Development Of gi Tract Flashcards
When does gi development occur
The GI tract development occurs during the 3rd to 12th weeks of embryonic age.
Brief ( describe gi tract )
It is a tube that is
initially associated with the trachea whereby during the time period mentioned above, it separates
and develops into its own tube.
What can occur if incorrect development occurs during fo tract development
Incorrect development during this stage can lead to aspiration
pneumonia where food ingested simply travels into the lungs (infant not able to feed). The
development of the GI is therefore crucial to its functionality.
What layers are firmed when zygote develops - and names
When the zygote develops into a blastocyst, three primary germ layers are formed. These are the
mesoderm, ectoderm and endoderm.
What does the mesoderm develop into
The mesoderm develops into muscles and connective tissue,
What does the ectoderm develop into
the ectoderm develops into external parts of the body (especially the back) and skeletal muscle and
What does the endoderm develop into
the endoderm develops into the internal structures of the body.
Where specifically do the three germ layers develop from
These three germ layers develop
from the initial blastocyst
What does an initial blastocyst comtain
that contains a trophectoderm around the outside, the hypoblast that will
form the primitive endoderm, and the epiblast develops into the three germ layers discussed.
What two components of the blastocyst develop to form the inner mass
The hypoblast and epiblast both develop from the inner cell mass.
What is the process where hypo and epi blasts develop to form inner mass called
This process of development into
these germ layers is known as gastrulation.
The embryo implants into the uterine wall at the
beginning of the 3rd week during which gastrulation occurs.
In terms of how these different germ layers contribute to the GI tract,
Where anatomically do epiblast cells lye - and is this visible name
the epiblast cells in the mid-
line of the embryo ingress from the caudal end and this is visible as the primitive streak.
What happens when the epiblast cells ingress
These
ingressing cells will develop into the mesoderm that form surrounding muscles, connective tissue,
mesenteries and blood vessels of the GI tract.
What do most epiblast suggest rise to and why
A large part of the epiblast gives rise to the ectoderm
that forms the ENS. This is because the ectoderm forms part of the spinal cord and the innervations
the come from the cord.
What do some epiblast sand hypoglasts form
Some of the epiblast and the hypoblast form the ectoderm that gives rise to
the epithelium of the gut tube and glands.
What happens after day 15 of the development of the gi tract
After day 15 of development, the GI tract has to move out of the region around the amniotic sac due
to the lack of space (no bigger than a couple of inches). The different organs of the GI tract then
rotate and develop before entering again into the now more developed stomach region of the
embryo.
How is the initial gut formed after day 15
The initial gut is formed by the folding of sheets of cells in two directions.
Describe the folds of the initial gut
These folds are
towards the midline along the cranial-caudal axis and the folding towards the yolk sac at the cranial
and caudal ends (see the gut tube towards the centre between days 12 and
30).
What do the folds of the initial gut form
The folds from this initial gut tube form the Bucco-pharyngeal membrane
that forms the head and pharynx, and the cloacal membrane that will form the
rectum and anus.
What forms the umbilical, cord
The yolk sac, allantois and stalk that are not yet integrated
into the gut tube form the umbilical cord.
What does the septum transversum form
The septum transversum will go on
to form the diaphragm (under the red moon shaped structure) and the liver
forms within this area.
What is the function of the mesenteries
The mesenteries anchor the GI tract to the post
abdominal wall preventing them from moving down into the pelvis.
Where do arteries pass to the gi tract
The
arteries that supply some of the organs of the GI tract run through the
mesentery and are known as mesenteric arteries.
What are the mesenteries formed of
The mesenteries are formed
from the somatic mesoderm and splanchnic mesoderm.
Describe e brief an embryo
The embryo is initially a solid flat disk attached to the hemispherical yolk sac.
How is a yolk stalk formed from an embryo
Part of the yolk sac cavity is then enclosed within the embryo by pinching off the yolk sac to form a
yolk stalk and a balloon-like yolk sac.
What happens to the intestinal portals in the embryo
Within the embryo, the cranial and caudal intestinal portals
extend the tube towards the mouth and anus. The prochordal and cloacal plates limit this extension
of the tube.
What is the gut tube in an embryo made up of
The gut tube is primarily made up of the endoderm that makes up the epithelia and the
glands, with the surrounding mesoderm making up muscles and connective tissue including the
mesentery.
What does the primary gut tube develop into
The primary gut tube then develops into the foregut, the midgut and
hindgut.
What does the foregut contain
The foregut contains all the structures from the mouth to the
cranial half of the duodenum. This includes the pharynx, oesophagus,
the stomach and the cranial half of the duodenum. This region also
contains the ampulla of Vater that joins the pancreatic duct with the common bile duct.
Where does the midgut begin and what does it contain
The midgut
begins at the caudal duodenum and extends to the proximal two-thirds of the transverse colon
containing the jejunum, ileum, caecum and the appendix.
What does the hindgut contain
The hindgut contains the distal third of the
transverse colon, the descending colon and the rectum.
What does the superior mesenteric artery supply
The superior mesenteric artery supplies the
midgut,
What does the inferior mesh enteric artery supply
the inferior mesenteric artery supplies the hindgut and the
What artery supplies the foregut
foregut is supplied by the celiac
artery (the first branch from the abdominal aorta).
What branches off from the aorta and where to
There are also 5 arterial branches to the thoracic
oesophagus from the descending aorta. This plexus of blood vessels joins the gut to the aorta.
What develops meat to major branches of aorta
Sympathetic ganglia develop next to the major branches of the aorta. The post-ganglionic
sympathetic axons innervate the same tissues that the respective arteries supply with blood.
What ganglia innervation the foregut superior mesenteric and midgut and hindgut
The
celiac ganglion supplies the foregut, the superior mesenteric ganglion supplies the midgut and the
inferior mesenteric ganglion supplies the hindgut.
What is foregut development
Foregut development (stomach development mainly) arises from stomach expansion and rotation.
When does foregut development occur and what happens
At the 4th week of development, the region of the tube that will become the stomach begins to dilate to
form an enlarged lumen.
• There is then a 90o turn about the cranio-caudal axis of this section of the
tube with its dorsal border not growing more rapidly than its ventral border.
• This forms the greater curvature of the stomach.
• The left and right vagus nerves flank the left and right sides of the stomach.
How is the dorsal wall of the stomach attached to the body
The dorsal wall of the stomach is attached to the body by mesentery called the dorsal
mesogastrium. This will form the greater omentum.
How is the ventral wall of the stomach attached to the ventral mesentery
The ventral wall of the stomach is attached to
ventral mesentery and this includes the liver. This will form the lesser omentum.
What happens and forms when the stomach rotates
When the stomach
rotates, the dorsal mesogastrium is drawn with it and this encloses a space known as the omental
bursa (lesser sac). The folded mesogastrium grows to form the greater omentum with the folds
fusing to obliterate the omental bursa.
How are the greater and lesser omentum connected
The greater and lesser omentum are connected by the
epiploic foramen.
What does the pyloric sphincter do - describe
The pyloric sphincter controls what enters the duodenum from the stomach. It is a smooth muscle
that contracts when the sphincter is closed.
What can occur if development of pyloric sphincter goes wrong - symptoms of disease
There can be an abnormality in development of this
region known as pyloric stenosis. Here, the gastric outlet into the duodenum is obstructed by smooth
muscle hypertrophy. There is a 3 in 1000 incidence of this abnormality. This condition causes
projectile vomiting shortly after feeding (vomit not stained in bile). During this mis development of
the pyloric sphincter, the channel can elongate such that it can be described as a railroad track. This
condition is diagnosed if the length of the passage is greater than 16mm, the wall width is greater
than 4mm and the diameter of the passage is greater than 14mm.
What organ buds off the foregut during development
An organ that buds off the foregut during foregut development is the liver.
How is the budding off of liber initiated
The inducing signal for
this budding comes from the heart and is sent to the ventral gut endoderm.
Describe budding and development of liver
The hepatic diverticulum
(initial part of liver) rises as a bud from the most caudal portion of the foregut and grows into the
mesenchyme of the septum transversum. Cords of hepatic endoderm, bile drainage ducts and blood
vessels proliferate from this diverticulum and these arrange into sinusoids. The liver then exceeds the
size of the septum transversum and expands into the ventral mesentery.
What does the remaining ventral mesentery of liver give rise to
The remaining ventral
mesentery gives rise to the falciform ligament between the liver and body wall as well as the lesser
omentum between the liver and the stomach.
What is the other organ that buds off from the foregut
Another organ that buds from the foregut is the pancreas.
What are the pancreatic buds
There are two pancreatic buds; a dorsal
and ventral one.
Where does the dorsal bud of the pancreas come from - what induces it
The dorsal bud comes from the duodenal endoderm and is induced by the
notochord
Where does the ventral bud if the lampreys comes from - what induces it
whilst the ventral bud from the hepatic diverticulum is induced by the hepatic mesoderm.
What happens to the pancreatic buds as the duodenum rotates
As the duodenum rotates during its development, the dorsal and ventral buds meet and fuse.
What happens if the ventral bud is bilobed and rotation is opposite
If the
ventral bud is bifid (bi-lobed) and rotates around the duodenum in opposite directions, an annular
pancreas eventually forms that can obstruct the duodenum.
How is the midgut anatomically attached
The midgut is attached throughout its
length by the dorsal (and not ventral)
mesentery and is supplied by the superior
mesenteric artery.
What feature provides the stereotypical folding of the gut
The mesentery and gut
grow at different rates leading to
stereotypical folding of the gut. The midgut increases in length very rapidly where it rotates around
the superior mesenteric artery.
What happens when the midgut needs fi be accommodated
The abdomen is now too small to accommodate the elongated
midgut so it herniates into the umbilical stalk in the yolk sac at 6 or 7 weeks. Here, the tube of the
midgut rotates another 180o
What happens at 10 weeks to the abdomen
By 10 weeks, the abdomen is big enough to accommodate the midgut
again so the intestines return into it.
How do umbilical hernias occur
Umbilical hernias occur when the intestines return into the abdomen normally, however the rectus
abdominas fails to fuse around the umbilicus. In this case, the gut is essentially only covered by skin.
What happens in omphalocele
In omphalocele, the intestinal loops fail to return into the abdomen with the hernia covered in
amnion. The cause of this condition is unknown however, it is associated with maternal obesity,
alcohol/tobacco and SRRI (antidepressants) use.
What is gastroschisis
Gastroschisis is the failure of the ventral body wall
to fuse where there is essentially no covering membrane to the midgut and it essentially falls out of
the abdomen. It has an increasing incidence (currently 1 in 3000) with a marked association with
young maternal age, low maternal BMI and recreational drugs (especially cocaine).
What us meckels diverticucl8m
Other
abnormalities of midgut development include Meckel’s diverticulum (2-4% of population). This is a
common intestinal abnormality that is usually asymptomatic. In this condition, the yolk duct attached
to the ileum near the ileo-cecal junction gets inflamed displaying symptoms that are clinically
indistinguishable from appendicitis (right ileac fossa pain). This point is around 2 feet from the
ileocecal junction. It may contain ectopic gastric cells that leads to ulceration and lower GI bleeding.
This connection can be to the umbilicus that can lead to several situations. Ulceration can occur as
well as volvulus upon gut rotation (expansion), diverticulum (narrowing), and incorrect development
(fibrous cord formed).
What happens in hirschsprung disease
In Hirschsprung’s disease, aganglionic megacolon forms (absence of parasympathetic ganglia). This
primarily affects the hindgut with dilations of sections of the colon due to lack of tone and peristalsis
caused by lack of neural crest cells (populate the developing gut and give rise to enteric ganglia) as
they fail to migrate to the correct location leading to absence of ganglion cells. Absence of inhibitory
innervation results in tonic contractions and colonic obstruction. This can lead to profound
constipation.
What is the cloaca
The cloaca is the transient common end of the digestive and urogenital systems that includes the
base of the allantois, the urogenital sinus. It is covered by cloacal (proctodaeal) membrane over an
ectoderm depression called the proctodaeum. This is then split by the urorectal septum giving rise to
the urogenital membrane and the anal membrane (perforation between week 7 and 8). Imperforate
anus can be persistence of the anal membrane or atresia of the anal canal, the rectum or both. There
are diseases where the gut system can interact with the urinary system due to interaction at the
cloaca (pooing and peeing).
