For our last talk on the derivatives of the trilaminar embryo,
we´re gonna return and discuss
how the endoderm forms the lining of the gastrointestinal,
respiratory, and urogenital tracts.
Recall that it´s located far below the ectoderm and then the mesoderm
and its development is tightly linked
with the visceral layer of lateral plate mesoderm.
So the splanchnopleure which is the term we employ for endoderm
and visceral layer of lateral plate mesoderm together
is going to pinch together and form an actual tube.
Simultaneously, we´re gonna have the formation of large vessels just dorsal to it.
These are gonna be the paired dorsal aortae
which are eventually going to fuse into a single aorta.
Now, as the somatopleure wraps around the body
and creates the anterior body wall, simultaneously, the splanchnopleure
and the endoderm that lines the inside of it pinched together to form a distinctive tube.
This is the gut tube and as it does so it separates from the yolk sac
and the portion of the yolk sac that´s remaining
is gonna now be called the secondary yolk sac.
Around day 25, we can also see that the intermediate mesoderm
is forming the early kidney alongside the dorsal aortae.
The dorsal aortae are getting closer and closer and are fusing as this process occurs.
By the 26th day, the gut tube has completely come together
and is connected to the dorsal body wall by what is called a dorsal mesentery.
Now, the dorsal mesentery is of vital importance
because it´s the way that the blood vessels and innervation to the gut tube
are actually able to reach it.
A byproduct of the folding is a ventral mesentery
that connects the gut tube to the anterior body wall.
This disappears pretty much everywhere except the foregut and liver
which is gonna be forming with something called the falciform ligament.
Now, the gut tube is connected to the anterior body wall
because the somatopleure has come together
and fused completely on the anterior aspect of the embryo
and completely enclosed the intraembryonic coelom.
In the process, it has pinched the secondary yolk sac off of the gut tube
and it only remains tethered to it by a small expansion from the midgut.
So if you look at this illustration,
it would be easy to think the yolk sac is completely separate from the body
but there is in fact one tiny point
where it´s connected at what is gonna become our umbilicus.
So it may be appearing outside but it sticks around for a little while,
but eventually, rescinds and doesn´t contribute anything
to the mature developing embryo.
Before we move on, it´s important to get an idea
about how the heart is actually tied to formation of the gut tube.
The heart does not actually come from the gut tube
but it develops in close association to it.
You may recall that the cardiogenic mesoderm migrated very far anteriorly
and in fact, our heart starts developing above our head
and eventually, folds down into our chest.
There´s gonna be little hallow tubes
that develop in the mesoderm called endocardial tubes
and they come together and form a single endocardial tube as the body wall folds.
So here, we can take a look at this illustration
and see the notochord right in the center.
Just below it is where we have the endoderm
and the rest of the splanchnopleure folding together to create a gut tube.
As this folding occurs, the heart tubes on either side are moving anteriorly
and are actually gonna take up residence just in front of the gut tube.
As they do so, they´re pulling a small space called the pericardial coelom along with them.
Now, as the folding of the splanchnopleure occurs,
it brings these heart tubes closer together and actually allows them to fuse
and we wind up with instead of two, just one endocardial tube
that has the heart muscle develop around it.
Now, don´t worry if that´s a little unclear.
We´re gonna have an entire series of lectures on development of the heart.
But what I want you to note now is that the heart
develops very much anterior or ventral to the foregut
and is surrounded by a space called the pericardial cavity.
The heart has to come online and start beating by day 22
because at that point, the cavities that exist inside the embryo
are no longer sufficient to allow diffusion of nutrients, waste products, and gasses,
and we actually need to have a circulatory system develop
and the heart being the pumping portion of that system
is vitally important for a continued development of the embryo.
So here we can see that it´s remaining attached to the gut tube
by a single dorsal mesocardium
but one thing that you wanna appreciate about this picture is
is that the heart´s maybe located ventral to the gut tube
but it´s going to send vessels that go to every single organ of the body
as development proceeds.
So as we move through this process, we come to a point where the embryo,
instead of just being a collection of cells or stacked layers of cells
is actually starting to look vaguely human.
So at day 22, we can see the neural tube folding together,
day 24, we can see several somites laid out
like beads on a string on either side of it.
And then, by day 26, if you squint,
you might convince yourself you see some sort of animal there
with the heart bulge sticking out in the front
and the body wall beginning to fold around and completely enclose everything.
But here, as we move to day 28, we can see limb buds,
distinctive head, a tail, which we eventually lose,
and then, by day 32, once again, doesn´t look completely human
but you can convince yourself that this is a developing animal
with limb buds, heart, and head.
And we´re gonna return to the trilaminar embryos,
the starting point for every lecture more or less moving on from here
because all the organ systems can be followed from that trilaminar stage forward
to make sense of it.
So thank you very much and we´ll return back here
and move forward through every single organ system in the next series of lectures.