

TGF-β / BMP (cushions)
NKX2-5
TBX5
GATA4

Clinical notes

Ventricular septal defect: the most common congenital heart defect; membranous (perimembranous) defects are the exam classic.
Atrioventricular canal defect: the cushions fail to fuse; strongly linked to trisomy 21.
Ebstein anomaly: the septal and posterior tricuspid leaflets fail to separate from the wall and sit displaced toward the apex.

Test yourself

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

Which structures give rise to the mitral and tricuspid valves?

The endocardial cushions of the AV canal, whose tissue forms the leaflets; hollowed-out myocardium forms the chordae and papillary muscles.

Which three tissues form the membranous interventricular septum?

Endocardial cushion tissue, the conal (outflow tract) ridges and the edge of the muscular septum.

Which syndrome is associated with atrioventricular canal defects?

Down syndrome (trisomy 21).

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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); Ventricular Septal Defect | Congenital Heart Defects (CHDs) | CDC (Centers for Disease Control and Prevention, 2024); Atrioventricular Septal Defect (AVSD) (Centers for Disease Control and Prevention, 2024).

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Representative model, for educational purposes only.