GI Embryology

Question 1

When does the primordial gut tube form and from what?

Answer 1

Week 4 after lateral folding of the embryo from the endoderm lining of the yolk sac

Question 2

What closes each end of the primordial gut?

Answer 2

An epithelial membrane derived from the ectoderm.
Cranial end = Oropharyngeal membrane
Caudal end = Cloacal membrane

Question 3

From where are the oropharyngeal and cloacal membranes derived?

Answer 3

Both from depressions in the ectoderm known respectively as the stomodeum and proctodeum (anal pit)

Question 4

What overall does the endoderm of the primordial gut form?

Answer 4

Most of the gut, epithelium and glands.

Question 5

From where is the gut muscle derived?

Answer 5

The splanchnic lateral plate mesoderm

Question 6

What are the derivatives of the foregut?

Answer 6

• Primordial Pharynx (& derivatives)
• Lower Resp. Tract
• Oesophagus & Stomach
  Duodenum (Section above the bile & pancreatic duct)
• Liver, Biliary apparatus & pancreas

Question 7

Name 3 pathologies related to development of the lower resp tract and oesophagus:

Answer 7

• Oesophageal Atresia
• Tracheo-Oesophageal -
  Fistula
• Polyhydramnios

Question 8

Define Oesophageal Atresia?

Answer 8

Failure of the oesophagus to recanalize, essentially becoming blocked

Question 9

Define Tracheo-oesophageal fistula>

Answer 9

The septum deviates abnormally causing a connection between the two tubes

Question 10

Define Polyhydramnios?

Answer 10

Oesophageal Atresia leads to fluid collecting in the amniotic sac due to a lack of swallowing

Question 11

How does the stomach move after creation and what does this cause?

Answer 11

The stomach rotates 90 degress clockwise

• Forms the lesser sac (or omental bursa)
• Causes the left vagus nerve to supply its anterior surface and the right its posterior surface

Question 12

What is hypertrophic pyloric stenosis?

Answer 12

Hypertrophy of smooth muscle in the pylorus of the stomach causes stenosis, so no fluids can exit the stomach to the duodenum.

• The child cant eat and will vomit everything up,
• Most common in men.
• Treated surgically

Question 13

What forms the liver and biliary apparatus?

Answer 13

The hepatic diverticulum from the foregut

Question 14

Name one developmental pathology of the liver and one of the bile duct?

Answer 14

Accessory Hepatic Ducts

Extrahepatic Biliary Atresia (Obliterated bile ducts)

Question 15

How does the pancreas form?

Answer 15

Dorsal and Ventral pancreatic bud form from the foregut and fuse when the stomach rotates

Question 16

Name 2 developmental pathologies of the pancreas?

Answer 16

Annular pancreas

• A failure of the ventral bud to migrate around the duodenum properly
• May cause duodenal stenosis

Accessory Pancreatic tissue
- Forms in other areas of foregut

Question 17

How does the spleen form?

Answer 17

End of week 4 & week 5, it develops from mesenchyme

Question 18

What are the derivitives of the midgut?

Answer 18

• Distal Duodenum
• Jejunum
• Ileum
• Caecum
• Appendix
• Ascending Colon
• proximal 2/3rds of transverse colon

Question 19

How does the midgut loop develop?

Answer 19

1) Physiological umbilical herniation - It herniates into the umbilical cord for more space
2) Rotation of midgut loop - rotates 90 degrees counterclockwise within the umbilical cord
3) Retraction of Intestinal loops - Pulls out of umbilical cord and goes 180 degrees further counter clockwise
4) Fixation of intestines - Intestines become fixed to the abdominal wall, particularly duodenum

Question 20

Name 5 developmental pathologies of the midgut?

Answer 20

• Meckels Diverticulum
• Omphalocele
• Gastroschisis
• Abnormal Rotation
• Reversed Rotation

Question 21

Explain Meckels Diverticulum:

Answer 21

A small vitelline duct persists attaching midgut to umbilicus
Can develop into a:
- Cyst
- Fistula 
- Ligament

Question 22

Explain Omphalocele?

Answer 22

Herniated loops fail to retract and abdominal viscera remains herniated through an enlarged umbilical ring

Viscera are still covered in a later of amnion

Question 23

Explain Gastroschisis::

Answer 23

Abdominal contents herniate directly through a the abdominal wall into the amniotic cavity.
They are uncovered by peritoneum or amnion
Occurs through a weak area right of the umbilicus

Question 24

What is included in the hindgut?

Answer 24

• Distal 1/3rd of the transverse colon
• Descending Colon
• Sigmoid Colon
• Rectum
• Anal Canal

Also forms the lining of the bladder and urethra

Question 25

What is the cloaca?

Answer 25

An endoderm lined sac at the end of the hindgut before it turns to drain out the allantois

Question 26

What are the connections of the cloaca?

Answer 26

Hindgut - Enters the cloaca at its dorsal section
Allantois - Leaves the cloaca at its ventral section
Cloacal membrane - Ectodermal membrane seperates the cloaca from anal pit

Question 27

What happens to the cloaca before birth?

Answer 27

• Urorectal septum comes to seperate cloaca into urogenital sinus and anorectal canal
• End of week 7 = Urorectal septum ruptures cloacal membrane, opening the hindgut to outside
• Ectoderm of the anal canal proliferates - closing it with an anal membrane
• Week 9 = Anal canal re-opens

Question 28

Explain some developmental abnormalities of the hindgut:

Answer 28

Causes by incomplete seperation pf cloaca by urorectal septum:

A) Urorectal Fistula
B) Rectovaginal Fistula
C) Rectoanal Atresia
D) Imperforate Anus (Anal membrane doesnt break down)