Hello and welcome to the first talk we're going to have
on the development of the vascular system.
We're gonna start by discussing the aortic arches and very large arteries
that are gonna be leaving the heart.
So it's hard to think about the time
when we didn't have blood and blood vessels in our body
but in early, early development
we have our cells fed by diffusion from fluid filled chambers.
But as we get larger and larger we eventually need a circulatory system
that allows us to get nutrients to and from the cells as our body gets larger and larger.
So formation of blood, blood vessels and the heart
all have to work in close coordination
so that when one is present, the others are present and the heart can pump
and actually have something to pump with and through.
You're no good to have a heart if you don't have vessels and vice versa.
So at the very earliest time we're gonna start seeing vessels at around about day 17.
Little things called blood islands are starting to form inside the yolk sac.
Now, proper term would be angiogenic cell clusters,
clusters of cells that are going to make blood.
And within day 18,
we're gonna start seeing the same sort of things appearing in the mesoderm,
both in the embryo and in the connecting stalk leading to the eventual placenta.
Now inside these angiogenic cell clusters,
we have mesoderm that's starting to hollow out
and create vessels and within them are little blebs of cells
and these are the hematopoietic cell clusters
that are gonna start generating the fetal blood supply,
but not fetal blood supply, embryologic blood supply
which then becomes the fetal blood supply.
Early on, this is where blood comes from but it's eventually gonna switch to the liver,
spleen, thymus and bone marrow
as those organs become available sites for hematopoiesis.
So let's talk about the vessels that leave the heart and we'll return
and talk about the veins in the subsequent lecture.
The vitelline artery is the artery that supplies the yolk sac
and while that's not gonna sound like a big deal right now,
it's a very big deal in the early embryo
and it's tied to the development of our gastrointestinal system.
The umbilical arteries are even more important
because they're taking poorly oxygenated blood
from the fetus to the placenta where it's gonna be oxygenated
and then return to the umbilical veins.
More on those in just a little while.
And last but not least, we've got arteries that are distributing all over the body.
So the dorsal aorta on the right and left fuse into a single dorsal aorta
but midway down the developing embryo
and there are going to be little segmental branches
that are leaving it all along its length supplying those tissues.
So we're gonna start by discussing how the heart pumps blood into the aortic sac
and then distributes it to the dorsal aorta through aortic arches.
Now there are gonna be 1, 2, 3, 4, 6 aortic arches that develop in humans.
The 5th arch does not tend to form because humans do not have
that 5th set of gill arches that some other animals do.
This is tied to the development of the pharyngeal arches
and we have a separate lecture on that
that you can check out if that sounds at all mysterious or unclear.
So I want you to note that the heart will pump blood in the aortic sac,
the blood will then travel posteriorly through the aortic arches
to reach the right and left dorsal aorta.
At that point, blood is gonna travel through intersegmental arteries
to reach the various portions
of the body that are developing like the neck, the torso, the limbs and so on
and then a little further down it'll fuse to a single dorsal aorta.
One very important relationship right now is that
the gut tube is forming posterior to the heart
but it's gonna be travelling anterior to the dorsal aorta.
That's gonna be important when we discuss vascular slings
and how malformations of the blood supply can occur.
Now if we take a look at this structure,
the early aortic arches to my mind looked a bit like one of those facehugger aliens
from the Aliens movies.
They are wrapping around to get to the dorsal aorta from the aortic sac.
The first and second arches are gonna tend to disappear very quickly
and they don't contribute a great deal to the mature vasculature.
The 1st arch will remain as a bit of the maxillary artery
and the 2nd arch will remain as a tiny little bit of the stapedial artery,
the artery that supplies the smallest of the inner,
pardon me, the middle ear bones, the stapedius.
The 3rd, 4th and 6th pharyngeal arteries
are the ones that really make a big difference.
I want you to know that the 3rd arch, 4th arch and 6th arch
all attach to the dorsal aorta,
however, the 3rd and 4th arch are going to have their connection
to the dorsal aorta separate.
So if I back up just a moment,
you can see here that the dorsal aorta
are continuous with all the arches feeding into each of them
but as the bone proceeds the connection between the 3rd arch and 4th arch
is going to disappear and the dorsal aorta will detach.
The 3rd arch is going to become the internal/external,
but primarily the common carotid artery,
so internal/external carotid arteries are branching off of it
but the 3rd arch is gonna make the common carotid artery leaving towards the head.
The 4th arch is connected to a more posterior and inferior portion of the dorsal aorta,
so it's going to arch out and head down the body.
And then the 6th arch does something very interesting.
It's going to disappear on the right,
but retain it's connection to the dorsal aorta on the left.
So follow the 3rd arch going to the head.
4th arch and 6th arch stand a little lower down on the body
and as we go a little further along,
the 4th arch is going to stay connected to the right side
which will become the right subclavian artery and on the left side it'll retain its connection
to the dorsal aorta and become part of the aortic arch.
Now, before we move a little further along to the 6th arch
I'd like to take a moment and look at how this blood supply interacts with the gut tube.
Remember that it's wrapping around the gut tube
from anterior to posterior to reach the dorsal aorta.
As this is happening, the lungs are starting to develop off of the gut tube
and the lung buds are gonna grow forward from the foregut and as that happens
the blood supply associated with the developing lungs
is going to be coming from the 6th aortic arch.
So the 6th aortic arch is going to give off the eventual pulmonary arteries.
So at this point on the picture on the right side of the screen,
we can see that the pulmonary arteries are connecting to the 6th arch
and the 6th arch is connecting to the dorsal aorta.
Eventually, we're gonna leave both pulmonary arteries uninterrupted
but we're going to detach the right 6th arch from the dorsal aorta,
whereas on the left it retains its connection to the dorsal aorta
and becomes a structure called the ductus arteriosus.