We’ll continue our investigation of how the foregut develops
by looking at how the organs of the foregut rotate, migrate,
and develop a set of mesenteries and specialized structures called omentum.
We’ve already seen how the stomach rotates
and balloons out to create a greater curvature and a lesser curvature.
It’s hanging from its dorsal mesentery
and as it moves, it’s gonna do some interesting things
to this dorsal mesentery
as other organs develop and force the stomach to rotate.
In particular, we’ve got the liver developing into the septum transversum
and the ventral mesentery that are located anterior to the stomach.
The gallbladder and ventral pancreatic bud
are also present and inferior to the liver.
We can’t see them in this illustration but do remember that they are there.
Whereas the dorsal pancreatic bud and the spleen
will be developing within the dorsal mesentery.
As the liver enlarges, it has room to migrate to the right
and that’s exactly what it does.
It’s tethered to the anterior body wall by the ventral mesentery
which changes its name to the falciform ligament
and connects the liver to the anterior body wall.
As the liver moves to the right side, it pulls on its connection to the stomach
and forces the stomach to rotate so that the stomach’s anterior region
is forced to rotate to the right.
As this occurs, that section of mesentery
will now be called the lesser omentum.
So the lesser omentum connects the liver to the stomach and duodenum.
Posterior to that, we have the spleen
developing within the dorsal mesentery
and because of that, the dorsal mesentery
between the stomach and the spleen
will be known as the gastrosplenic ligament.
This is not a ligament as we’d expect in the musculoskeletal system
that allows muscles to pull on bones but we simply name it
because it is a stretch of connective tissue
connecting two different structures.
The liver continues to enlarge, the stomach continues to rotate,
and as the greater curvature balloons,
the pyloric region and proximal duodenum are forced to the right.
This process creates a space posterior to the liver
and posterior to the stomach
that is called the lesser sac or the omental bursa.
It is part of the peritoneal cavity
but the peritoneal cavity as a whole is only connected to this lesser sac
by a small gap running right underneath the lesser omentum.
This is known as the omental foramen or more commonly to surgeons,
the foramen of Winslow.
So the lesser omentum is going to be containing the entryway
to the lesser sac or omental bursa
from the rest of the peritoneal cavity
which is often referred to as the greater sac.
Now, the dorsal mesentery that’s going to be running
between the gastrosplenic ligament
is going to expand tremendously along the stomach’s greater curvature.
As it expands, it kind of folds forward
and loops back creating a great sheet of tissue that overlies the intestines.
This sheet of tissue is going to become the greater omentum
and initially, it’s continuous with the space posterior to the stomach,
the lesser sac or the omental bursa.
In this image, we can see in the sagittal cut
how the lesser sac is present posterior to the stomach
but anterior to the pancreas
and that space between folds of the greater omentum is continuous with it.
As the greater omentum develops,
those two folds will fuse together
decreasing the space that’s present in the omental bursa.
At the same time, the greater omentum fuses
with the mesentery of the transverse colon.
Because of that, when you encounter the greater omentum
in the abdominal cavity
and lift it up, you may move the stomach,
but you’ll also move the transverse colon
which is present just posterior to the greater omentum.
Now, I remember the first time I investigated the abdomen of a cadaver,
I had no idea what the greater omentum was.
It was not an organ I’d ever heard of.
So upon opening the abdominal cavity,
I saw nothing but a sheet of tissue
and I had a minor freak out until my adviser came by,
lifted up that apron of the omentum,
and showed me that the intestines were indeed there.
Now, as the greater omentum is forming,
we have a fusion of other regions of the mesentery.
The pancreas and spleen develop partially
within the dorsal mesentery of the body
but as the space in the abdomen becomes tighter and tighter,
the pancreas and spleen get pushed towards the posterior body wall.
As that occurs, the mesentery of the pancreas
is going to fuse with the mesentery lining the posterior body wall.
When that happens, the pancreas will become secondarily retroperitoneal.
It’s no longer freely hanging out inside the abdominal cavity,
it’s stuck to the posterior body wall
and the connection of mesentery between it
and the spleen will then be called the splenorenal ligament.
Splenorenal because the left kidney is very close by
and to follow that ligament posteriorly
will take you into the vicinity of the left kidney
and its connection to the spleen.
Thank you very much for your attention and I’ll see you on our next talk.