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Summary the development of the heart and its conduction system
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Summary of the development of the heart and its conduction system
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The Development of the Heart and its Conduction SystemTopics covered:
The origins of the heart tube and the chambers of the heart
Atrioventricular, interatrial, interventricular and outflow tract septations
Atrioventricular septal defects (common in Down's syndrome)
Atrial septal defects, including persistent forament ovale (PFO)
Ventricular septal and outflow tract defects
The features of Tetralogy of Fallot
The development of the conduction system
The circulatory changes that take place at birth
Congenital heart disease incidence is around 8 in every 1000 births. CHD is one of the
most common of all life threatening birth defects.
Close to one in every hundred live births has a congenital heart defect, they vary in
severity, but are all are serious, some require surgical intervention and many will
result in the death of the child.
Defects include-
Ventricular septal defects (VSD) – most common
-Membranous
-Muscular
Pulmonary stenosis (PS)
Atrioventricular septal defects (AVSD)
Atrial septal defects (ASD)
Transposition of great arteries (TGA)
Tetralogy of Fallot (ToF)
The most common cause of death is VSD, because they are the most common heart
defects, many aren’t lethal but many are.
The heart starts of as a tube and develops into the complex heart structure and it
does this while the heart is actually functioning (acting as a pump). Very soon after
its initial formation, the heart starts to beat (and it has to, in order to keep embryo
alive) and function as well as going through the complex development processes.
The heart is the first organ to develop and function and occurs in the third to tenth
weeks post-conception. The heart is first recognisable as a crescent shaped tube of
myocardium enclosing an endocardially lined lumen.
The heart is of mesodermal origin, with the exception of a couple of extra cell
populations.
Early Stages
, Mouse embryo
myocardium stained blue, development of outflow tract, ventricles and atria
In order to understand the early stages of development of heart, we can use the
mouse embryo heart development pictures above (very similar to human heart
development).
The heart starts of as a crescent shaped tube, that has a lumen on the inside and
muscle on the outside and an endothelial lining to the lumen, which is called the
endocardium of the heart.
The medial part of the tube as can be seen expands, which will become the left
ventricle. The endocardial tube becomes attached to developing aortic arches
(arterial vessels) cranially (of which there are two, each leading on to an aorta, so we
start of with two paired aortas), this is the outflow (shown with arrow) of the heart.
It also attaches to the systemic veins (developing venous system, venous vessels)
caudally, which is the inflow (shows with broken arrow).
The heart is built up from this stage onwards from cells that flow in to the heart,
these “add” to the heart at arterial end and venous end and add extra bulk and length.
It is not true that all the features of the heart are already present in the crescent.
The heart tube is made up of an outer layer of muscle, myocardium, an inner layer
that lines the lumen, the endocardium. In between there is an acellular layer called
cardiac jelly, this is important as its going to disappear along most of the heart tube,
but in a couple of regions will thicken up and develop into cushions, which is
important in two parts of septum formation.
, Slightly inaccurate length of
grey tube
The whole heart tube is now going to elongate by the addition of cells from the
“second heart field”, at both the outflow and inflow.
Because the ends of the tube are fixed to vessels in the embryo, the heart must bend
as it elongates. This is called “looping”
The heart loops so much, that the ventricles come in front of the atria
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