Development of the GI Tract

Question 1

What is primary germ layer formation?

Answer 1

The primary germs layers are formed during the process of gastrulation
This occurs at the beginning of the third week, the embryo has implanted into the uterine wall
The embryo is a flat disc, comprised of two cell layers at the implantation stage: Epiblast and hypoblast

Question 2

Describe the formation of the primitive streak

Answer 2

Gastrulation begins at the caudal end of the embryonic disc, within the cells of the epiblast.
These epiblast cells move in towards the midline from either side of the embryo and form the primitive streak.
Those cells then involute, ingress and migrate outwards whilst differentiating at the same time to give us the three primary germ layers

Question 3

What are the primary germ layers?

Answer 3

Cells of the primitive streak give rise to:
Mesoderm (1st layer): surrounding muscle, connective tissue and mesenteries
Endoderm (2nd layer): epithelium of the gut tube
Epiblast gives rise to Ectoderm (3rd layer): gut innervation
Hypoblast gives rise mainly to extra-embryonic tissue

Question 4

Describe initial gut folding

Answer 4

The gut tube is formed by folding of sheets of cells in 2 directions:
Folding towards the midline along the cranial-caudal axis
Folding towards the yolk sac at the cranial and caudal ends
During this process gastrulation is also occurring at the same time

The gut tube gets closed at both ends.
At the cranial end it’s closed by a structure called differential membrane and at the caudal end it’s closed by the clocal membrane.

Question 5

The neural ectoderm folds…

Answer 5

The neural ectoderm folds upwards and comes together to form an entire closed neural tube that will then be covered over by the remaining ectoderm.
At the same time hay folding from the 2 lateral sides of the embryo in towards the midline to enclose the yolk sac.

Question 6

What is the mesoderm and the origin of the mesenteries?

Answer 6

Splanchnic mesoderm fuses juntos to form the boundaries of the ventral and dorsal mesenteries

The mesentry is mesoderm that attaches the gut to the body wall
Between the dorsal aspect of the embryo and the gut=dorsal dysentery. Between the ventral aspect of the embryo and the gut=ventral mesenteries.

Question 7

Label this

Answer 7

Question 8

What is the primary gut tube made up of?

Answer 8

Primary gut tube made up of:
sheet of endoderm, which makes the epithelia
surrounding mesoderm, which makes muscle and connective tissue (including mesentery)

Question 9

What specifies the position of GI tract structures?

Answer 9

•Boundaries of Hox gene expression along the cranial-caudal axis specify the position of GI tract structures

Question 10

Describe arterial supply to the GIT

Answer 10

Gut surrounded by plexus of blood vessels, joining vitelline vessels to aorta.
Plexus forms the arteries that supply the GIT from the aorta
These define the boundaries of the gut:
5 arterial branches to the thoracic oesophagus
celiac artery to the foregut
superior mesenteric artery to the midgut
inferior mesenteric artery to the hindgut

Question 11

Where do the sympathetic ganglia develop?

Answer 11

Sympathetic ganglia develop next to major branches of the aorta
Post-ganglionic, sympathetic axons innervate the same tissues that the arteries supply with blood:
Celiac ganglion – foregut
Superior mesenteric ganglion – midgut
Inferior mesenteric ganglion – hindgut

Question 12

Describe the development of the stomach

Answer 12

So the gut tube initially starts to swell and that swelling is asymmetric so it’s more extensive dorsally than it is ventrally

The gut tube is also rotating 90 degrees clockiwise . Diagram D shows further rotation along the cranial-caudal axis so that the greater curvature lies more quarterly and the lesser curvature of the stomach lies more cranially.

Question 13

Describe mesentry attachment of the stomach

Answer 13

Dorsal wall of stomach is attached to the body by mesentery: the dorsal mesogastrium
Ventral wall of somach is attached by ventral mesentery, which includes the liver

Question 14

What happens to the stomach as it starts to undergo rotation?

Answer 14

Because the stomach is not free-floating and because it’s attached to mesenteries, as the stomach starts to undergo rotation, mesenteries attached to it are also drawn in that same movement.
In particular, the dorsal mesogastrium is going to be drawn around as the stomach rotates and encloses the omental bursa
The folded mesogastrium grows to form the greater omentum
Fusion of the mesenteries with the posterior abdominal wall fixes the mature gut in place and also make them retroperitoneal

Question 15

What is pyloric stenosis?

Answer 15

Gastric outlet obstruction caused by smooth muscle hypertrophy
Projectile vomiting shortly after feeding
Pyloric channel elongation, “railroad track”

Question 16

How does the liver organ bud from the foregut?

Answer 16

Signals from the heart to ventral gut endoderm induces the liver to bud from the foregut
Hepatic diverticulum grows into mesenchyme of septum transversum
Cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoids
Liver expands into ventral mesentery
Remaining ventral mesentery gives rise to the falciform ligament and lesser omentum

Question 17

How does the pancreas bud from the foregut

Answer 17

Hay 2 pancreatic buds:

• Dorsal from duodenal endoderm (induced by notochord)
• Ventral from hepatic diverticulum (induced by hepatic mesoderm)

As the duodenum rotates, ventral and dorsal buds meet and fuse so that there will be a common outlet of the pancreas

Question 18

What is an annular pancreas?

Answer 18

As the duodenum rotates, ventral and dorsal buds meet and fuse so that there will be a common outlet of the pancreas
Annular pancreas, which can constrict the duodenum, develops from bi-lobed ventral buds that migrate in opposite directions

Question 19

Describe the development of the intestines in the midgut

Answer 19

Attached throughout length by dorsal but not ventral mesentery
The mesentery and gut grow at different rates, leading to stereotypical folding of the gut
SMA supplies the mid gut. The intestines rotate around the SMA
Abdomen is too small to accommodate, so herniates into umbilical stalk at 6 or 7 weeks
By 10 weeks, the abdomen is bigger, and the intestines return within the body wall

Question 20

What is an umbilical hernia?

Answer 20

Umbilical hernia: Intestines return normally, but rectus abdominis fails to fuse around umbilicus. Therefore the gut covered in skin

Question 21

What is Omphalocele?

Answer 21

Omphalocele is the failure of intestinal loops to return into abdomen, so the hernia is covered in amnion

(causes unknown, but associated with maternal obesity, alcohol/tobacco, SSRI use)

Question 22

What is gastroschisis?

Answer 22

Gastroschisis is the failure of ventral body wall to fuse: no covering
Marked association with young maternal age, low maternal BMI, recreational drugs (especially cocaine)

Question 23

What is Meckels diverticulum?

Answer 23

Meckel’s diverticulum: yolk duct is not obliterated, but is attached to ileum, near the apex of midgut loop. Usually asymptomatic
But the diverticulum can contain ectopic gastric cells which can lead to ulceration
The yolk duct can also be connected to umbilicus by a ligament. In this case volvulus occurs- mid gut rotation can’t proceed properly bc of the attachment to the ligament, which can obstruct the ileum

Question 24

What is Hischsrung’s disease?

Answer 24

Hischsprung’s disease: ganglionic cells of the myenteric and submucosal plexuses in the bowel aren’t present from the anus to a variable length along the large intestine.
Absence of parasympathetic enteric ganglia= lack of parasymp innervation
Caused by lack of neural crest cells
The aganglionic segment remains in a tonic state, leading to failure in peristalsis and bowel movements, causing severe constipation

Question 25

What are neural crest cells?

Answer 25

Neural crest cells originate from the dorsal region of the neural tube
They contribute to a wide variety of tissues in the embryo, and to parasympathetic innervation of the gut
Hay 2 main streams of neural crest cells : the vagal and the sacral neural crest cells.

Question 26

Why is the hindgut region that’s most commonly affected in Hirschsprung disease?

Answer 26

Hay 2 main streams of neural crest cells : the vagal and the sacral
The cells in the vagal stream will enter through the foregut and migrate along the entire length of the neural tube.
Cells that populate the hindgut will primarily enter through this sacral stream.
Defects in this sacral stream in the hindgut are most commonly seen in Hirschsprung disease.

Question 27

What is cloaca? describe its development

Answer 27

The cloaca: the transient common end of digestive and urogenital systems, including the base of the allantois (urogenital sinus)
Covered by a membrane over ectoderm depression called the proctodeum
That cloacal membrane is split by the urorectal septum
This gives rise to the urogenital and anal membrane (perforate at 7-8 weeks so they can open the GI and GU tract)

Question 28

What are the causes of an imperforate anus?

Answer 28

•Imperforate anus can be:

• Persistence of anal membrane
• Atresia of anal canal, rectum or both