development of GI tract

Question 1

When does GI tract development occur?

Answer 1

During 3rd to 12th weeks of embryonic age

Question 2

Briefly describe the formation of the GI tract

Answer 2

1. Primary germ layer formation
   
   2. Gut tube formation
   3. Regionalised changes caused by rotation, swelling and elongation
2. Organs and glands produced by budding from the gut tube

Question 3

Describe the formation of the blastocyst

Answer 3

Zygote —> 2 cells —> 4 cells —-> 8-16 cells —-> blastocyst

Question 4

Why is folate needed in the first trimester of pregnancy?

Answer 4

Folic acid is needed for the correct formation of the spinal cord

Question 5

What are the 3 primary germ layers?

Answer 5

MESODERM = muscles and connective tissue 
ENDODERM = everything inside 
ECTODERM = skeletal and outside

Question 6

Describe the formation of the primary germ layer

Answer 6

• The primary germ layer forms during the process of gastrulation
• Occurs at the 3rd week; the embryo has implanted into the uterine wall
• The embryo is a flat disc, comprised of two cell layers: epiblast and hypoblast. Epiblast cells in the mid-line of the embryo begin to ingress, starting from the caudal (tail) end; this is visible as the primitive streak.
• The ingressing cells differentiate into the mesoderm (that will become the surrounding muscles, connective tissue and mesenteries and blood vessels).
• The epiblast gives rise to the ectoderm (neural crest; it provides innervation to the gut, the enteric NS).
The hypoblast (and epiblast) gives rise to the endoderm (with forms the epithelium and the gut tube and the glands).

Question 7

What is the composition of the embryo?

Answer 7

The embryo is a flat disc, comprised of 2 cell layers

Epiblast
Hypoblast

Question 8

When do most genetic abnormalities occur?

Answer 8

During the primary germ layer formation in the 3rd week

Question 9

Briefly describe the directions of the formation of the gut tube.

Answer 9

The gut tube is formed by the folding of sheets of cells in two directions:
• folding towards the midline along the cranial-caudal axis
folding towards the yolk sac at the cranial and caudal ends

Question 10

What are the subdivisions of the gut, and where do their boundaries lie?

Answer 10

FOREGUT: from the pharynx to the Ampulla of Vater (duodenal papilla)

MIDGUT: from the duodenal papilla to the proximal 2/3rds of the transverse colon

HINDGUT: from the distal 1/3rd of the transverse colon to the rectum

Question 11

What are the blood supplies to the different parts of the gut?

Answer 11

Gut surrounded by plexus of blood vessels, joining vitelline vessels to aorta
The coeliac artery supplies the foregut.
The superior mesenteric artery supplies the midgut.
The inferior mesenteric artery supplies the hindgut.
Sympathetic ganglia develop next to the major branches of the aorta.
Post-ganglionic, sympathetic axons unnerved the same tissues that the arteries supply with blood.
- celiac ganglion: foregut
- superior mesenteric ganglion: midgut
- inferior mesenteric ganglion: hindgut

Question 12

Describe the rotation of the stomach.

Answer 12

First, at the level at which the stomach will form, the tube begins to dilate, forming an enlarged lumen.
It makes a 90° turn about the cranio-caudal (vertical) axis, so the dorsal border grows more rapidly than the ventral border, which established the greater curvature of the stomach.
This means that the anterior part becomes the right, and the posterior becomes the left.

Question 13

What genes determine the position (of the structures) along the cranio-caudal axis?

Answer 13

Homeotic (HOX) genes.

Question 14

What fixes the mature gut in place?

Answer 14

Fusion of the mesentries with the posterior abdominal wall fixes the mature gut in place.
Themesenterysecretes serous fluid, creating lubrication to prevent friction between the internal organs. It is also responsible for holding theabdominalorgans in place. Truemesenteriesconnectsabdominalorgans to the posterior wall of theabdominal cavity.

Question 15

Describe the formation of the liver, budding from the gut tube.

Answer 15

Inducing signal = heart to ventral gut endoderm
The hepatic diverticulum grows into the mesenchyme of the septum transversum. The cords of the hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoids.
The lives exceeds the size of the septum transversum, and expands into the ventral mesentery.
The remaining ventral mesentery gives rise to:
- the falciform ligament between the liver and the body wall
- the lesser omentum between the liver and stomach

Question 16

Describe the formation of the pancreas, budding from the gut tube.

Answer 16

There are two pancreatic buds:

• dorsal, from the duodenal endoderm (induced by the notochord)
• central, from the hepatic diverticulum (induced by the hepatic mesoderm)

As the duodenum rotates, the dorsal and ventral buds meet and fuse.
If the ventral bud bifid (bi-lobed) and one rotates around duodenum, annular pancreas formed, which can obstruct duodenum

Question 17

What is an annular pancreas?

Answer 17

An annular pancreas develops from a bi-lobed ventral bud that migrates in separate directions, coming around and fusing to constrict the duodenum.

Question 18

Describe the development of the intestines.

Answer 18

The mesentery and the gut grow at different rates, leading to the stereotypical folding of the gut.
The abdomen is too small to accommodate this, so the gut herniates into the umbilical stalk at 6/7 weeks.
• Attached throughout length by dorsal mesentery
• The mesentery and gut grow at different rates, leading to stereotypical folding of the gut
• A ventral branch of the aorta supplies the mid gut: superior mesenteric artery (SMA)
With a very rapid increase in length, the intestines rotate around the SMA

Question 19

What is an omphalocele?

Answer 19

It is the failure of the intestinal loops to return to the abdomen, so it ends up as a hernia covered in amnion.
Its causes are unknown, but it is associated with maternal obesity, alcohol/ tobacco, SSRI use.

Question 20

Describe gastroschisis

Answer 20

It is a defect in the abdominal (belly) wall, a failure of the ventral body wall to fuse. This results in no covering of the gut as it sits outside the body.
There is a marked association with young maternal age, low maternal BMI, and recreational drugs (especially cocaine).

Question 21

Describe Hirschsprung’s disease.

Answer 21

• Aganglionic megacolon 
	• Primarily affects 
	• Dilatation of sections of the colon, with lack of tone and peristalsis, leading to profound constipation 
	• Absence of parasympathetic ganglia 
Caused by lack of neural crest cells

Question 22

In anal development, what is the cloaca?

Answer 22

The cloaca is the transient common end of the digestive and urogenital systems, including the base of the allantois (urogenital sinus).
It is covered by the cloacal (proctodeal) membrane over the ectoderm depression, the proctodeum.

It is split by the urorectal septum. This gives rise to the urogenital membrane and anal membrane (that perforate at 7-8 weeks).

Question 23

What could cause an imperforate anus?

Answer 23

An imperforate anus can be:

• persistence of the anal membrane
• atresia of the anal canal, rectum or both

Question 24

What does the cloaca give rise to?

Answer 24

Rectum and urogenital sinuses