Hello and welcome to the first of our discussions on development of the heart.
This amazing process is gonna 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 1% of all births and is a leading cause
of both infant morbidity and mortality,
especially, if it's not diagnosed prior to delivery.
In the United States alone, between 1999 and 2000,
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 organogenic period,
that when the visceral layer of lateral plate mesoderm folds anteriorly
to create the gut tube,
it has two tubes within it called endocardial tubes,
and these are going to be the early primordia 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 gonna pull a small cavity with them.
That cavity is part of the intraembryonic coelom
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 the subsequent talk.
The endocardial 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 beat 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 mesocardium,
which is essentially a mesentery of the heart
connected to the dorsal side of the embryo.
As the heart continues to develop,
it moves from its initial location to its final location
and one amazing thing about the heart,
is 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 mature position.
As it does so, it pulls the pericardial cavity with it,
along with the strip of mesoderm called the septum transversum.
That mesoderm is gonna travel with it
and take up residence inferior to the heart and the septum transversum
is the earliest primordia 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 the subsequent talk.
Now the heart has more or less found its normal position
in the eventual thorax,
and it starts beating about 22 days into development.
This is important because at this point
the embryo can't get much larger without a heart.
Simple diffusion of gasses and nutrients from cells
is no longer sufficient to handle the size of the embryo.
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 22 days
because we absolutely have to have it in order to get any larger.
So the heart pumps bringing blood in from inferior,
peristaltically pumps it out superiorly
and into a paired set of dorsal aorta on the posterior body wall.
So blood that's deoxygenated comes in inferiorly, gets pumped superiorly,
and is gonna 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 gonna develop some distinctive areas that bulge outward.
The first and most inferiorly, is the part that receives blood from the body.
They're gonna be a left and right side of the sinus venosus
and the sinus venosus pumps blood to the primitive or primordial atrium.
From there, the atrium is gonna pump blood to the embryologic ventricle
and then to an area called the bulbus cordis.
Now, thereafter, blood is gonna travel through aortic arches
to reach the dorsal aortae,
and at this point, day 21, 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 bulbus cordis
and distribute it through aortic arches to the dorsal aortae.
Now, as the heart enlarges and gets those distinctive five segments,
the dorsal mesocardium is gonna start to break down
and that's going to allow the heart to essentially sag ventrally
in the developing chest.
And it's gonna sag into the pericardial sac that's already present around it.
As that happens, the ventricle and bulbus cordis wind up more anterior.
The sinus venosus and atria wind up more posterior
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 bulboventricular loop.
And you've got the primordial ventricle just on the back side of that bulge
and the bulbus cordis on the anterior side of that bulge.
And as development proceeds,
the ventricles continue to move a bit more anteriorly.
Here's an early electron micrograph of a developing heart.
Posteriorly, we have the atria,
which has been straddled by the outflow tracks.
It's more posterior and as we move more anteriorly,
we see the ventricle and then the bulbus cordis.
So we're looking at this from an anterior view
and can see that the bulbus 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 gonna make a single linear flow of blood into four separate chambers
and two completely separate circuits.
What we have to do is follow the development of each one of these areas
as they move from the embryologic structure to the adult structure.
The sinus venosus, 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 auricles, the little dog-eared appendages
that are just hanging out on the side of each atrium,
along with quite a bit of the left atrium's wall.
The primordial ventricle becomes the left ventricle
and the bulbus cordis is gonna 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
to the large vessels leaving the heart.
So at this point, I'd like to take a moment
and follow the flow of blood through the embryologic 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 gonna come in the sinus venosus.
From there, it's gonna 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 ventricle to the bulbus cordis,
and the bulbus cordis has two sub-divisions;
the conus cordis, a smoothened area
and the truncus arteriosus, the initial outflow track.
Thereafter, it's gonna move to the aortic sac and be distributed
to however many aortic arches are present at that stage of development.
So here, we're gonna 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.