Previously: Cardiovascular development

On the 22^nd day of the embryo’s development, the cardiac tube forms constrictions and enlargements, called curvatures. These are the very primitive structures of the heart to come.

The heart tube folds back on itself

The uneven growth of the cardiac tube leads it to fold back on itself in a precise manner on the 23^rd day.

The primitive atrium and sinus venosus move behind the primitive ventricle and arterial bulb.

A wall is formed to give the two ventricles

A septum forms in the atrioventricular canal to separate it into two parts, the right and the left, in the middle of the fourth week.

The ventricular septation occurs from bottom to top, at the same time as the newly formed left and right ventricular cavities expand.

This ventricular junction is a muscular structure, of the same nature as the left ventricle, which becomes thicker than its neighbour.

However, the muscular junction is not complete. An orifice persists in the upper part of the ventricles up to the 7^th week.

During this time, there is interventricular communication in the fetal heart, that fluids circulate freely between the ventricles.

Aorta and pulmonary artery are formed

Shortly after the beginning of the atrioventricular separation, at the 5^th week, another separation is starting at the level of the arterial bulb.

It is spiral shaped, unlike the atrioventricular septum, which is rather straight. This spiral separation forms the aorta and pulmonary artery, hence its name: the aorticopulmonary septum.

The opening between the two ventricles closes

A part of the arterial bulb incorporates in the ventricles. It closes the remaining orifice in the upper part of the muscular interventricular septum, the muscular wall separating the two ventricles.

This new portion of the septum is short and thin. It is fibrous rather than muscular, like the adventitia, the outermost layer of the blood vessels.

Mitral and tricuspid devices take shape

The development of the interior of the ventricles culminates with the development of spongy muscular bundles.  Some of them thin and others are transformed into papillary muscles and into chordae tendineae for keeping the flaps of the atrioventricular valves in place, the tricuspid valve on the right and the mitral valve on the left.

Related chapiter: Atrial development and veinous return