Lecture 6 - Development of the Gut Continued

Question 1

Describe the layout of the midgut

Answer 1

• Starts at 2nd point of duodenum, ends 2/3 way along transverse colon
• Large bowel sits to right of small bowel, and transverse colon superior to small bowel, this lecture will explain how this layout occurs.

Question 2

What occurs during the 6th week of development in the midgut?

Answer 2

• Intestinal loop elongates rapidly, alongside development of liver
• intestinal loop herniates out of umbilical cord in order to gain space

Question 3

What does the cranial and caudal portion of the intestinal loop go onto form?
What rotations does the midgut undergo to produce the layout of the midgut?

Answer 3

• Cranial = future small intestine
• Caudal = future large intestine
• A 90 degree anticlockwise rotation to cause SI to sit to the right of the large intestine.
• 2 x further 90 degree anticlockwise rotations to cause normal layout, SI on L of large intestine

Question 4

After rotation of the midgut, what occurs in week 10 of development?

Answer 4

Midgut herniation returns to abdomen in its final position

Question 5

What developmental problems can occur if the correct rotations of the midgut do not occur?

Answer 5

• If there’s only 1 x 90 degree rotation you get a left sides colon
• If you get only 1x 90 degree rotation and its clockwise, transverse colon will be posterior to small intestine

Question 6

What is omphalocele and gastroschisis & how are they caused?

Answer 6

• Omphalocele is persistence of abdominal viscera in the umbilical cord after 10th week. Has peritoneal covering still so not exposed to amniotic fluid. High mortality rate due to other developmental abnormalities
• Gastroschisis is failure of abdominal wall to form through incomplete lateral folding. Herniated contents not covered in peritoneum, affecting gut development. Mortality rate much lower than omphalocele

Question 7

What is the vitelline duct & what occurs if it is patent or partially patent?

Answer 7

• Vitelline duct is the connection between midgut and developing yolk sac
• If patent, or partially patent, can get Meckel’s diverticulum causing an outpouching or a bulge in the lower part of the SI.
• Rule of 2: 2% of population, 2 feet proximal to ileo-caecal valve, detected in under 2’s, 2:1 M:F ratio

Question 8

Why does recanalisation of the midgut occur?

What happens if this recanalisation process is unsuccessful?

Answer 8

• Gut tube is occluded by endodermal proliferation, needs to be recanalised during week 9 to form definitive hollow gut tube.
• Partial recanalisation = stenosis (narrowing)
• Unsuccessful recanalisation = atresia (often affects duodenum)

Question 9

What does the hindgut consist of?

Answer 9

• Distal 1/3 of colon to anus

- Descending colon, sigmoid colon, rectum, superior anal canal & bladder epithelia

Question 10

How does the cloaca divide during the 4th to 7th week

Answer 10

• Urorectal septum descends caudally (downwards) dividing cloaca into urogenital and anorectal spaces.
• Cloacal membrane ruptures in 7th week opening up the anorectal canal

Question 11

What is the proctodeum and pectinate line?

Answer 11

• Proctodeum is a bit of stratified squamous ectoderm that covers where the anus would be
• Pectinate line separates proctodeum from columnar endoderm

Question 12

What causes the following hindgut abnormalities:

1) Imperforate anus
2) Anorectal agenesis
3) Fistulae

Answer 12

1) failure to rupture anal membrane
2) failure of blood supply means anus doesn’t form
3) abnormalities during development could cause connection between rectum & anus or even bladder & vagina