Development & Congenital Abnormalities of the Upper GI tract

Question 1

How is the GI tract subdivided?

Answer 1

• Esophagus
• Stomach
• Small intestine
  
  • duodenum
  • jejunum
  • ileum
• Large intestine
  
  • cecum
  • appendix
  • colon
  • rectum
  • anal canal

Question 2

What do the 3 GI germ layers give rise to?

• Ectoderm:
• Mesoderm (Splanchnic mesoderm):
• Endoderm:

Answer 2

• Ectoderm** ⇒ enteric nervous system**
• Splanchnic mesenchyme ⇒ muscle, connective tissue, and other layers of the wall of the gut
• Endoderm ⇒ epithelial components of the gut

Question 3

Following gastrulation, small indentations develop first in the anterior and then in the posterior of the embryo producing the ….

Answer 3

foregut diverticulum (anterior intestinal portal) and the hindgut diverticulum (caudal intestinal
portal)

Question 4

What is the result of folding of the embryo for the GI tract?

Answer 4

1. Folding in the transverse plane creates:
   
   • primitive gut tube from the endoderm
2. Folding in the sagittal plane creates:
   
   • 3 subdivisions of the primitive gut tube:
     
     1. foregut
     2. midgut
     3. hindgut

Question 5

What is the associated ventral branch of the dorsal aorta for each subdivision of the primitive gut tube?

Answer 5

1. Foregut: celiac trunk
2. Midgut: superior mesenteric artery
3. Hindgut: inferior mesenteric artery

Question 6

What does the foregut give rise to?

Answer 6

1. esophagus
2. thyroid
3. lung
4. stomach
5. liver
6. pancreas
7. proximal duodenum (1^st and 2^nd parts)

Question 7

What does the midgut give rise to?

Answer 7

1. distal duodenum (3^rd and 4^th parts)
2. jejunum
3. ileum
4. cecum
5. appendix
6. ascending colon
7. 1/3-1/2 of the transverse colon

Question 8

What does the hindgut give rise to?

Answer 8

1. remaining transverse colon
2. descending and sigmoid colon
3. rectum
4. superior part of the anal canal

Question 9

The stomach is a ______ ___ between the esophagus and intestine where food undergoes both _________
and _______ digestion to form _____

Answer 9

The stomach is a dilated sac between the esophagus and intestine where food undergoes both mechanical
and chemical digestion to form chyme

Question 10

1. When does the simple gut tube begin to dilate?
2. How is it attached to the body wall?
3. What flanks the stomach to the left and right?

Answer 10

1. The simple gut tube in the foregut region destined to form the stomach begins to dilate around week 4
2. Attached to the body wall by the dorsal and ventral mesenteries
3. Left and right vagus nerves flank the the left and right sides of the stomach, respectively

Question 11

What happens between weeks 5-7 for the formation of the stomach?

Answer 11

Week 5:

• The primordial stomach enlarges and broadens along the dorsal-ventral axis
• The dorsal wall of the stomach expands more quickly than the ventral wall ⇒ creating the greater curvature of the stomach and the lesser curvature of the stomach

**Week 7: **

• stomach rotates 90° clockwise around its longitudinal axis
• greater curvature, which arose from the embryonic dorsal side, now faces the left side of the body
• lesser curvature, which arose from the embryonic ventral side, faces the the right side of the body

Question 12

Results from rotation of the developing stomach:

• Left vagus nerve ⇒
• Right vagus nerve ⇒
• What is produced behind the stomach?
• How does it affect the postitioning of stomach and the duodenum?
• Dorsal mesentery extends from the greater curvature forming the _______ _______
• Ventral mesentery also attached to the developing liver is known as the ______ _______

Answer 12

• left vagus nerve ⇒ supplies the anterior wall of the mature stomach (anterior vagal trunk nerve)
• right vagus nerve ⇒ supplies the posterior wall (posterior vagal trunk nerve)
• Produces a space behind the the stomach referred to as the lesser sac or omental bursa
• Pulls the stomach and duodenum upward placing these organs in their final body position
• Dorsal mesentery extends from the greater curvature forming the greater omentum
• Ventral mesentery also attached to the developing liver is known as the lesser omentum

Question 13

• The space posterior to the stomach is referred to as the _____ ___ or ______ _____
• The space anterior and inferior to the stomach is known as the ______ ___

Answer 13

• The space posterior to the stomach is referred to as the lesser sac or omental bursa
• The space anterior and inferior to the stomach is known as the greater sac

Question 14

What type of epithelium is in the mature stomach?

Answer 14

simple columnar

Question 15

The liver, gallbladder, and the biliary duct system develop as an outgrowth of the ______ ______ _______

Answer 15

The liver, gallbladder, and the biliary duct system develop as an outgrowth of the ventral foregut
endoderm

Question 16

What are the three distinct phases of liver bud (hepatic diverticulum) formation?

Answer 16

1. Foregut endoderm composed of polarized columnar epithelial cells protrudes into the surrounding septum transversum mesenchyme
   
   • derives from the splanchnic mesoderm between the heart and the midgut
   • gut lumen and the basal surfaces contact laminin-rich basement membrane
2. Simple columnar epithelium transforms to a pseudostratified epithelium encased in basement membrane
3. Basement membrane is degraded, and bipotential hepatoblasts delaminate and migrate into the septum transversum mesenchyme forming cords of hepatic cells within the mesenchyme
   
   • hepatoblasts have the potential to differentiate into ⇒
     
     • hepatocytes
     • cholangiocytes

Question 17

• Why are FGFs and BMPs important to the developing liver?
• What are endothelial cells required for?

Answer 17

• FGFs and BMPs secreted from the heart and the septum transversum mesenchyme are essential for proper specification and outgrowth the the primordial liver bud
• Endothelial cells surround the pseudostratified bud
  
  • required for delamination and expansion of the liver

Question 18

• Why is the fetal liver important?
• What are sinusoids?

Answer 18

• The fetal liver is also an important site of hematopoiesis
  
  • key site of hematopoiesis prior to the onset of marrow hematopoiesis.
  • gives the liver its bright red appearance
• Sinusoids: blood vessels residing at the basal surface of polarized hepatocytes
  
  • derive from vessels resident in the septum transversum mesenchyme via angiogenesis

Question 19

1. What forms the gall bladder?
2. What forms the cystic duct?
3. How is the bile duct formed?

Answer 19

1. A small caudal region of the liver bud gives rise the the gall bladder
2. The stalk of the bud forms the cystic duct
3. The stalk connecting the hepatic and cystic ducts to the duodenum becomes the bile duct
   
   • This duct initially attaches to the ventral aspect of the duodenal loop
   • As the duodenum grows and undergoes rotation ⇒ bile duct is carried to the dorsal aspect of the duodenum

Question 20

• Where do the two buds of the developing pancreas arise from?
• How does rotation affect the pancreatic buds?
• What eventually happens to the buds?

Answer 20

• The pancreas develops as two buds emanating from the dorsal and ventral foregut endoderm
  
  • The endoderm gives rise to the exocrine and endocrine epithelial cells of the pancreatic parenchyma
• As the developing duodenal region of the small intestine rotates rightward and takes on a “C” shape, the ventral bud is carried dorsally to lie posterior to the dorsal pancreatic bud
• The buds eventually fuse

Question 21

Formation of the small and large intestines:

1. endoderm:
2. splanchnic mesenchyme:
3. neural crest:

Answer 21

• **endoderm: **
  
  • simple columnar epithelium that covers the villi and crypts of Lieberkuhn
  • duodenum
• splanchnic mesenchyme:
  
  • smooth muscle and connective tissue
  • duodenum
• neural crest:
  
  • neurons that innervate the gut
    
    • i.e. enteric nervous system
  • duodenum

Question 22

1. As the duodenum expands, it transforms from a ______ tube to a __ ______ tube
2. As the developing stomach rotates, the duodenum also rotates to the ____
3. How is development of the duodenum similar to esophageal development?
4. ___________ results in restoration of the lumen.

Answer 22

1. As the duodenum expands, it transforms from a straight tube to a “C” shaped tube
2. As the developing stomach rotates, the duodenum also rotates to the right
3. Similar to esophageal development, the epithelial cells derived from the endoderm proliferate to occlude the lumen of the gut tube
4. Re-canalization results in restoration of the lumen.

Question 23

As the midgut elongates during week 5 of development, it forms a ventral U-shaped tube termed the ….

Answer 23

As the midgut elongates during week 5 of development, it forms a ventral U-shaped tube termed the midgut loop or primary intestinal loop

Question 24

• The midgut loop has a _____ limb and _____ limb
• What arises from each limb of the midgut loop?
• What normally happens by week 6 of midgut loop development?

Answer 24

• The midgut loop has a cranial limb and caudal limb
• cranial limb ⇒
  
  • bulk of the small intestine (distal duodenum, jejunum, and most of the ileum)
• caudal limb ⇒
  
  • distal ileum
  • cecum
  • appendix
  • parts of the colon (ascending colon and part of the proximal transverse colon)
• by week 6 of development the midgut loop:
  
  • physiological umbilical herniation
    
    • organs expand more quickly than the body cavity itself expands ⇒
    • herniates through the umbilicus into the umbilical cord

Question 25

* What happens to the midgut loop while herniated? * What is the result?

Answer 25

* While herniated, the midgut loop **rotates 90° counterclockwise around the axis of the superior mesenteric artery ** * Results in positioning of: * **cranial limb** on the **right** * **caudal limb** on the **left **

Question 26

What normally happens by the **10^th week** of midgut development?

Answer 26

* Body cavity has grown sufficiently to contain the intestine * **Midgut retracts into the body cavity ** * cranial limb retracts prior the the caudal limb * During retraction, the **midgut undergoes further rotation of 180° counterclockwise** around the axis of the superior mesenteric artery * **cranial limb is localized to the left** side of the body * **caudal limb occupies the right** side of the body cavity

Question 27

What happens from week 11 onwards, with regards to the midgut?

Answer 27

* transverse colon rests in front of the duodenum. * Initially, the cecum and a short ascending colon rest underneath the liver. * As the ascending colon grows and elongates, the cecum descends placing the cecum and appendix in the lower right quadrant of the body. * During the fetal period, the vitelline duct regresses and disappears

Question 28

What is the division between midgut-derived and hindgut-derived ascending colon is marked by?

Answer 28

transition of the blood supply from: ## Footnote **superior mesenteric artery ⇒ inferior mesenteric artery**

Question 29

What is the **cloaca**?

Answer 29

expanded terminal region of the hindgut

Question 30

**Partitioning of the cloaca** occurs as mesenchyme known as the ________ \_\_\_\_\_\_ grows and expands

Answer 30

Partitioning of the cloaca occurs as mesenchyme known as the **urorectal septum** grows and expands.

Question 31

* **dorsal cloaca ⇒ ** * **ventral cloaca ⇒ **

Answer 31

* dorsal cloaca ⇒ **rectum and part of the anal canal** * ventral cloaca ⇒ **urogenital sinus**

Question 32

* **superior anal canal** develops from \_\_\_\_\_\_ * **inferior anal canal **develops from the \_\_\_\_\_\_\_\_, which is \_\_\_\_\_\_\_\_\_\_-derived

Answer 32

* superior anal canal develops from **hindgut** * inferior anal canal develops from the **proctodeum**, which is **ectodermally-**derived

Question 33

**Failure or incomplete re-canalization of the duodenum leads to:**

Answer 33

* **Duodenal atresia:** complete occlusion * rare and is usually associated with other congenital anomalies * **Polyhydramnios** (excess amniotic fluid in the amniotic sac) * **occurs with duodenal atresia** * complete blockage prevents intestinal absorption of swallowed amniotic fluid * **Duodenal stenosis:** partial occlusion * Blockage causes _vomiting of stomach contents as well as bile_

Question 34

What is **extrahepatic biliary atresia**?

Answer 34

* **Most serious anomaly of extrahepatic biliary system development** * Most commonly, **obliteration of bile ducts** is the cause (85% of cases) * **Jaundice** occurs _soon after birth_ * **Stools are acholic** (_clay colored_) * If unable to repair ducts surgically, biliary atresia will be fatal without a liver transplant.

Question 35

**Gastroschisis** 1. Result of? * Consequences? 2. Clinical presentation: 3. Etiology: 4. What can it be mistaken for?

Answer 35

1. **Results from defect in anterior abdominal wall** * Abdominal viscera extrudes through wall **without involvement of the umbilical cord** * Viscera exposed to amniotic fluid resulting in **serositis** (inflammation of serosa) 2. **Usually occurs on the right side lateral to the umbilicus** 3. Etiology not well understood: * multifactorial * vascular event * environmental factors 4. **NOT A HERNIA!**

Question 36

**Omphalocele: ** 1. Definition: 2. What organs are involved? 3. Clinical associations:

Answer 36

1. **Herniation of the abdominal viscera into the proximal umbilicus** 2. **Intestine fails to return to the abdomen** * can also include liver, stomach, gonads 3. Usually associated with other congenital anomalies including cardiac and urogenital defects

Question 37

**Rotational defects:** 1. **Nonrotation or malrotation** 2. **Reverse rotation**

Answer 37

1. **Nonrotation or malrotation** * occur when the **gut fails to rotate completely** upon returning to the body cavity * Large intestine ⇒ more leftward. * Small intestine ⇒ more rightward. 2. **Reverse rotation** (clockwise vs. counterclockwise) * results in **anterior positioning of the duodenum in relation to the tranverse colon**

Question 38

**Mispositioned intestine** can lead to ....

Answer 38

**volvulus** ⇒ obstruction, infarction, tissue death

Question 39

**Meckel's diverticulum:** 1. Definition: 2. What can it mimic? 3. What is the problem with containing all layers of the ileum?

Answer 39

1. **Meckel’s diverticulum** = _outpocketing of the_ _ileum_ resulting from **persistence of the vitelline duct** * Cysts and fistula can also occur 2. Can become inflamed and _mimic appendicitis_ 3. Can include epithelial cell types of the stomach and pancreas ⇒ enzymes!

Question 40

**Hirschsprung’s disease:** 1. Result of? 2. Clinical presentation: 3. Why does dilation occur? * What is contained in the dilated regions? 4. Clinical complication:

Answer 40

1. **Aganglionosis of the colon** 2. Presents as **megacolon** 3. **Dilation occurs because the affected tissue, which lacks ganglion cells, fails to relax** * Enlarged dilated region contains normal neural crest derived ganglion cells 4. **Prevents movement of bowel contents**