Development of the GI tract

Question 1

Give an overview of the development of the GI tract

Answer 1

GI tract development occurs during 3rd to 12th weeks embryonic age (5 to 14 weeks LMP)

Primary Germ Layer Formation

Gut tube formation

Regionalised changes caused by rotation, swelling, elongation

Organs and glands produced by budding from the gut tube

Question 2

Describe the formation of the zygote

Answer 2

1. Zygote to 16th cell

2. We then have a blastocyte including the inner cell mass, epiblast and hypoblast and trophectoderm

Question 3

Describe the process of primary germ layer formation - 1

Answer 3

The primary germs layers are formed during the process of gastrulation
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:
1. Epiblast
2. Hypoblast

Question 4

Describe the process of primary germ layer formation - 2

What does the epiblast do and what does it form?

What does the Mesoderm form?

What does the epiblast form?

What does the hypoblast form?

Answer 4

Epiblast cells in the mid-line of the embryo begin to ingress, starting from the caudal end
Visible as the Primitive Streak
Ingressing cells differentiate into Mesoderm
surrounding muscles, connective tissue and mesenteries and blood vessels.
Epiblast gives rise to Ectoderm (neural crest)
innervation of the gut (Enteric NS)
Hypoblast (and epiblast) gives rise to Endoderm
epithelium of the gut tube and glands

Question 5

What is the purpose of gastrulation?

Answer 5

Gastrulation generates the three primary germ layers

Question 6

Describe the process of initial gut folding and tube formation

What does the somatic mesoderm do?

What does the splanchnic mesoderm do?

Answer 6

The gut tube is formed by 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

Think of the Gut as a tube

The gut tube forms at 12-30 days. Look at slides of the structure of it.

Somatic mesoderm = voluntary

Splanchnic mesoderm = gut motility, nervous system

Question 7

Give an overview of formation of the gut tube?

What is the gut tube made out of?

Answer 7

The embryo is initially a solid flat disk attached to the hemispherical yolk sac (& similarly to amnion)

Part of the yolk sac cavity is enclosed within the embryo by pinching-off the yolk sac to form a yolk stalk and balloon-like yolk sac

Within the embryo, the cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the prochordal and cloacal plates

Primary gut tube made up of:

1. sheet of endoderm, which makes the epithelia and glands
2. surrounding mesoderm, which makes muscle and connective tissue (including mesentery)

Question 8

Describe the regionalization and subdivision of the primary gut tube

Answer 8

Foregut:
Pharynx
Oesophagus
Stomach
Cranial half of duodenum
Ampulla of Vater
(joining of common bile duct  and pancreatic duct)

Midgut
Caudal duodenum (From duodenal papilla )
Jejunum
Ileum
Caecum
Appendix
Ascending colon
Proximal 2/3 of transverse colon

Hindgut
Distal 1/3 of transverse colon
Descending colon
Rectum

Question 9

How does arterial supply defines regions of the gut?

Answer 9

Foregut is up to the celiac artery, it branches anteriorly, and other branches going to the liver, spleen and stomach. Mouth to the second part of the duodenum

The midgut is up to the mesenteric artery. Third part of the duodenum to 2/3rds or transverse colon.

Hindgut is up to the mesenteric artery.

Question 10

Describe the arterial blood supply to the GI tract

What does the arterial blood supply develop into?

Answer 10

Gut surrounded by plexus of blood vessels, joining vitelline ( relating to, or associated with the yolk of an egg) vessels to aorta.

Plexus resolves to form the arteries that supply the GI tract from the aorta

These define the boundaries of the gut:

1. ~5 arterial branches to the thoracic oesophagus (from the descending aorta)
2. celiac artery to the foregut
3. superior mesenteric artery to the midgut
4. inferior mesenteric artery to the hindgut

Question 11

How does sympathetic ganglia develop next to major branches of the aorta?

Answer 11

Post-ganglionic, sympathetic axons innervate the 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 foregut - Stomach development

What does it arise by?

When does it arise and where?

What does the stomach attach to and form?

Answer 12

Stomach arises by expansion and rotation

During week 4 at the level where the stomach will form the tube begins to dilate, forming an enlarged lumen

Initially concave ventral, convex dorsal:

1. 90 degree turn about cranio-caudal axis
2. The dorsal border grows more rapidly than ventral, which establishes the greater curvature of the stomach

Dorsal wall of stomach attached to body by mesentery: the dorsal mesogastrium (will form greater omentum)
Ventral wall attached by ventral mesentery, which includes the liver (will form lesser omentum)

Question 13

Describe the mesenteric attachments from the stomach to the liver

Answer 13

Mesenteric arteries are arteries of the small intestine

It connects the liver and the stomach

Question 14

Describe the foregut development

Answer 14

As the stomach rotates, the dorsal mesogastrium is drawn with it

This mesogastrium encloses a space, the omental bursa (lesser sac)

The folded mesogastrium grows to form the greater omentum, the folds fusing to obliterate the bursa

Question 15

What is peritoneum?

Answer 15

The serous double membrane lining the cavity of the abdomen and covering the abdominal organs.

Not all organs are covered by it.

Question 16

What is Pyloric Stenosis?

What does it cause?

What is the incidence level?

What occurs as a result of this abnormality?

Answer 16

An abnormality of foregut development.

Gastric outlet obstruction caused by smooth muscle hypertrophy
3 in 1000 incidence
Projectile vomiting shortly after feeding (not bile-stained)
Pyloric channel elongation, “railroad track”
L >16mm, wall >4mm, diameter >14mm

Question 17

Describe Organ budding from the foregut - liver

What initiates it?

What does the Hepatic diverticulum grow?

Answer 17

Inducing signal: heart to ventral gut endoderm

Hepatic diverticulum grows into mesenchyme of septum transversum

Cords of hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoid

Liver exceeds size of septum transversum, expands into ventral mesentery

Remaining ventral mesentery gives rise to:

1. falciform ligament between liver and body wall
2. lesser omentum between liver and stomach

Question 18

Describe organ budding from the foregut - pancreas

Answer 18

Two pancreatic buds:

1. Dorsal from duodenal endoderm (induced by notochord)
2. Ventral from hepatic diverticulum (induced by hepatic mesoderm)

As duodenum rotates, ventral and dorsal buds meet and fuse

Question 19

Describe the formation of the midgut: intestines

Answer 19

Attached throughout length by dorsal mesentery (but not ventral mesentery)

The mesentery and gut grow at different rates, leading to stereotypical folding of the gut

With a very rapid increase in length, 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

Question 20

What is an Umbilical hernia?

Answer 20

Intestines return normally, but rectus abdominis fails to fuse around umbilicus: gut covered in skin

Question 21

What is Omphalocele?

What causes it?

Answer 21

Failure of intestinal loops to return into abdomen:
hernia covered in amnion
(causes unknown, but associated with maternal obesity, alcohol/tobacco, SSRI use)

Question 22

What is Gastroschisis?

What causes it?

Answer 22

Failure of ventral body wall to fuse: no covering
Increasing incidence (1 in 3000).  Marked association with young maternal age, low maternal BMI, recreational drugs (especially cocaine)

Question 23

Give some more abnormalities of intestinal (midgut) development

Answer 23

Persistence of yolk duct (normally obliterated)

Most common intestinal abnormality
Yolk duct attached to ileum, near ileo-cecal junction - apex of midgut loop (about 2 feet from ileocaecal junction)

Meckel’s diverticulum (2-4% population) usually asymptomatic

Can get inflamed (Meckel’s diverticulitis clinically indistinguishable from acute appendicitis)

May contain ectopic gastric cells: ulceration and lower GI bleeding

Can be connected to umbilicus by ligament
Gut rotation causes volvulus
Can form umbilical fistula

Question 24

Abnormalities of intestinal development: Hirschsprung’s disease - describe this disease

Answer 24

Aganglionic megacolon

Primarily affects the hindgut

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 25

Describe the development of the hindgut: Anal development - the cloaca

Answer 25

The cloaca is the transient common end of digestive and urogenital systems, including the base of the allantois (urogenital sinus)

Covered by cloacal (proctodeal) membrane over ectoderm depression, the proctodeum

Split by the urorectal septum

This gives rise to the urogenital membrane and anal membrane (perforate at 7-8 weeks)

Imperforate anus can be:

1. Persistence of anal membrane
2. Atresia of anal canal, rectum or both