Development of the midgut and hindgut (use IPAD pic WC 06.02.23) Flashcards
Where does the midgut start and end?
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)
Where does the hindgut start and end?
Start: Distal 1/3 of transverse colon
End: Upper anal canal
What is the arterial supply to the mid and hindgut?
Mid: Superior Mesenteric artery
Hind: Inferior mesenteric artery
What is the innervation symp/parasymp of mid and hindgut?
Mid:
S: lesser splanchnic (T10-11)
P: Vagus
Hind:
S: Least splanchnic (T12 +/- L1) and lumbar splanchnic nerves
P: Pelvic splanchnic
Where is visceral pain felt from the mid and hindgut?
Mid: Umbilical region
Hind: Suprapubic region
Where does the gut tube initially form from?
The yolk sac
- Initially cylindrical then tube differentiates into stomach, intestines etc
What are the stages of the midgut development?
- 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
What happens in elongation of the midgut?
- This is the formation of the primary intestinal loop
- Connection to the yolk sac (vitelline duct) maintained, but narrows
What are the two parts of the primary intestinal loop?
Cephalic limb + Caudal limb
What does the Cephalic limb become?
- Distal part of the duodenum
- Jejunum
- Part of the ileum
(TOP LINE OF TUBE)
What does the caudal limb become?
- Distal part of the ileum
- Caecum
- Appendix
- Ascending colon
- Proximal 2/3 of transverse colon
(BOTTOM LINE OF TUBE)
Why does the intestinal loop herniate into the umbilical cord?
The abdominal cavity is too small for the gut loops and the liver, which are both rapidly growing at this time
What rotation occurs during herniation of the intestinal loop?
- It elongates and coils up
- It rotates 90 degrees anticlockwise (viewed from the front) around axis of SMArtery
What happens to the segment destined to become the large intestine?
It elongates but does not coil
What happens during retraction of the gut loop?
- At week 10, gut loop returns to abdomen
- Gut loop rotates another 180 degrees anticlockwise
- This rotation brings things into their final place
What sections of the gut loop are retracted first?
- 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
How is this gut loop then fixed?
- 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
How does the caecal bud and appendix form?
- 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
Why is appendicitis pain felt where it is?
- 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
What abnormalities can occur with this formation of the midgut?
- Gut rotation
- Return of loops to the abdomen
- Mesenteries
What is Meckel’s diverticulum?
- 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
What does the last part of the hindgut communicate with?
The cloaca (which will becomes the anorectal canal)
What is the cloacal membrane?
The boundary between the endoderm lining the cloaca and the surface ectoderm is the cloacal membrane
What does the urorectal septum do?
- It grows towards the cloacal membrane and separates the allantois (foetal membrane) from the cloaca
How does the anorectal canal form?
- 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
How do abnormalities of the urorectal septum occur?
When separation of the hindgut from the urogenital sinus is not complete
