We will begin discussing
how the gastrointestinal system develops by looking at the endoderm
and how it folds to create the original gut tube and its associated structures.
Now, the gastrointestinal system begins as a simple tube from mouth to anus
but some very interesting things happen along the way.
It has to extend, elongate, and develop a variety of glands
that are associated with it such as the liver and the pancreas.
As this happens, the gut actually runs out of space in the abdominal cavity
and has to herniate temporarily into the umbilical cord.
The foregut organs not only grow larger but migrate, shift around,
and pull their mesenteries along with them creating some bizarre structures
such as the omentum and various mesenteries
that connect different parts of the gut tube to the body wall.
And last but not least, the common chamber for the reproductive, urinary,
and digestive tracks, the cloaca, has to be subdivided
to keep those systems separate from each other at the time of birth.
So let’s begin by returning to an earlier stage of development,
a roundabout the trilaminar embryo stage as Neurulation is occurring
and as the neural folds are moving in to create the central nervous system,
the splanchnopleure and its underlying endoderm are folding together to create a gut tube
and in the process, extend the yolk sac off of and out of the body wall.
Now, the body wall is going to move forward the somatopleure
surrounding the splanchnopleure and it’s going to form a definitive body wall
both in the thorax, abdominal, and later, the pelvic region.
But as it does so, it needs to allow a little bit of space for two structures to exit the body.
The first is the yolk sac and the second is the umbilical cord.
Within the umbilical cord is another small extension of the endoderm called the allantois
or if you’re a stickler for the proper French pronunciation, the allantois.
As the gut tube continues to turn and twist with development and folding of the body,
the yolk sac gets stretched further and further away
and we wind up with mesoderm
intervening between the endoderm and the ectoderm everywhere
except two places, the oropharyngeal membrane
which is going to rupture to become the mouth,
the stomodeum, and the cloacal membrane which will eventually rupture
to form the anal and opening of the urogenital systems.
So the anal opening and the opening of the urogenital systems
come from the common chamber, cloaca, and the membrane that covered it.
Now, the yolk sac continues elongating out of the body
and will eventually rescind and disappear.
But before it does so,
it remains connected to the developing midgut by a long vitelline duct.
Now, let’s return to a cross-section of the body
and see how the gut tube is suspended by its dorsal mesentery.
The dorsal mesentery comes into existence
because the splanchnopleure is wrapping around
as the gut tube kind of becomes an actual tube
and it allows blood vessels to travel to and from the gut tube.
Nerves, vessels, and other structures that need to support the gut tube
and carry nutrients away,
and carry arterial rich blood to the organs of the GI system
can only get there if there’s a mesentery connecting it to the posterior body wall.
The blood supply to the gut is going to fall into three major vessels,
supplying three major regions of the gut.
The celiac artery or celiac trunk supplies the foregut and the foregut is the stomach,
the proximal duodenum, the liver, the gallbladder, and the majority of the pancreas.
Thereafter, we have the superior mesenteric artery supplying the midgut.
The midgut is the distal duodenum, a part of the head of the pancreas,
and then, the jejunum, the ileum, the cecum, appendix,
ascending colon, transverse colon,
and then, we come to the last segment, the hindgut.
The hindgut organs get their blood from the inferior mesenteric artery
and they are the descending colon, sigmoid colon, and rectum.
So we have three separate arteries supplying three separate
but continuous portions of the gut tube.
Becaues we have three separate blood supplies,
the points where those arteries meet from watershed regions
which are more prone to ischemic events than other places.
An ischemic event would be an interruption in the blood
and therefore, oxygen supply to tissues and can cause the gut tube to become narrowed
causing atresia if it’s completely narrowed
and blocked off or simple stenosis if it’s only narrowed
and just has a difficult time passing tissue or food from one area to the next.
Watershed areas occur where we have two separate blood supplies meeting.
In the esophagus, we have thoracic arterial branches of the esophagus
that are coming from the aorta meeting branches of the stomach
and very distal esophagus coming from the celiac trunk.
So esophageal stenosis may be noted here
due to interruption of blood supply between those two.
The foregut and midgut meet in the duodenum
and because of that, there can be disruption of the duodenum causing atresia
or actual interruption of the duodenum
if there’s insufficient blood supply
coming from either the celiac or superior mesenteric arteries.
The midgut and hindgut meet at the transition
of the transverse colon and the descending colon.
Because of that, we can have ischemic events more frequently there
as branches of the superior and inferior mesenteric artery
are unable to meet and effectively supply the area.
And lastly, the inferior mesenteric artery supplies the hindgut
and where the rectum transitions to the anal region,
we have a separation of blood supply with the inferior mesenteric artery above
and branches of the internal iliac artery below.
So these are the regions that are more prone to ischemic events
than others within the gut tube.
Thank you very much for your attention, and I’ll see you at our next talk.