Last updated 30 March 2006

• Meanwhile, the inflow and outflow tracts of the heart are also developing, attached to the endocardial tubes, including:
• Paired dorsal aortae, the outflow tracts, on either side of the midline, connected to the cranial ends
• 3 pairs of inflow veins at the caudal ends, from medial to lateral:

1. The vitelline veins, which drain the yolk sac
2. The umbilical veins, carrying oxygenated blood from the placenta
3. Common cardinal veins, formed from the union of the posterior cardinal veins draining the trunk and the anterior cardinal veins draining the head.

• Cephalic folding of the embryo brings these tubes together in the future thoracic region, pulling the cranial ends of the dorsal aortae ventrally into a pair of dorsoventral loops forming the 1st aortic arches.

• The primary heart tube is subdivided by a number of sulci into a series of chambers, including, from the inflow (caudal) to the outflow (cranial) end:

1. The sinus venosus, consisting of the left and right sinus horns into which the inflow veins drain, which will eventually become the right atrium
2. The primitive atrium, that will give rise to the auricles
3. The ventricle, separated from the primitive atrium by the atrioventricular sulcus, that will form most of the future left ventricle
4. The bulbus cordis, separated from the ventricle by the bulboventricular (interventricular) sulcus which will form:
• The right ventricle from its inferior part
• The conotruncus from the remainder, which itself will form the outflow tracts of the future ventricles, including:
  • The conus cordis at the proximal end
  • The truncus arteriosus at the distal end, which will eventually split to form the ascending aorta and pulmonary trunk
5. The aortic sac most cranially, leading to the first aortic arches and, eventually, the other 4 aortic arches.

• The wall of the primary heart tube develops 4 layers:

1. The endothelium, derived from the primary heart tube itself
2. The myocardium, derived from a layer of splanchnopleuric lateral plate mesoderm that invests the heart tube on day 22
3. A layer of cardiac jelly secreted by the myocardium
4. The epicardium (visceral pericardium), derived from another wave of splanchnopleuric mesoderm.

• The dorsal mesocardium from the foregut that initially held the heart tube in the primitive pericardial cavity ruptures, creating the transverse and oblique pericardial sinuses, and the heart is now held in the pericardial cavity by the inflow and outflow tracts.
• On day 23, the heart tube begins to elongating, folding and looping that will bring the primitive chambers into their proper future positions.

• In sinistral (normal) folding, the bulbus cordis is displaced inferiorly, ventrally and to the right; the ventricle is displaced to the left and the primitive atrium and sinus venosus are displaced posteriorly and superiorly.

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Remodelling of the heart tube

• Whilst the heart tube folds, the initially bilaterally symmetrical circulation is remodelled into the asymmetric systemic and pulmonary circulations, with the inflow end generally developing before the outflow end.
• On day 22, the heart begins beating, and circulation begins on about day 24, with blood initially returning equally to the sinus horns, but the venous return will eventually be shifted to the right, emptying into the right atrium via the superior and inferior venae cavae.

• On about day 50, all but the right vitelline vein and the right anterior cardinal vein degenerate.
• By day 56:

1. The right vitelline vein has become the superior part of the inferior vena cava (IVC), draining blood from the lower body and yolk sac
2. The anterior cardinal vein has become the superior vena cava (SVC), draining blood from the upper body.

• The left sinus horn transforms into the coronary sinus, draining the coronary vessels, and the oblique vein of the left atrium.

Remodelling of the Atria

• The right sinus horn forms a smooth walled region called the sinus venarum in the right posterior wall of the primitive atrium, displacing the right side of the primitive atrium anteriorly and to the right to form the right auricle.
• The sinus venarum drains the SVC, IVC and coronary sinus, and will give rise to the future right atrium.
• The crista terminalis divides the right auricle, which is trabeculated, from the right atrium proper, which is smooth.

• The primitive atrium also sprouts a pulmonary vein that bifurcates twice, forming 2 left and 2 right branches, whose roots are incorporated into the left posterior wall of the primitive atrium.
• This will become the left atrium, whilst the left side of the primitive atrium is displaced anteriorly and the left to become the left auricle.

Atrial septation

• In the fourth week, the left, right, superior and inferior endocardial cushions appear inside the atrioventricular canal.
• The superior and inferior endocardial cushions fuse to form the septum intermedium, creating left and right atrioventricular canals.

• The membranous septum primum grows down from the posterosuperior roof of the primitive atrium towards the septum intermedium.
• The opening between these two septa, the ostium primum, forms the initial interatrial shunt between the right and left atria.
  • This allows oxygenated blood to flow as follows: IVC ---> Right Atrium ---> Left Atrium ---> Left Ventricle ---> Ascending Aorta.
  • Another shunt, the ductus arteriosus, channels blood from the pulmonary trunk into the descending aorta, sparing the constricted foetal pulmonary circulation.
• As the ostium primum gets eliminated, the ostium secundum appears superiorly in the septum primum, allowing shunting to continue.

• In the fifth week, the thick, muscular septum secundum also grows down from the roof of the primitive atrium, to the right of the septum primum, stopping short of the septum intermedium to form the foramen ovale.
• Blood shunts by entering the space between the two atrial septa ventrally through foramen ovale, and exiting dorsally through ostium secundum.
• As these two openings don’t overlap, the sudden rise in left atrial pressure at birth clamps the membranous and muscular atrial septa together, closing the shunt.

Alignment of the atrioventricular canals and outflow tracts

• Initially, in the folded heart tube, the atrioventricular canal connects the common atrial chamber to the future left ventricle, and only the primitive right ventricle has direct access to the truncus arteriosus.
• Remodelling:
  1. Shifts the atrioventricular canal to the right, aligning its right and left sides to the primitive atria and ventricles on each side.
  2. Brings the left side of the truncus arteriosus into line with the future left ventricle.

Ventricular septation, development of the valves and septation of the outflow tracts

• At the end of the fourth week, the muscular ventricular septum appears on the inferior wall of the heart between the two primitive ventricles, but doesn’t completely divide the ventricles.

• In weeks 5-8, the atrioventricular valves develop from the myocardium around the atrioventricular canals, with the chordae tendinae and papillary muscles.

• 2 longitudinal trunconal septa appear on opposite walls of the conotruncus, developing spirally both proximally and distally, eventually fusing to split the outflow tract into the ascending aorta and pulmonary trunk.
• Inferiorly, the trunconal septa also fuse with the superior endocardial cushion, forming the membranous ventricular septum, and finally fuse with the muscular ventricular septum, completing the division of the ventricles.