Embryology midgut and hindgut

Question 1

What is the midgut continuous with initially in the embryo?

Answer 1

The yolk sac at the vitelline duct

Question 2

What happens at week 5 in midgut develpoment?

Answer 2

Midgut and dorsal mesentery undergo rapid elongation to form primary intestinal loop which has cranial and caudal limbs.

Question 3

What will the cranial limb of the primary intestinal loop form?

Answer 3

Distal duodenum, jejunum, proximal ileum

Question 4

What will the caudal limb of the primary intestinal loop form?

Answer 4

Distal ileum, caecum, appenxix, ascending colon, proximal 2/3 transverse colon.

Question 5

What happens at week 6 in midgut development?

Answer 5

Elongation of primary intestinal loop - herniates into umbilical cord and rotates 90 degrees anticlockwise.
Liver also growing which takes up space
Jejunal loops form

Question 6

What happens at week 10 in midgut development?

Answer 6

Midgut returns to abdomen and rotates a further 180 degrees anticlockwise placing the proximal jejunal loops on the left side and the caecum inferior to the liver.
Caecum develops vermiform appendix
Vitelline duct obliterated

Question 7

What is the total rotation of the midgut?

Answer 7

270 degrees anticlockwise

Question 8

What happens after week 10 in midgut development?

Answer 8

The caecum descends into the right iliac fossa oulling the ascending and transverse colon into place.
The dorsal mesentery of the ascending and descending colons shortens and pulls them against the posterior abdominal wall making them secondarily retroperitoneal.

Question 9

What 6 problems can result from abnormal midgut development?

Answer 9

1. Variable position of the appendix
2. Meckel’s diverticulum
3. Omphalocele
4. Non-rotation of midgut
5. Reversed rotation of midgut
6. Volvulus

Question 10

What position are 64% appendices in?

Answer 10

Retrocaecal

Question 11

What position are 32% appendices in and what implication might this have in appendicitis?

Answer 11

Projects inferiorly towards the pelvic brim. Referred pain to pelvis in appendicitis.

Question 12

What is Meckel’s dicerticulum?

Answer 12

Remnant of vitelline duct creating an outpocketing on the ileal wall. May contain ectopic pancreal or gastric tissues causing inflammation/ulceration/bleeding. Rule of 2s

Question 13

What are the rule of 2s associated with Meckel’s divertiulum?

Answer 13

1. Affects 2% population
2. 2x more likely to affect males
3. 2ft from ileocaecal junction
4. 2 inches long
5. Symptomatic in 2% cases

Question 14

What is omphalocele?

Answer 14

In week 10 midgut fails to return to the abdomen. Covered by skin unlike gastroschisis.

Question 15

What is non-rotation of the midgut?

Answer 15

Initial 90 degree rotation normal but no further 180 degree rotation. Results in small intestine on right, colon on left. Asymptomatic.

Question 16

What is reversed rotation of the midgut?

Answer 16

Initial 90 degrees normal, next 180 degrees is clockwise. So total rotation 90 degrees clockwise. Gut all in correct place except duodenum infront of colon.

Question 17

What is a volvulus?

Answer 17

Abnormal rotation can cause sections that would normally be retroperitoneal to remain intraperitoneal e.g. duodedenum. This can lead to a volvulus (twisting of the midgut) –> acute bowel obstruction, bilious vomiting. If constricts arterial supply –> ischaemia.

Question 18

What is the anorectal canal formed from?

Answer 18

Dorsal part of the cloaca

Question 19

What are the bladder, urethra and caudal part of the vagina formed from?

Answer 19

Ventral part of the cloaca (urogenital sinus)

Question 20

What forms in weeks 4-6 to separate the urogenital sinus from the anorectal canal?

Answer 20

Urorectal septum

Question 21

What happens in week 7 in hindgut development?

Answer 21

The cloacal membrane ruptures forming an anal opening and a ventral opening of the urogenital sinus.

Question 22

what is the proctodeum?

Answer 22

The anal pit

Question 23

What layer is the upper 2/3 of the anal canal formed from?

Answer 23

Endoderm (hindgut)

Question 24

What layer is the lower 1/3 of the anal canal formed from?

Answer 24

Ectoderm (proctodeum/anal pit)

Question 25

How do the upper and lower anal canal become continuous?

Answer 25

Cloacal membrane degenerates

Question 26

What is the join between the upper and lower anal canal marked by?

Answer 26

The pectinate line

Question 27

What 2 problems can arise from abnormal hindgut development?

Answer 27

1. Fistulae (rectourethral or rectovaginal) due to too small cloaca or failure of urorectal septum to extend caudally.
2. Imperforate anus (failure of anal membrane to degenerate)

Question 28

What is the innervation of the GI tract?

Answer 28

Enteric NS. 2 plexi- myenteric and submucosal

Question 29

Where is the myenteric plexus and what does it control?

Answer 29

Between the circular and longitudinal muscle layers and it controls muscle contraction

Question 30

Where is the submucosal plexus and what does it control?

Answer 30

Between the circular muscle layer and the mucosa. Regulates secretion.

Question 31

What is the enteric nervous system derived from?

Answer 31

Neural crest cells

Question 32

What is Hirschsprung disease/ congenital aganglionic megacolon?

Answer 32

Neural crest cells fail to migrate to the GI tract. Absence of enteric ganglia so no peristalsis so dilation of aganglionic part. Usually rectum or sigmoid colon. Associated with trisomy 21. Only treatment is to remove that part of the bowel.