Hello and welcome to the first of our
discussions on development of the heart.
This amazing process is going to give rise
to an incredibly complex pumping
machine hanging out in our thoracic cavity.
But how we get there is very convoluted and
you might ask yourself, why should
we care? Reason is, heart malformations and
heart defects are incredibly
common and prevalent.
Some form of cardiac malformation tends to
occur in about one percent of all births
and is a leading cause of both infant
mortality, especially if it's not diagnosed
prior to delivery.
In the United States alone, there were
roughly 6000 deaths associated with
cardiac anomalies and malformations.
So let's go into the process by which the
heart moves from being a single
tube to becoming the incredibly complex
mechanism that it is.
So you may recall from the lecture we had on
the organogenesis period that
when the visceral layer of lateral plate
museum folds anteriorly to create the gut
tube, it has two tubes within it called
And these are going to be the early
primordial of the heart as they're brought
together, as we can see on the right side.
They're going to come into existence just
ventral to the developing gut tube, and
they're going to pull a small cavity with
That cavity is part of the intra embryonic
sea limb, and specifically in this
region, we refer to it as the pericardial
cavity, although it's slightly different from
the mature pericardial cavity for reasons
we'll go into in a subsequent talk.
The endocarditis tubes are brought together,
fuse and develop
some what's referred to as cardiac jelly
around them and around.
The cardiac jelly is developing myocardium
or muscle of the
heart. This myocardium is going to be able
to be relatively
early, but it's not until those tubes fuse
that it's going to be able to do anything
with the chamber that's inside and propel
blood through it.
Now, the heart is suspended in the
pericardial cavity by a dorsal
myocardium, which is essentially a mesentery
of the heart connected to the dorsal
side of the embryo as the heart continues to
It moves from its initial location to its
And one amazing thing about the heart is
that it actually develops anterior to our
face. It grows right about in this area
early on, and as the embryo
enlarges and the forebrain gets larger and
larger, it folds the heart
down into the thorax into its eventual
As it does so, it pulls the pericardial
cavity with it, along with a
strip of mesoderm called the septum
That mesoderm is going to travel with it and
take up residence inferior to the
heart, and the septum transversum is the
earliest primordial we have of
the diaphragm. So we'll discuss how the
septum transversum becomes the diaphragm
and separates the pericardial cavity from
the peritoneal cavity in a subsequent talk.
Now, the heart has more or less found its
normal position in the eventual
thorax, and it starts beating about twenty
two days into development.
This is important because at this point, the
embryo can't get much larger without a
heart. Simple diffusion of gases and
nutrients from cells is no
longer sufficient to handle the size of the
We can't just create cavity after cavity for
these things to diffuse.
We need to have a circulatory system and the
heart starts pumping at twenty two days
because we absolutely have to have it in
order to get any larger.
So the heart pumps bringing blood in from
inferior peristaltic pumps it
out early and into a paired set of dorsal
aorta on the
posterior body wall.
So blood that's de oxygenated comes in and
really gets pumped
superior and is going to travel on either
side of the gut tube through what are called
aortic arches, and those aortic arches carry
the blood dorsally to a
pair of aorta.
Now, at this point, the heart, instead of
being a simple cylindrical
tube, is going to develop some distinctive
areas that bulge outward.
The first and most inferior really is the
part that receives blood from the body.
You're going to be our left and right side
of the sinus venosus and the sinus espinoza's
pumps blood to the primitive or primordial
From there, the atrium is going to pump blood
to the embryologist ventricle
and then to an area called the bulbous
Now, thereafter, blood is going to travel
through aortic arches to reach the dorsal
aorta. And at this point day twenty one, we
only have a single aortic arch.
But as we develop more aortic arches, we
have a common chamber called the aortic sac
that's going to receive the blood from the
bulbous Cordis and distribute it through
aortic arches to the dorsal aorta.
Now, as the heart enlarges and gets those
segments, the dorsal myocardium is going to
start to break down, and that's going to
allow the heart to essentially sag ventrally
in the developing
chest, and it's going to sag into the
pericardial sac that's already present
around it. As that happens, the ventricle
and bulbous cordis wind up
The sign espinoza's and atria wind up more
And if you think about how the heart appears
in the mature human, the atria are
posterior to the ventricles, which are more
anterior, so that little
fold between them is called the Boubou
And you've got the primordial ventricle just
on the backside of that bulge and the
bulbous Cordis on the anterior side of that
And as development proceeds, the ventricles
continue to move a bit more
anteriorly. Here's an early electron
micrograph of a developing heart
posterior. We have the atria, which has been
straddled by the outflow,
tracks its more posterior, and as we move
more anteriorly, we see the ventricle
and then the bulbous courtis.
So we're looking at this from an anterior
view and can see that the bulbous cordis and
ventricle are actually located in front of
the atria, which are receiving
the blood. Now, when we discuss formation of
the heart, we're going to
make a single linear flow of blood into four
chambers and two completely separate
What we have to do is follow the development
of each one of these areas as they move from
the embryo logic structure to the adult
The Sinus V.A.C.S is going to form the right
atrium along with the
superior vena cava, the inferior vena cava
and the blood drainage of the heart
itself. The coronary sinus.
The primitive atrium or primordial atrium is
going to form the oracles, the little
dog eared appendages that are just hanging
out on the side of each atrium,
along with quite a bit of the left atriums
The primordial ventricle becomes the left
ventricle and the bulbous Cordis is going to
transition to become the muscular portion of
the right ventricle, along with the outflow
tracks of both ventricles.
So the initial smooth portion of the aorta
and the pulmonary trunk as they
leave the ventricles and travel up to become
the very proximal pulmonary trunk
and proximal aorta.
Thereafter, the aortic sac is going to
divide to become a
very proximal, but just a little further
along portion of the aorta
and pulmonary artery.
Thereafter, the aortic arches will contribute
most of the large vessels, leaving the
heart. So at this point, I'd like to take a
follow the flow of blood through the embryo
logic heart on the left side of your
screen, we've got a sagittal cut.
We've taken the lid off the left side of the
heart and on the right side of your screen.
We've taken a coronal cut through the
anterior wall of the heart.
So as blood enters, it's going to come in
the sinus V.A.C.S.
From there, it's going to enter the atrium,
the primordial atrium and
then pass into the next chamber, which is
the ventricle so atrium
to ventricle. From there, it passes from the
the bulbous Cordis, and the bulbous Cordis
has two subdivisions.
The cone is Cordis, a smoothened area and
the trunk is archdiocese.
The initial outflow track thereafter, it's
going to move to the aortic sac and be
distributed through. However, many aortic
arches are present at that stage of
development. So here we're going to take a
break and then move on to
the further subdivisions of the heart and
how this single flow of blood in a
relatively linear sense becomes two separate
circuits in a four chambered heart.
Thank you very much for your attention.