Development of the GI tract

Question 1

When does development of the primitive gut tube begin? What is the internal and external lining derived from?

Answer 1

At the 3rd week:

Internal lining derived from endoderm (future epithelial lining)

External lining derived from splanchnic mesoderm

Question 2

What does the splanchnic mesoderm give rise to?

Answer 2

Future musculature and visceral peritoneum, the mesentery is also formed from a condensation of this mesoderm

Question 3

What are the 2 layers of lateral plate mesoderm?

Answer 3

1. Splanchnic: covers GI tube

2. Somatic: lines body wall

Question 4

Which section of the gut tube has a ventral mesentery? What is the ventral mesentery derived from and what else does it give rise to?

Answer 4

The Foregut, derived from the septum transversum; undifferentiated mesoderm that gets carried to its ventral position during longitudinal folding. Also gives rise to the thoracic diaphragm

Question 5

What forms the greater and lesser peritoneal sacs?

Answer 5

The dorsal and ventral mesenteries divide the foregut cavity into R and L sacs:

• The L sac becomes the greater sac and continues as the bulk of the peritoneal cavity
• The R sac becomes the lesser sac, rotates and ends up lying behind the stomach

Question 6

What does the Lesser peritoneal sac create?

Answer 6

The greater omentum; fold of visceral peritoneum that hangs down from the stomach

Question 7

Name 3 functions of the greater omentum

Answer 7

1. Physically limits the spread of intraperitoneal infections
2. Immune contribution
3. Fat deposition

Question 8

When does the lung bud develop and what does it derive from?

Answer 8

The 4th week from the endoderm

Question 9

Name 2 conditions that can occur as a result of abnormal positioning of the tracheoesophageal septum

Answer 9

1. Proximal blind ended esophagus

2. Tracheoesophageal fistula

Question 10

Which glands are formed from the ventral and dorsal mesentery?

Answer 10

Ventral: Liver, biliary system, pancreas (uncinate process and inferior head)

Dorsal: pancreas majority (superior head, neck, body, tail)

Question 11

How does the stomach’s rotation influence positioning of the surrounding nerves?

Answer 11

The VAGUS n follows the stomach’s rotation:

• L vagus: ends up on the anterior surface
• R vagus: ends on the posterior surface

Question 12

What organs/structures does the foregut give rise to? (6)

Answer 12

1. Esophagus
2. Stomach
3. Pancreas, liver, gallbladder
4. Duodenum (proximal to the entrance of the bile duct)

Question 13

What organs/structures does the midgut give rise to? (7)

Answer 13

1. Distal duodenum, jejunum, ileum, cecum, appendix, ascending colon, proximal 2/3 transverse colon

Question 14

Why does physiological herniation occur?

Answer 14

Since the abdominal cavity is too small to accommodate the rapid simultaneous growth of the liver and the intestinal loop, the intestines must herniate into the umbilicus temporarily

Question 15

How does the rotation of the stomach influence the duodenum?

Answer 15

As the stomach rotates the duodenum grows in a C shaped loop, gets pushed right and then against the posterior abdominal wall (some parts are retroperitoneal)

Question 16

Why is the descent of the cecal bud important and what happens if this fails to occur?

Answer 16

Ensures the ascending colon lengthens, without this descent you may have a sub-hepatic cecum

Question 17

Name 3 things that can occur as a result of a persisting vitelline duct

Answer 17

1. Cyst that can become inflamed
2. Fistula (contents can dribble out)
3. Meckel’s diverticulum: a persisting pouch of endoderm

Question 18

What are the potential consequences of having a Meckel’s diverticulum?

Answer 18

The mucosa can contain gastric and pancreatic tissue that can produce cells secreting acid and/or proteolytic enzymes. If the secretions are high enough it can cause an ulceration

Question 19

What is the rule of 2’s in Meckel’s diverticulum? (5 things)

Answer 19

2% population, 2 feet from the ileocecal valve, 2 inches long, usually detected in under 2s, 2:1 male: female

Question 20

What does Meckel’s diverticulum mimic?

Answer 20

Early stages of appendicitis

Question 21

What is recanalization? What can happen if it’s unsuccessful? Where is it most common?

Answer 21

Cell growth can become so rapid that the lumen may become partially or completely obliterated - posing the need to restore the lumen

If it’s unsuccessful can lead to:

• Stenosis; narrowing of the lumen
• Atresia; lumen obliterated `

Question 22

What is the most common cause of Upper and lower duodenum atresia?

Answer 22

Upper: failure of recanalization
Lower: vascular accident

Question 23

What is gastroschisis?

Answer 23

A defect in the anterior abdominal wall (fails to close), gut tube and derivatives are outside body cavity

Question 24

What is omphalocele?

Answer 24

Persistence of a physiological herniation: SI comes out of the umbilicus

Question 25

What structure is responsible for cloacal partitioning and what is the result?

Answer 25

The urorectal septum provides an anteroposterior division, forming the urogenital sinus and anorectal canal

Question 26

What is an imperforate anus and anorectal agenesis?

Answer 26

Imperforate anus: failure of anal membrane to rupture

Anorectal agenesis: high blind-ending rectum, absence of anus and anal canal

Question 27

What structures have a fused mesentery?

Answer 27

duodenum, ascending and descending colon, rectum