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we will begin discussing how the
gastrointestinal system develops for
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
forged organs not only grow larger but
migrate shift around and pull their
mesentery x' along with them creating
some bizarre structures such as the
omentum and various mesentery x' 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 tracts 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 around about the trilemma
nor embryo stage as neural ation is
occurring and as the neural folds are
moving in to create the central nervous
system the splanchnic lor 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 somatic lure
surrounding the splanchnic floor 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
alan toys or if you're a stickler for
the proper French pronunciation the Alan
tois as the gut tube continues to turn
and twist with development and folding
of the body the
yolk sac get stretched further and
further away and we wind up with Mesa
germ intervening between the endoderm
and the epiderm everywhere except two
places the oral pharyngeal membrane
which is going to rupture to become the
mouth the stoma diem and the cloaca
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 along gating
out of the body and will eventually
rescind and disappear but before it does
so it remains connected to the
developing mid-cut by a long villain
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 splanchnic floor 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 forged 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
mid gut the mid gut is the distal
duodenum a part of the head of the
pancreas and then the jejunum the ileum
the cecum appendix a sending colon
transverse colon and then we come to the
last segment the hindgut
the hindgut organs and 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
because we have three separate blood
supplies the points where those arteries
meet form 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 to 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
mid gut 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 mid gut 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
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you