Download CRRM1.11 - Embryology of the Heart

CRRM1.11: EMBRYOLOGY OF THE HEART
06/11/07
LEARNING OUTCOMES
Describe the function and position of the heart tube
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By the third week of gestation, the developing embryo has formed the primitive heart tube within the
pericardial cavity (formerly the intraembryonic coelom)
The heart tube differentiates to form the foetal heart
The original endocardial heart tube is first surrounded by embryonic mesoderm which begins to sink
into the pericardial cavity
The pericardial cavity engulfs the heart tube with a surrounding layer of mesoderm which then
becomes the myocardium
The heart tube now consists of three distinct layers: the outermost to the pericardial cavity,
epicardium (a serous mesothelial layer); the muscular myocardium (former mesoderm) and the
innermost endocardium
At one end of the tube is the aortic sac from which arterial outflow arises and at the other is the sinus
venosus which is the point of venous entry
At this point it is possible to ‘reach around’ the heart tube within the pericardial cavity due to the
presence of the transverse sinus of the pericardium
The heart tube is ‘fixed’ at the base of the aortic sac and sinus venosus so growth occurs from the
centre of the tube which expands and folds centrally
Describe differentiation of the heart tube
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The heart tube differentiates to form an atrium immediately after the sinus venosus followed by a
narrowing of the tube after which the primitive ventricle gives rise to the conus cordis and truncus
arteriosus which will later become the aorta and pulmonary arteries
Explain formation of the cardiac septa
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Sub-endocardial cushions form at the narrowing of the developing atrio-ventricular canal
The cushions grow towards one another and fuse to form the atrio-ventricular septum, dividing the
canal into two distinct channels
Describe septum formation in the atrioventricular canal and the common atrium
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The developing heart now takes on a four-compartment shape similar to an egg-box from which the
two atria and ventricles will eventually form
The primary septum forms between the developing atria from proliferating cells of the atrioventricular septum
Highly oxygenated blood flowing from the placenta into the IVC enters the right atrium and flows
against the primary septum creating a hole called the foramen ovale
A secondary septum forms alongside the primary septum but does not fuse completely, allowing
limited blood flow through the foramen ovale into the left atrium
Explain how atrial septal defects (ASDs) arise
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The foramen ovale is normally closed off at birth by the fusion of the primary and secondary septa,
separating the atria for normal cardiac function
Defects can arise when either this hole is not closed up properly or others form along the atrial septa
which usually require surgical closure
Describe septum formation in the ventricle, truncus arteriosus and conus cordis
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In the ventricle, the muscular inter-ventricular septum forms from the endocardium up to the atrioventricular septum
The aortico-pulmonary septum then begins to form down the centre of the truncus arteriosus to the
conus cordis in a spiral, fusing to the atrio-ventricular septum
The two channels formed by the APS will become the main pulmonary artery and aorta
Explain how ventricular septal defects (VSDs) arise
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If the aortico-pulmonary septum does not fuse properly to the atrio-ventricular septum then it is said
to be offset and gives rise to an over-expressed (over-riding) aorta and pulmonary artery stenosis
Fallot’s tetralogy is a condition which comprises of the above ventricular septal defect and right
ventricular hypertrophy (since the right ventricle has to work harder against the pulmonary stenosis)
This condition is otherwise known as blue baby syndrome (cyanosed) because of the mixture of
oxygenated and deoxygenated blood flowing through the aorta