8. Gastrointestinal - Development of the Midgut and Hindgut

Question 1

What structures are derived from the midgut?

Answer 1

1. Duodenum (beyond bile duct entry)
2. Jejunum
3. Ileum
4. Caecum and appendix
5. Ascending colon
6. Proximal 2/3 transverse colon

Question 2

How does the primary intestinal loop form?

Answer 2

1. The midgut elongates enormously and runs out of space
2. Forms this loop
3. The loop has the superior mesenteric artery as its axis, connected to the yolk sac via the vitelline duct, has cranial and caudal limbs
4. Physiological herniation: during 6th week, intestines herniate into the umbilical cord (as liver is also growing rapidly and abdominal cavity is too small to accommodate both)

Note: green line = vitelline duct

Question 3

Describe midgut rotation

Answer 3

1. 3 rotations, 90 degrees each time
2. First rotation within umbilicus
3. Second and third after returning to abdominal cavity (in week 10). Note: cranial limb returns to the cavity first, moving to the left side
4. Rotations account for positions of the small and large intestines, as well as the twisted appearance of the mesentery of the small intestine

Question 4

Describe some consequences of malrotation of the midgut

Answer 4

1. Incomplete rotation = midgut only makes one 90 degree rotation = left-sided colon
2. Reversed rotation = midgut loop makes one 90 degree rotation clockwise = transverse colon passes posterior to the duodenum
3. Volvulus = obstruction caused by twisting of stomach/intestine either around itself or a mesentery

Question 5

What occurs if the vitelline duct persists?

Answer 5

1. Vitelline cyst
2. Vitelline fistula (direct communication with umbilicus and intestinal tract
3. Meckel’s diverticuum (ileal diverticulum) - rule of 2’s = 2% of pop, 2feet from ileocaecal valve, usually detected in under 2’s, 2:1 ratio male:female. may be free or attached to the umbilicus. midgut so often confused with appendicitis. Can contain ectopic gastric or pancreatic tissue.

Question 6

What is recanalisation, where does it occur?

Answer 6

1. Recanalisation occurs to restore the lumen
2. e.g. in the duodenum which grows rapidly and thickened obliterating the lumen. recanalisation occurs to restore the lumen.
3. Also occurs in oesophagus and bile duct

Question 7

What problems can occur if recanalisation is not successful?

Answer 7

1. Can be wholly or partially unsuccessful
2. = Atresia (lumen obliterated) or stenosis (lumen narrowed)
3. Most occur in duodenum
4. Upper duodenum atresia = failure of recanalisation
5. Lower duodenum atresia = vascular accidents (due to malrotation, volvulus, body wall defect)

Question 8

Discuss pyloric stenosis

Answer 8

1. Not due to recanalisation failure!
2. Hypertrophy of circular muscle in the region of the pyloric sphincter
3. Common abnormality in stomach of infants
4. Causes characteristic projectile vomiting

Question 9

Discuss anterior abdominal wall defects

Answer 9

1. Gastrochisis - failure of closure of abdominal wall during folding of embryo = leaves gut tube and derivatives outside body cavity. No peritoneum or amnion covering of viscera. can become necrotic.
2. Omphalocele - persistence of physiological herniation. differs from umbilical hernia as hernias have skin + subcutaneous tissue covering. Omphalocele has peritoneal covering.

Question 10

What structures are derived from the hind gut?

Answer 10

1. Distal 1/3 transverse colon
2. Descending colon
3. Rectum
4. Superior part of anal canal
5. Epithelium of urinary bladder

Question 11

Describe the partitioning of the cloaca and the development of the anal canal

Answer 11

1. At first, the hindgut ends blindly at the cloacal membrane, which separates it from the proctadaeum
2. Cloaca undergoes anterioposterior division
3. A wedge of mesoderm (urorectal septum) grows down into the cloaca, dividing it into the urogenital sinus and anorectal canal posteriorly

Question 12

What is the anal canal divided into, and what by? What is it’s functional relevance?

Answer 12

1. Superior and Inferior parts
2. By the pectinate line
3. Indicates differences in arterial supply, venous and lymphatic drainage and innervation

Question 13

What is the blood supply, lymphatic drainage and innervation of the anal canal 1) above pectinate line and 2) below pectinate line?

Answer 13

Question 14

Relate the different areas of the gut with their respective pain patterns.

Answer 14

1. Visceral pain is poorly localised
2. Foregut and its derivative = epigastric pain
3. Midgut = periumbilical
4. Hindgut = suprapubic

Note: parietal peritoneum receives somatic innervation

Question 15

Describe some hindgut abnormalities

Answer 15

1. Imperforate anus - failure of anal membrane to rupture
2. Anal/anorectal agenesis
3. Hindgut fistulae

Question 16

What tissues do the cranial and caudal limbs of the primary intestinal loop give rise to?

Answer 16

Cranial

1. Distal duodenum
2. Jejunum
3. Proximal ileum

Caudal

1. Distal ileum
2. caecum
3. appendix
4. ascending colon
5. proximal 2/3 transverse colon

Question 17

Describe the timeline for gut development

Answer 17