Development of GI Tract

Question 1

What is the significance of the gut tube formation?

Answer 1

Organs and glands are produced by budding from the gut tube.

Question 2

Describe primary germ formation.

Answer 2

The primary germ layers are formed during the process of gastrulation.

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).

Most genetic abnormalities occur at this time.

Question 3

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

Answer 3

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 4

The gut tube doesn’t form as a tube the first time round. Describe this phenomenon.

Answer 4

The gut cells will grow to the point where there’s no lumen, just a solid tube. This is the point at which the gut needs rearrangement (scaffolding).
Recanalisation is where some cells die in a manner that leaves behind a large diameter in the middle of the tube.

Sometimes, the middle part of the gut doesn’t recanalise, leading to duodenal atresia (if duodenal, it’s a short-term issue, if jeujenal, then a long-term issue). Soon after birth, babies start projectile vomiting green vomit (green due to bile).

Question 5

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

Answer 5

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 6

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

Answer 6

The coeliac artery supplies the foregut.
The superior mesenteric artery supplies the midgut.
The inferior mesenteric artery supplies the hindgut.

Question 7

What is the innervation of the gut?

Answer 7

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 8

Describe the rotation of the stomach.

Answer 8

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 9

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

Answer 9

Homeotic (HOX) genes.

Question 10

What fixes the mature gut in place?

Answer 10

Fusion of the mesentries with the posterior abdominal wall fixes the mature gut in place.

Question 11

Describe pyloric stenosis.

Answer 11

It is when there is a gastric outlet obstruction caused by smooth muscle hypertrophy (ie. the smooth muscle forming the pyloric sphincter overgrows).
It can be spotted in newborns, as they will start projectile vomiting shortly after feeding (the vomit will NOT be bile-stained).

Question 12

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

Answer 12

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 mesentry.

The remaining ventral mesentry gives rise to:

• the falciform ligament between the liver and the body wall
• the lesser omentum between the liver and stomach

Question 13

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

Answer 13

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.

Question 14

Describe and annular pancreas.

Answer 14

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

Question 15

Describe the development of the intestines.

Answer 15

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.
By 10 weeks, the abdomen is bigger, and the intestines return.

This part of gut development can lead to congenital gut disorders.

Question 16

Describe an umbilical hernia.

Answer 16

It is when the intestines return normally, but the rectus abdominis fails to fuse around the umbilicus, leading to gut covered in skin.

Question 17

Describe an omphalocele.

Answer 17

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 18

Describe gastroschisis.

Answer 18

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 19

Briefly, list and describe other possible congenital gut disorders.

Answer 19

• persistence of the yolk duct (most common)

- Meckel’s diverticulum

Question 20

Describe Hirschsprung’s disease.

Answer 20

It is where the person has an aganglionic megacolon.
It primarily affects the hindgut.

There is an absence of parasympathetic ganglia, caused by a lack of neural crest cells.
There also is a dilatation of sections of the colon, with a lack of tone and peristalsis, leading to profound constipation.

Because of the failure of the development of enteric ganglia, there is a section of the gut that is completely absent of ganglia.

Question 21

In anal development, what is the cloaca?

Answer 21

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 22

What could cause an imperforate anus?

Answer 22

An imperforate anus can be:

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