We will now discuss the development of the midgut
and how it relates to the other portions of the gastrointestinal system.
The midgut begins as a U-shaped loop of the gut tube
which stretches out along the umbilical cord and has the vitelline duct
connecting it to the yolk sac, extending, continuing through the umbilical cord.
The midgut will become the distal portion of the duodenum,
a portion of the pancreas, and then, the jejunum, ileum, cecum,
vermiform appendix, the ascending and transverse colon
and its blood supply comes through the superior mesenteric artery.
The major events that are going to occur in midgut formation
are relatively simple in theory.
They are rotation and elongation.
The rotation of the midgut occurs along its blood supply.
The superior mesenteric artery acts as the axis for the rotation of the midgut
and it undergoes a 270 degree rotation.
And if you were looking at me from the front as you are,
that would appear to go in the counterclockwise direction.
The proximal cephalic loop,
the part that’s on the top, is going to move inferiorly
to become the distal duodenum, the jejunum, and the ileum.
The more caudal loop,
the part on the underside is going to move superiorly into the right
to become the rest of the small intestine, the ileum,
as well as the cecum, appendix, ascending and transverse colon.
Now, in addition to rotating along that 270 degree axis,
there’s gonna be a tremendous amount of elongation of the midgut at this point.
This elongation and just pure space that the midgut is taking up
causes it to herniate out into the umbilical cord during week six.
During normal development however, the abdominal cavity will enlarge
and the gut will return into the abdomen by week 10.
This is called a physiologic herniation
and it is a normal event in development so long as it goes away.
If it maintains its presence in the umbilical cord for an extended period,
that will be problematic.
Now, things that can go wrong is that you can have the gut rotate improperly
and lay down in the wrong position.
Before we can understand the things that go wrong,
let’s do a little more review
on how the organs of the midgut wind up in their mature position.
So if we think about the midgut extending out from my umbilicus,
I’m gonna take a hard right turn like I’m driving a car
and the more distal portion in my left hand
is going to move to the right side of my abdomen.
That’s why we find the cecum and appendix
in the lower right quadrant of the abdomen.
Likewise, the more cephalic structures like the jejunum
are going to be curling this way and lay down on the left side of my abdomen.
Typically, the cecum comes to rest in the lower right hand corner and the jejunum
tends to rest in the upper left corner of my abdomen
and the ileum is laid down progressively more to the right as time goes by.
As the cecum is moving into the lower right quadrant,
it has the appendix bud off of it and elongate.
Because of that, the appendix is coming into existence
as the cecum is coming into its final position.
And so, the appendix may be floating lose out in the mesentery,
have its own attachment to the body wall,
or it may be tucked under the cecum and relatively obscured.
That’s why finding an appendix
when you’re trying to remove one can sometimes be problematic.
One thing to note is that there are smooth muscle bands
on the surface of the large intestine
called teniae coli and if you find them
and follow them down to the base of the cecum,
they always terminate at the vermiform appendix.
So if you’re having trouble finding it, locate one of the teniae coli
and it’s your roadmap to the appendix.
As the gut tube reaches its final position
and the midgut is going to get to the place where it belongs,
portions of it will fuse to the posterior body wall and lose their mesentery.
The mesentery doesn’t literally disappear
but as it folds back on the posterior body wall, it does fuse to a limited degree
and those portions of the midgut are now immobile.
These are called secondarily retroperitoneal structures.
Secondarily because they didn’t used to be retroperitoneal
but they became that way.
And this would be the ascending colon, the descending colon,
as well as most of the duodenum, very little of the duodenum has a mesentery.
Structures that do have a mesentery
like the jejunum, the ileum, the appendix,
the sigmoid colon, and the transverse colon
are all referred to as intraperitoneal structures.
Meaning, that they’re loose in the abdomen and suspended by a mesentery.
Now, just to fill out the end of that scheme,
primary retroperitoneal structures
are those that never existed in the abdominal cavity
and were always located posterior to it such as the kidney and the adrenal glands
and we’ll discuss those in their own separate lecture.
As the midgut is returning into the abdomen,
it has the vitelline duct stretching to the yolk sac present coming off of the ileum.
Typically, that connection is going to disappear as the yolk sac disappears
and there’ll be no trace of it.
However, you can have what’s known as an ileal or Meckel’s diverticulum.
In this case, there’s a tiny little pouch sticking off of the ileum
which is a remnant of that vitelline duct and its connection to the yolk sac.
This is occurring in what we referred to as the rule of twos.
About 2% of the population are affected.
It affects males in a two to one ratio versus females.
You find it within two feet of the ileocecal valve where the ileum meets the cecum.
They’re usually approximately two inches long
and you often find two types of mucosa in there.
Accessory gastric or pancreatic tissue
can be found alongside normal intestinal tissue
and generally, they’re going to occur
and present before the patient is two years of age.
Now, these ileal diverticula can also be relatively clinically invisible.
They don’t always manifest and some people have them and don’t know it.
Problems that can occur are that occasionally, you don’t just have a little pouch.
You can actually have a little cord of tissue anchoring that ileal diverticulum
to the umbilicus from within.
So it’s tethered to the anterior body wall.
The problem with this is that it can occasionally get twisted around that cord
causing ischemia and volvulus.
Volvulus means twisting and in this case, it’s going to cause a great deal of pain
as that portion of the gut is deprived of its blood supply and becomes ischemic.
Other problems that can occur
is you can have little remnants of that connection
if the vitelline duct maintain their connection to the ileum
and form little cysts either at the umbilicus or just inside the body wall.
These may enlarge and become painful,
and once again, serve as a locus for volvulus
to occur if the intestine starts wrapping around that connection.
And you can also have what are referred to as vitelline fistulas.
A fistula is any inappropriate connection from one thing to another.
A connection that shouldn’t be there.
So a fistula is something that punched through from one space to another
and in this case, an ileal diverticulum
has a vitelline fistula connecting it to the umbilicus
and you can guess how this is going to present.
The infant will have fecal material or partially digested food
leaking out of his or her umbilicus.
Now, malrotation of the midgut is one other thing
that can go wrong in the process of midgut development.
Remember, we typically have a 270 degree rotation
like I’m turning my car to the right.
So counterclockwise from your view.
But sometimes, the gut only turns 90 degrees and then, moves back in.
In this case, the more distal portion of the midgut winds up on the left side.
So I’m going to have all of my large intestine
and cecum packed into the left side of my abdomen
and more proximal structures like the jejunum and ileum on the right side.
We can also have gut rotation go the opposite direction
and go 270 degrees clockwise from your view or from my view.
I’m taking a hard left turn as I turn the steering wheel
and my gut will wind up strangely in about the right place.
The organs will wind up more or less
where we’d expect them to but the duodenum
instead of being posterior to the transverse colon is going to be anterior to it
and can sometimes block the transverse colon
and cause some intestinal blockage and a bit of colonic obstruction.
Now, these malrotations are often asymptomatic
and are found incidentally during abdominal operations or imaging.
Things that will manifest however when midgut rotations and malrotations
go in very strange directions and wind up blocking the passage of food
and fecal material through the intestines.
Mixed rotation where one segment rotates
but the other does not can produce a variety of strange appearances
to where the gut contents were laid down.
But the important thing to know is that all the gut components are still there,
they’re just in the wrong place.
The problem is we can wind up with volvulus
as one portion of the intestine wraps itself
around another fixed structure, or another organ that’s in the wrong place.
This volvulus as we mentioned before
with the ileal fistula and ileal diverticulum
can wrap the intestine around itself,
close off its blood supply and result in ischemia and death if it’s not treated.
Malrotation can also produce some problems
where a portion of the gut that wants to be retroperitoneal
attempts to fuse to the body wall
and winds up blocking another portion.
If our gut rotates normally, the parts that are going to fuse
to the posterior body wall have a relatively unobstructed pathway there.
If malrotation occurs,
they may compress other portions of the gut on their pathway
to seal themselves off with the body wall.
And as before, that can obstruct movement of fecal material
and food through the gut
or compromise the blood supply to the affected area.
Thank you very much for your attention and we’ll return with another topic.