Development Of gi Tract

Question 1

When does gi development occur

Answer 1

The GI tract development occurs during the 3rd to 12th weeks of embryonic age.

Question 2

Brief ( describe gi tract )

Answer 2

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.

Question 3

What can occur if incorrect development occurs during fo tract development

Answer 3

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.

Question 4

What layers are firmed when zygote develops - and names

Answer 4

When the zygote develops into a blastocyst, three primary germ layers are formed. These are the
mesoderm, ectoderm and endoderm.

Question 5

What does the mesoderm develop into

Answer 5

The mesoderm develops into muscles and connective tissue,

Question 6

What does the ectoderm develop into

Answer 6

the ectoderm develops into external parts of the body (especially the back) and skeletal muscle and

Question 7

What does the endoderm develop into

Answer 7

the endoderm develops into the internal structures of the body.

Question 8

Where specifically do the three germ layers develop from

Answer 8

These three germ layers develop

from the initial blastocyst

Question 9

What does an initial blastocyst comtain

Answer 9

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.

Question 10

What two components of the blastocyst develop to form the inner mass

Answer 10

The hypoblast and epiblast both develop from the inner cell mass.

Question 11

What is the process where hypo and epi blasts develop to form inner mass called

Answer 11

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,

Question 12

Where anatomically do epiblast cells lye - and is this visible name

Answer 12

the epiblast cells in the mid-

line of the embryo ingress from the caudal end and this is visible as the primitive streak.

Question 13

What happens when the epiblast cells ingress

Answer 13

These
ingressing cells will develop into the mesoderm that form surrounding muscles, connective tissue,
mesenteries and blood vessels of the GI tract.

Question 14

What do most epiblast suggest rise to and why

Answer 14

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.

Question 15

What do some epiblast sand hypoglasts form

Answer 15

Some of the epiblast and the hypoblast form the ectoderm that gives rise to
the epithelium of the gut tube and glands.

Question 16

What happens after day 15 of the development of the gi tract

Answer 16

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.

Question 17

How is the initial gut formed after day 15

Answer 17

The initial gut is formed by the folding of sheets of cells in two directions.

Question 18

Describe the folds of the initial gut

Answer 18

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).

Question 19

What do the folds of the initial gut form

Answer 19

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.

Question 20

What forms the umbilical, cord

Answer 20

The yolk sac, allantois and stalk that are not yet integrated
into the gut tube form the umbilical cord.

Question 21

What does the septum transversum form

Answer 21

The septum transversum will go on
to form the diaphragm (under the red moon shaped structure) and the liver
forms within this area.

Question 22

What is the function of the mesenteries

Answer 22

The mesenteries anchor the GI tract to the post

abdominal wall preventing them from moving down into the pelvis.

Question 23

Where do arteries pass to the gi tract

Answer 23

The
arteries that supply some of the organs of the GI tract run through the
mesentery and are known as mesenteric arteries.

Question 24

What are the mesenteries formed of

Answer 24

The mesenteries are formed

from the somatic mesoderm and splanchnic mesoderm.

Question 25

Describe e brief an embryo

Answer 25

The embryo is initially a solid flat disk attached to the hemispherical yolk sac.

Question 26

How is a yolk stalk formed from an embryo

Answer 26

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.

Question 27

What happens to the intestinal portals in the embryo

Answer 27

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.

Question 28

What is the gut tube in an embryo made up of

Answer 28

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.

Question 29

What does the primary gut tube develop into

Answer 29

The primary gut tube then develops into the foregut, the midgut and
hindgut.

Question 30

What does the foregut contain

Answer 30

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.

Question 31

Where does the midgut begin and what does it contain

Answer 31

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.

Question 32

What does the hindgut contain

Answer 32

The hindgut contains the distal third of the

transverse colon, the descending colon and the rectum.

Question 33

What does the superior mesenteric artery supply

Answer 33

The superior mesenteric artery supplies the

midgut,

Question 34

What does the inferior mesh enteric artery supply

Answer 34

the inferior mesenteric artery supplies the hindgut and the

Question 35

What artery supplies the foregut

Answer 35

foregut is supplied by the celiac

artery (the first branch from the abdominal aorta).

Question 36

What branches off from the aorta and where to

Answer 36

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.

Question 37

What develops meat to major branches of aorta

Answer 37

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.

Question 38

What ganglia innervation the foregut superior mesenteric and midgut and hindgut

Answer 38

The
celiac ganglion supplies the foregut, the superior mesenteric ganglion supplies the midgut and the
inferior mesenteric ganglion supplies the hindgut.

Question 39

What is foregut development

Answer 39

Foregut development (stomach development mainly) arises from stomach expansion and rotation.

Question 40

When does foregut development occur and what happens

Answer 40

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.

Question 41

How is the dorsal wall of the stomach attached to the body

Answer 41

The dorsal wall of the stomach is attached to the body by mesentery called the dorsal
mesogastrium. This will form the greater omentum.

Question 42

How is the ventral wall of the stomach attached to the ventral mesentery

Answer 42

The ventral wall of the stomach is attached to

ventral mesentery and this includes the liver. This will form the lesser omentum.

Question 43

What happens and forms when the stomach rotates

Answer 43

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.

Question 44

How are the greater and lesser omentum connected

Answer 44

The greater and lesser omentum are connected by the

epiploic foramen.

Question 45

What does the pyloric sphincter do - describe

Answer 45

The pyloric sphincter controls what enters the duodenum from the stomach. It is a smooth muscle
that contracts when the sphincter is closed.

Question 46

What can occur if development of pyloric sphincter goes wrong - symptoms of disease

Answer 46

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.

Question 47

What organ buds off the foregut during development

Answer 47

An organ that buds off the foregut during foregut development is the liver.

Question 48

How is the budding off of liber initiated

Answer 48

The inducing signal for

this budding comes from the heart and is sent to the ventral gut endoderm.

Question 49

Describe budding and development of liver

Answer 49

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.

Question 50

What does the remaining ventral mesentery of liver give rise to

Answer 50

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.

Question 51

What is the other organ that buds off from the foregut

Answer 51

Another organ that buds from the foregut is the pancreas.

Question 52

What are the pancreatic buds

Answer 52

There are two pancreatic buds; a dorsal

and ventral one.

Question 53

Where does the dorsal bud of the pancreas come from - what induces it

Answer 53

The dorsal bud comes from the duodenal endoderm and is induced by the
notochord

Question 54

Where does the ventral bud if the lampreys comes from - what induces it

Answer 54

whilst the ventral bud from the hepatic diverticulum is induced by the hepatic mesoderm.

Question 55

What happens to the pancreatic buds as the duodenum rotates

Answer 55

As the duodenum rotates during its development, the dorsal and ventral buds meet and fuse.

Question 56

What happens if the ventral bud is bilobed and rotation is opposite

Answer 56

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.

Question 57

How is the midgut anatomically attached

Answer 57

The midgut is attached throughout its
length by the dorsal (and not ventral)
mesentery and is supplied by the superior
mesenteric artery.

Question 58

What feature provides the stereotypical folding of the gut

Answer 58

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.

Question 59

What happens when the midgut needs fi be accommodated

Answer 59

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

Question 60

What happens at 10 weeks to the abdomen

Answer 60

By 10 weeks, the abdomen is big enough to accommodate the midgut
again so the intestines return into it.

Question 61

How do umbilical hernias occur

Answer 61

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.

Question 62

What happens in omphalocele

Answer 62

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.

Question 63

What is gastroschisis

Answer 63

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).

Question 64

What us meckels diverticucl8m

Answer 64

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).

Question 65

What happens in hirschsprung disease

Answer 65

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.

Question 66

What is the cloaca

Answer 66

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).