We’ll begin our exploration of how the foregut develops
by looking at how the foregut and its associated glands separate from other organ systems.
Initially, the foregut is simply the area of the gut tube
leading from the mouth a little further down
to where the respiratory diverticulum buds off
and later, the hepatic or liver bud is going to bud off of it as well.
Initially, it’s going to be a simple tube
but will develop into the various foregut organs
including the distal esophagus, the stomach, the liver,
the gallbladder, the spleen, the proximal duodenum, and the large portion of the pancreas.
It receives its blood from the celiac trunk or celiac artery coming off of the aorta.
In the process of foregut development, we have to break it into multiple steps
because there’s quite a lot going on.
In this talk, we’re gonna look at how the esophagus separates from the airway,
the larynx and trachea,
and then, follow how the stomach goes from a simple tube
to ballooning out and rotating to create its mature shape.
Subsequent talks will look at how the liver, gallbladder, pancreas, and spleen
develop from outgrowths of the gut tube from the foregut
and then, how the foregut organs migrate as the liver,
stomach, and other organs take their final position inside the abdominal cavity.
Now, this lecture is going to reiterate some of the points
that were made when we discussed the respiratory system’s development and in particular,
the development of the large airways.
Early on, we have a respiratory diverticulum growing off of the foregut
and it is going to extend down to create the larynx, the trachea and the bronchi.
But to fully separate from the developing esophagus,
we have a couple of tracheoesophageal ridges growing in from either side
to meet on the midline to create a full tracheoesophageal septum
and that’s what finally separates the airway anteriorly
from the food carrying esophagus posteriorly.
Amongst things that can go wrong in the process of separating the airway
from the esophagus are going to be tracheoesophageal fistulas
or tracheoesophageal anomalies.
The most common version of this is gonna result in an esophageal atresia proximally
and connection of the esophagus to the airway a bit more distally.
What’s going to happen in this case is that an infant who’s affected will attempt to feed
but will then immediately regurgitate formula or milk
because it literally has nowhere to go.
This could be diagnosed by trying to pass an oral gastric tube down the mouth
and since it cannot get to the stomach,
it will loop back and create a very distinctive looped shape on chest x-ray.
Other versions of tracheoesophageal fistulas
can connect the esophagus to the airway and cause aspiration
and may also be indicative of problems with the formation of the tracheoesophageal ridges.
Now, amongst other problems that can occur in this very proximal part of the foregut
are gonna be congenital hiatal hernias.
This is simply the esophagus being too short.
So since the esophagus doesn’t reach the abdominal cavity,
the very proximal portion of the stomach is inside the thoracic cavity
and interestingly, many people have these
and they are completely asymptomatic.
It’s not an uncommon finding amongst other things
when you’re looking at chest x-rays
to note that someone has a congenital hiatal hernia
that they may be completely unaware of.
You wanna distinguish this from a sliding hiatal hernia
where the esophagus is long enough to reach the abdomen
but increased abdominal pressure pushes
the very proximal stomach up into the thoracic area.
So it’s as though it’s just sliding through that opening of the esophagus
into the abdominal cavity through the diaphragm.
It is important to distinguish congenital hiatal hernias
and sliding hiatal hernias from a para-esophageal hernia.
In this case, the esophagus is travelling into the abdomen normally
and does not slide in and out but rather, there’s a weak spot nearby
that allows a portion of the stomach,
likely, the fundus to herniate through alongside the esophagus.
This can occur congenitally and if so, can compress the lungs causing lung hypoplasia
and not allow them to fill completely or develop normally.
We’ll now move on to how the stomach develops from a simple tube,
into its interesting C-shaped and ballooned state.
The stomach begins by rotating in such a way that it’s left side moves anteriorly
and its right side moves posteriorly.
So if I’m the stomach, something like this happens.
As that occurs, the stomach is going to twist around its longitudinal axis
and will then start expanding posteriorly into the left.
Now, note here that the stomach is not hanging out in isolation.
It has a left vagus nerve and a right vagus nerve
travelling down the esophagus to reach it
and as it rotates, it’s going to pull that left vagus nerve anteriorly
and the right vagus nerve posteriorly
and that’s gonna create the left and right vagus nerve’s contribution
to the anterior and posterior gastric nerves respectively.
Continued development of the stomach occurs when the side that’s facing to the left
and slightly posteriorly balloons outwards
and it balloons outward to a much more marked degree than the right side.
So instead of expanding uniformally,
it balloons out to one side and stays pinched on the other
and that’s what causes the stomach
to adopt a relatively C-shaped appearance as it develops
bringing its distal end no longer down but facing to the right.
Rotation of the midgut and migration of the liver
are also involved in taking the stomach to its normal position on the left side of the abdomen
but growth of its greater curvature on the left
and failure of growth of its lesser curvature facing to the right
and somewhat superiorly are gonna be what allows it to take on this ballooned
but slightly unusual shape that we’re used to seeing.
The stomach’s pyloric region empties to the duodenum
which will then have a C-shaped appearance
and will be discussed further when we get to the midgut.
The pyloric region has a very thick smooth muscle sphincter within it
and the pyloric sphincter is what normally allows food to stay in the stomach
until it’s very gradually released as that sphincter relaxes.
Hypertrophy of the pyloric sphincter makes stomach emptying very difficult
and we can see here on ultrasound,
an extended pyloric sphincter with a very narrow lumen
pointing down and a little bit to the left,
and here we can see a barium swallow study
with the fundus in the stomach very well represented and the stomach full of barium
but a very, very thin trickle of barium through that stenotic pyloric canal.
This is typically something that develops between weeks two and six
and is often described as a hard olive-shaped mass
in the upper middle portion of the abdomen.
So it can be preliminarily diagnosed with palpation
but you’d likely follow it up with an ultrasound or a barium swallow study to be definitive.
Moving into the duodenum,
we mentioned in a prior talk that the duodenum is one place where we have a watershed
between the superior mesenteric artery and the celiac artery.
Celiac artery for the foregut and superior mesenteric artery for the midgut.
Because there’s a failure of blood to supply this area cleanly,
there can occasionally be some problems leading to atresia.
Another thing that makes the duodenum prone to trouble
is that during normal development,
its internal lining proliferates so tremendously
that it actually blocks the actual lumen of the duodenum and it becomes impassible.
Normally, apoptosis is going to occur,
the cells will die off and little vacuoles will form, enlarge, and eventually, fuse
to recanalize the duodenum and if development proceeds normally,
we have a complete continuous pathway from the stomach, through the duodenum,
to the rest of the small intestine.
Sometimes these vacuoles do not form properly
and they only partially recanalize the duodenum.
Other times, we can have narrowing of it,
particularly, if there’s an ischemic event associated with the blood supply.
This makes travel of food through the duodenum difficult.
Even more extreme, we can have complete failure of recanalization because the vacuoles,
spaces that form don’t connect or it’s so stenotic, it becomes atretic.
There’s actual atresia of the duodenum.
This is typically diagnosed through what’s known as the double bubble sign
and it’s a funny name but a very good clinical finding to diagnose duodenal atresia
since food and gas cannot pass through the midpoint of the duodenum,
we tend to get gas bubbles trapped in the fundus of the stomach and the initial portion,
the first portion of the duodenum and this double bubble is there
because the gas and food cannot pass any further.
So double bubble sign can happen in any infant with duodenal atresia
but is more common in those with Down syndrome.
Thank you very much for your attention,
and we’ll follow-up on the rest of foregut development shortly.