Cardiac Embryology for Imagers by John Partridge. This is an imager’s guide to the formation of the heart. I have tried to slim the topic down to those aspects that I have found useful in my interest in the imaging of congenital heart disease.
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The first appearance of the heart
is a cardiogenic plate of
mesodermal tissue at the extreme head end of the embryonic disc.
Rapid development and flexion
of the head cause this cardiac
anlage to come to lie below the
head and mouth, in front of the
Two lateral extensions of cardiac
tissue become hollowed out to
form a pair of endothelial tubes,
which soon fuse to form the
primitive cardiac tube.
Paired veins from the trunk (the
cardinal system), liver, yolk sac
and placenta enter the heart tube from below and a series of
arterial arches emerge from the
The primitive cardiac tube has five zones:
the arterial trunk
the bulbus cordis )
} some would call these two together
the ventricle ) the primitive ventricle, with inlet
and outlet portions
and the sinus venosus
The arterial trunk will divide to separate the pulmonary and systemic supply.
The bulbus and the ventricle will differentiate into the right and left ventricles, but
let me say at once that it is not a matter of a septum growing up the middle of the primitive
ventricle; the real story is rather more complicated, but an understanding of it will assist
greatly in your analysis of congenital cardiac malformations.
The cardiac tube grows at a greater
longitudinal rate then the rest of the
embryo, causing it to fold. As it does this
it falls to the right. This is known as
d-looping. It may fall to the left in an
l-loop: this will lead to a malformed heart.
Below are chick embryo dissections showing
the two types of loop.
normal d-loop l-loop
This is an actual looped heart
Note that the ventricular mass is
now in line with the atria
This is a cutaway showing
the beginnings of the ventricular
septum. The ventricles will develop
as outpouchings from this position,
in the direction of the arrows.
so that we go from this
to this. Rather crude graphics
but I hope you get the point
let us call the top of the septum
the “septal crest”
Now look at the area which
was the lumen of the original tube,
It now forms a communication
between the ventricles: persistence of
it will result in the commonest
of ventricular septal defects, the
this figure is rather simplistic but might help
Now for the arterial trunk.
This structure does truly septate,
but embryologically it is a simple
coronal division in its embryonic
It will, as we will discuss, end up
as a spiral, but this is achieved by
The septation extends upwards from
the valves to end just beyond
the origin of the paired sixth aortic arches,
where it seals off against the posterior truncal wall.
As the sixth arch vessels are destined to be the
branch pulmonary arteries, the posterior channel is now
the main pulmonary artery. The anterior channel is the aorta.
This is why the aorta always arches over the pulmonary
arteries from anterior to posterior, no matter what
other cardiac abnormality is present. We will not discuss
aortic branching problems here, we must concentrate on the
ventricles and how the great vessels connect to them.
Because of the looping, the septating
arterial trunk will be dragged to the
right , and twisted as well.
As a result the ascending aorta
comes to lie to the right of the
Note that the looping brings the
trunk close to the AV canal.
The aorta is now poorly placed to attach itself to the
left ventricle and some mechanism is needed to
drag it to the left but still leave the PA over the right
ventricle. (One might wonder why the truncal septum does not seal off anteriorly above the sixth aortic arches, and so make the anterior channel the
Anyway, the relocation of the aorta to the left requires an appreciation of the modelling power
of differential growth.
All this is happening as the embryo is rapidly growing, even though it is only millimetres long.
At this stage, as we saw before, the ventricular mass is centralising in front of the AV canal so that separate atria can serve each ventricle. If we take a view downwards onto the crest of the septum, looking from the atria, we see something like this:
See how close the outlet
is to the inlet. If the gap
between them fails to grow
with the rest of the heart, in
the fully formed heart the two
will be in continuity.
The next stage is the most
difficult to describe or illustrate,
I hope I can make it reasonably
A surge of growth beneath the pulmonary artery
pushes it up, forward and right (black arrows). The gap
between the aorta and the inlet valve remains small
and fibroses (dotted line). These processes pin the
aortic valve to the rim of the developing mitral valve
as everything around them expands.
As a result, the aorta arises from the
left ventricle while the pulmonary
artery has risen over the right ventricle.
Once the gap between the truncal
septum and the septal crest obliterates,
the systemic and pulmonary supplies
will have been separated, and
connected to the correct ventricle.
And so now you can compare the flow scheme on the left with the more lifelike image
on the right
RPA = right pulmonary artery
LPA = left pulmonary artery
APS = aortopulmonary
RVO = RV outflow
LVO = LV outflow
Now we have described how the ventricles position themselves and the great
vessels spiral down to cross the circulation before the truncal septum fuses with the
superior margin of the septal crest.
Inferior to this, the posterior part of the septal crest is heading towards the AV valve,
which itself is dividing into the mitral and tricuspid valves
Four cushions (AVC) have
developed at the A/V junction; the
superior and inferior cushions will
meet to divide the AV orifice (AVO)
into the tricuspid and mitral valves.
The inferior septal crest (VS)
will aim to meet the divided valve
where the cushions fuse.
Viewing the mature anatomy form the atrial side, the two atrioventricular valves
have assumed their circular orifice shapes. The aortic valve, as we have discussed,
is in continuity with the mitral annulus: the AV valves have separated slightly at the
top, allowing the aortic valve to wedge between the mitral and tricuspid annuli, coming
to rest very close to the tricuspid annulus. The pulmonary valve remains pushed
up and forward, though still in continuity with the aortic valve.
This pattern of connections between the annuli of the four cardiac valves constitutes the
fibrous “skeleton” of the
heart, here viewed from
the front. This is a useful
image to carry in your head,
as much of ventricular anatomy
can be “dressed” on to this
Note that the commissures of the
aortic and pulmonary valves
reflect their common origin with
one commissure of each still in
line with its old partner. The
coronary artery origins will always
be from the sinuses adjacent to
the common commissure, even in
congenital abnormalities of aortic
position and/or connection.
Opposite the dividing atrioventricular
valve, the posterior walls of the atria
are beginning to lateralise. The
symmetrical systemic venous system
biases its growth to the right and
many of its left sided structures
disappear or involute. Thus the
systemic veins drain to the right side.
A septum is developing down the
middle of the atrium, probably in a
similar way to the ventricular septum
in that it is a ridge left behind as the
atrial walls grow away from it.
The to the left of the septum, the primary
pulmonary vein grows and seeks out
the primitive pulmonary venous complex.
As growth proceeds, the primary
vein is absorbed into the atrial wall
as showed here, to achieve the adult
form of separate left and right lung
All that is left now is to cover the development of the atrial septum in more detail.
This is another difficult topic, requiring some effort in all four dimensions.
This diagram is a simplified two dimensional version. The septum primum grows
downwards towards the developing AV valves, but “fenestrates” posteriorly to
form the ostium secundum, which is closed by the later-developing septum secundum.
This diagram is a little more true. The septum secundum is not really a true intracavitary
septum, but is a fold of atrial wall invaginating from the superior surface.
Here is someone else’s interpretation
Actually, I subscribe to the feeling that the septum
primum does not actually fenestrate, but that it
and the septum secundum form eccentrically
In any event, where the two cross in the middle
is the oval fossa if they overlap completely, or is a
secundum atrial septal defect if they leave a gap.
I feel this orientation of the septa explains best why
on transoesophageal echo the septa around a PFO
do not quite look like they should
from the diagrams
And to finish, a word on the AV valves. Looking back
on this image from a few slides ago, you may have
noticed that the way the septum seals off the “VSD”
space is not a simple line.
The area in question becomes the membranous septum, and is offset towards the mitral valve
resulting in a portion that is interventricular (MSV) and one that is between the LV and the
right atrium (MSA). You will meet this anatomy again in echocardiography and in your
understanding of the atrioventricular septal defects (“canal” defects). It allows the
wedging of the aortic valve between the mitral and tricuspid valves described before.
Well, that’s it. I do hope it has helped. On the next slide I have classified some congenital
malformations on the underlying embryological fault: feel free to give it a try.
- then you get
the truncal septum fails to fuse with the septal crest?
- perimembraneous VSD
the truncal septum is deviated to the PA side?
- tetralogy of Fallot
the truncal septum fails to develop?
- truncus arteriosus
the ventricular septum fails to reach the AV valve?
- AV septal defects
the arterial trunk stays over the RV but does divide?
- double outlet RV
the aortic valve pushes up and right instead of the pulmonary?
- transposition of the great vessels
the ventricles fail to centralise over the AV valve
- double inlet left ventricle (commonest form of single ventricle)
the loop is to the left?
- ventricular inversion (RV on the left, LV on the right)
and of course, combinations exist!
This is just a rough summary, but I hope you get the idea. Can you see now why
double outlet RV is common and double outlet LV is very rare? Similarly double inlet LV
is common, double inlet RV rare? And why a VSD so commonly accompanies problems of
connection of the ventricles to the great vessels.