Development of the midgut and hindgut (use IPAD pic WC 06.02.23)

Question 1

Where does the midgut start and end?

Answer 1

Start: Halfway along the duodenum (distal to the entrance of the bile duct - 3rd and 4th parts of duodenum are midgut)
End: Junction of the proximal 2/3 of the transverse colon with the proximal 1/3 (first 2/3 of the transverse colon are midgut)

Question 2

Where does the hindgut start and end?

Answer 2

Start: Distal 1/3 of transverse colon
End: Upper anal canal

Question 3

What is the arterial supply to the mid and hindgut?

Answer 3

Mid: Superior Mesenteric artery
Hind: Inferior mesenteric artery

Question 4

What is the innervation symp/parasymp of mid and hindgut?

Answer 4

Mid:
S: lesser splanchnic (T10-11)
P: Vagus
Hind:
S: Least splanchnic (T12 +/- L1) and lumbar splanchnic nerves
P: Pelvic splanchnic

Question 5

Where is visceral pain felt from the mid and hindgut?

Answer 5

Mid: Umbilical region
Hind: Suprapubic region

Question 6

Where does the gut tube initially form from?

Answer 6

The yolk sac
- Initially cylindrical then tube differentiates into stomach, intestines etc

Question 7

What are the stages of the midgut development?

Answer 7

• Elongation (growth)
• Physiological herniation (It protrudes out of the abdomen into the umbilical cord - week 6)
• Rotation (around axis of superior mesenteric artery)
• Retraction (back into abdomen - week 10)
• Fixation

Question 8

What happens in elongation of the midgut?

Answer 8

• This is the formation of the primary intestinal loop
• Connection to the yolk sac (vitelline duct) maintained, but narrows

Question 9

What are the two parts of the primary intestinal loop?

Answer 9

Cephalic limb + Caudal limb

Question 10

What does the Cephalic limb become?

Answer 10

• Distal part of the duodenum
• Jejunum
• Part of the ileum
  (TOP LINE OF TUBE)

Question 11

What does the caudal limb become?

Answer 11

• Distal part of the ileum
• Caecum
• Appendix
• Ascending colon
• Proximal 2/3 of transverse colon
  (BOTTOM LINE OF TUBE)

Question 12

Why does the intestinal loop herniate into the umbilical cord?

Answer 12

The abdominal cavity is too small for the gut loops and the liver, which are both rapidly growing at this time

Question 13

What rotation occurs during herniation of the intestinal loop?

Answer 13

• It elongates and coils up
• It rotates 90 degrees anticlockwise (viewed from the front) around axis of SMArtery

Question 14

What happens to the segment destined to become the large intestine?

Answer 14

It elongates but does not coil

Question 15

What happens during retraction of the gut loop?

Answer 15

• At week 10, gut loop returns to abdomen
• Gut loop rotates another 180 degrees anticlockwise
• This rotation brings things into their final place

Question 16

What sections of the gut loop are retracted first?

Answer 16

• Jejunum first to the left side
• Ileum next, settles towards the right
• Caecum returns last to the right upper quadrant
• It then descends to the right iliac fossa
• Caecal hood last part to be retracted

Question 17

How is this gut loop then fixed?

Answer 17

• Mesenteries come into close contact with the posterior abdominal wall and become fused
• this is retroperitoneal
• A fascial layer develops between the parietal peritoneum on the posterior body wall

Question 18

How does the caecal bud and appendix form?

Answer 18

• Caecal bud descends to the right iliac fossa as ascending colon lengthens
• The appendix develops during this descent
• It comes to lie in a variety of positions

Question 19

Why is appendicitis pain felt where it is?

Answer 19

• Dull pain initially around umbilicus (T10 dermatome)
• This is because sympathetic supply is lesser splanchnic (T10- T11)
• Then the pain localises to the right iliac fossa as sharp pain
• This is because appendix starts to irritate the parietal peritoneum which is innervated by somatic nerves overlying skin of anterior abdominal wall

Question 20

What abnormalities can occur with this formation of the midgut?

Answer 20

• Gut rotation
• Return of loops to the abdomen
• Mesenteries

Question 21

What is Meckel’s diverticulum?

Answer 21

• Vitelline duct normally degenerates
• This duct is between midgut loop and umbilical cord
• However it can persist to form an out-pouch which can ulcerate and bleed in some cases
• Pain mimics that of appendicitis

Question 22

What does the last part of the hindgut communicate with?

Answer 22

The cloaca (which will becomes the anorectal canal)

Question 23

What is the cloacal membrane?

Answer 23

The boundary between the endoderm lining the cloaca and the surface ectoderm is the cloacal membrane

Question 24

What does the urorectal septum do?

Answer 24

• It grows towards the cloacal membrane and separates the allantois (foetal membrane) from the cloaca

Question 25

How does the anorectal canal form?

Answer 25

• Ectoderm of cloacal membrane invaginates inwards which forms the anal pit
• The cloacal membrane ruptures so the upper and lower parts of anal canal become continuous
• Because of their different origins the upper and lower parts of anal canal have different blood supplies and epithelia

Question 26

How do abnormalities of the urorectal septum occur?

Answer 26

When separation of the hindgut from the urogenital sinus is not complete