cardiac embryology for imagers by john partridge l.
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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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cardiac embryology for imagers by john partridge
Cardiac Embryologyfor ImagersbyJohn 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.
  • Many of the illustrations are from Leon Gerlis, my collaborator in the cardiac section of “A Textbook of Radiology”, the copyright of which has been released. Others have come to me in various ways over the years and their provenance is uncertain. If you recognise any, do let me know so I can acknowledge them.

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

upper end.


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

the atrium

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


The fold of the loop is principally at the junction of bulbus cordis and ventricle. Note in panel C that the two end up side by side.

  • Now is the time to realise that the left ventricle will develop from the ventricle, and the right ventricle will develop from the bulbus cordis. (And an l-loop will
  • result in ventricular inversion with the left ventricle on the right.)(for more, see “The Anatomy of Ventricular Looping….Jorg Manner Clinical Anatomy Jan 2009 21-35)
  • Note also that the arterial trunk is above the developing right ventricle.
  • Now we must ask, where does the interventricular septum come from?

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.


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

perimembraneous VSD

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

straight position.

It will, as we will discuss, end up

as a spiral, but this is achieved by

differential growth.

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

pulmonary artery.

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

pulmonary artery.)


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.

day 9

day 13


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.


pulmonary artery

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

(truncal) septum

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.



tricuspid mitral


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.


Actually, I subscribe to the feeling that the septum

primum does not actually fenestrate, but that it

and the septum secundum form eccentrically

overlapping flanges.

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

septum secundum




septum primum


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.


What if?..............

- 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.