Development of the Heart
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Direct answer
The heart develops from paired cardiogenic areas of splanchnic mesoderm in front of the buccopharyngeal membrane, folding into a single heart tube by day 21 which begins to beat on day 22 and loops into an S-shape in the following week. Between weeks five and eight endocardial cushions divide the atrioventricular canal, septa primum and secundum close the atrial septum except the foramen ovale, a membranous septum completes the ventricular septum, and neural crest ridges spiral through the truncus to separate aorta from pulmonary trunk. Errors in these steps produce most congenital heart defects.
What you must remember
- Regions of the tube, caudal to cranial: sinus venosus, primitive atrium, primitive ventricle, bulbus cordis and truncus arteriosus; the bulboventricular loop bends to the right, placing the atrium dorsal to the ventricle.
- Fates: sinus venosus becomes the smooth part of the right atrium and the coronary sinus; primitive atrium forms the auricles; bulbus cordis forms the outflow tracts; truncus arteriosus divides into aorta and pulmonary trunk.
- Atrial septation: septum primum grows down, its ostium primum closes as perforations above become the ostium secundum; septum secundum grows to the right leaving the foramen ovale, whose flap seals after birth.
- Ventricular septation: a muscular septum grows upwards, completed by the membranous septum of endocardial cushion origin — the commonest site of ventricular septal defect.
- Aorticopulmonary septum: neural crest cells populate the truncal and bulbar ridges, which fuse in a spiral to divide the outflow tract.
- Defects: ostium secundum atrial septal defect is the commonest atrial defect; ostium primum defects accompany cushion defects; membranous ventricular septal defect is the commonest congenital cardiac malformation; neural crest failure gives persistent truncus, transposition and tetralogy of Fallot.
- Fallot tetralogy: anterosuperior deviation of the aorticopulmonary septum produces pulmonary stenosis, overriding aorta, ventricular septal defect and right ventricular hypertrophy.
- Foetal circulation: the foramen ovale and ductus arteriosus (left sixth arch) bypass the lungs, becoming the fossa ovalis and ligamentum arteriosum after birth; failure gives a patent ductus with a machinery murmur.
Common confusion
The two atrial septal defects are perpetually swapped: an ostium secundum defect lies high in the septum, an ostium primum defect lies low and marks an endocardial cushion defect. Students also blame cushion failure for tetralogy and transposition — these are outflow septation (neural crest) faults. The ductus arteriosus runs from the left pulmonary artery to the aorta distal to the left subclavian artery.
Exam-focused takeaway
Theory questions ask for the dilatations of the heart tube and their fates, the steps of septation, and the embryological basis of septal defects, Fallot tetralogy and transposition. In the viva, trace foetal circulation and the closure of the foramen ovale and ductus arteriosus.
Frequently asked questions
When does the heart start beating?
On about day 22, soon after the paired heart tubes fuse into an S-shaped tube.
How is the interatrial septum formed?
Septum primum grows down and its ostium primum closes as perforations form the ostium secundum; septum secundum then covers it, leaving the foramen ovale as a one-way valve in foetal life.
Which is the commonest site of ventricular septal defect?
The membranous part of the septum, from failure of endocardial cushion tissue to close the gap left by the growing muscular septum.
What is the embryological basis of tetralogy of Fallot?
Anterosuperior deviation of the spiral aorticopulmonary septum, giving pulmonary stenosis with overriding aorta, plus the ventricular septal defect and secondary right ventricular hypertrophy.
What closes the ductus arteriosus after birth?
Falling prostaglandins and rising oxygen tension cause contraction in the first days, with anatomical obliteration to the ligamentum arteriosum over weeks; indomethacin closes a patent ductus.