Development of the gastrointestinal tract: Midgut and hindgut

Question 1

What are the boundaries of the midgut?

Answer 1

Second half of the duodenum to 2/3 of transverse colon.

Question 2

Where is the midgut continuous with the yolk sac?

Answer 2

The vitelline duct.

Question 3

What happens to the midgut in week 5?

Answer 3

The midgut and associated dorsal mesentery undergo rapid elongation to form the primary intestinal loop, which communicates with the yolk sac through the vitelline duct.

Question 4

What structures develop from the cranial limb of the primary intetinal loop?

Answer 4

Distal duodenum, jejunum, and proximal ileum.

Question 5

What structures develop from the caudal limb of the primary intestinal loop?

Answer 5

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

Question 6

Aside from the gut tube, what other GI organ starts to grow during week 6?

Answer 6

Liver.

Question 7

Into what structure must the primary intestinal loop herniate and why?

Answer 7

Umbilical cord - not enough space in the abdo cavity.

Question 8

What also happens to the midgut as is herniates into the umbilical cord.

Answer 8

Rotates 90 degrees anticlockwise bringing the cranial limb to the right and the caudal limb to the left. Jejunal loops also form.

Question 9

Describe the rotation and position of the midgut in week 10.

Answer 9

Returns to the abdomenand rotates a further 180 degrees anti-clockwise. Brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver.

Question 10

What is the name of the diverticulum formed by the caecum?

Answer 10

Vermiform appendix

Question 11

In what week is the vitelline duct obliterated and what does it form?

Answer 11

Week 10 - Meckel’s diverticulum.

Question 12

What is the extent of the anti-clockwise rotation of the midgut by week 11?

Answer 12

270 degrees

Question 13

To where does the caecum decend following the return of the midgut to the abdomen?

Answer 13

Right iliac fossa

Question 14

How do the ascending and descending parts of the colon become secodarily retroperitoneal?

Answer 14

Dorsal mesentery of ascending and descending colon shortens and degenerates pulling them against the posterior abdominal wall - secondarily retroperitoneal.

Question 15

How is the appendix position in the majority of individuals?

Answer 15

Retrocaecally - 64%.

Question 16

Does the appendix have its own mesentery?

Answer 16

Yes - makes it fairly mobile.

Question 17

What other positions may the appendix be found in?

Answer 17

May project inferiorly towards the pelvic brim.

Question 18

How common is Meckel’s diverticulum?

Answer 18

Occurs in 2% of the population. 2x more common in males.

Question 19

Where in the GI tract does Meckel’s diverticulum occur?

Answer 19

2 feet from the ileocaecal junction

Question 20

How long is Meckel’s diverticulum?

Answer 20

2 inches

Question 21

What is the rule of 2s?

Answer 21

Meckels diverticulum: 2% of people, 2x more common in males, 2 feet from ileocaecal junction, 2 inches long.

Question 22

What is omphalocele?

Answer 22

Failure of the midgut to return to the abdomen in week 10. Associated with increased risk of mortality and other malformation eg cardiac or neural tube defect.

Question 23

What is the aetiology of non-rotation of the midgut?

Answer 23

Undergoes initial 90 degree rotation but fails to rotate further 180 when gut it retracted. Results in small intestine on right side and large on the left. Usually asymptomatic.

Question 24

What happens to cause reverse rotation of the midgut?

Answer 24

Initial 90 anti-clockwise rotation occurs but gut then rotates 180 degrees clockwise. Results in a total 90 degree clockwise rotation instead of 270 degrees. Colon in right place but duodenum lie ventral to transverse colon.

Question 25

How can abnormal rotation of the midgut lead to volvulus?

Answer 25

Can lead to parts that would normally be retroperitoneal to be suspended by dorsal mesentery. This can lead to twisting of the midgut and volvulus.

Question 26

What are the symptoms of volvulus?

Answer 26

Bowel obstruction and bilious vomiting. May also constrict arterial supply to the gut causing ischaemia and infarction.

Question 27

What structures does the hindgut give rise to?

Answer 27

Distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and cranial 2/3 of the anal canal.

Question 28

What happens to the distal part of the hindgut?

Answer 28

The anorectal canal entering the dorsal part of the cloaca.

Question 29

What happens to the ventral part of the cloaca?

Answer 29

Becomes the urogenital sinus - forms the bladder, pelvic urethra, penile urethra (males) and caudal part of the vagina.

Question 30

How does the anorectal canal become seperate from the urogenital sinus?

Answer 30

During weeks 4-6 a layer of mesoderm extends caudally seperate the two - uro-rectal septum.

Question 31

What happens as the urorectal septum approaches the cloacal membrane (ectoderm derivative)?

Answer 31

In week 7 the cloacal membrane ruptures creating the anal opening and ventral opening and a ventral opening for the urogenital sinus.

Question 32

What forms the perineal body?

Answer 32

The tip of the urorectal septum.

Question 33

From what is the upper 2/3 of the anal canal formed from?

Answer 33

Hindgut (endoderm)

Question 34

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

Answer 34

Protodeum (anal pit) - ectoderm

Question 35

When do the two parts of the anal canal become continuous?

Answer 35

When the cloacal membrane degenerates.

Question 36

What marks the junction between endoderm and ectoderm in the anal canal of an adult?

Answer 36

Pectinate line

Question 37

How common are congenital rectourethral/rectovaginal fistulas?

Answer 37

1 in 5,000 births

Question 38

What is the aetiology of rectourethral/rectovaginal fistulas?

Answer 38

Caused by abnormal cloaca e.g. too small or failure of urorectal septum to extend caudally. Opening of hindgut is shifted ventrally to the urethra in males and the vagina in females.

Question 39

What is an imperforate anus?

Answer 39

Failure of anal membrane to degenerate. Usually requires immediate surgery. Good long-term prognosis.

Question 40

What is the name of the division of the autonomic nervous system innervating the GI tract?

Answer 40

The enteric nervous system.

Question 41

Name the two enteric plexi?

Answer 41

Myenteric (Auerbach’s): between the circular and longitudinal muscle layers. Coordinates muscle contraction.

Submucosal (Meisser’s): between the circular muscle and mucosa and regulates secretion.

Question 42

From what is the enteric nervous system derived?

Answer 42

Neural crest cells

Question 43

What is Hirschprung disease/congenital aganglionic megacolon?

Answer 43

Failure of neural crest cells to migrate to bowel. Abscence of enteric ganglia leads to bowel obstruction due to lack of peristalsis. This causes dilation of the aganglionic of the bowel - usually rectum and sigmoid colon.

Genetic condition most commonly associated with trisomy 21. Only effective treatment is to remove the affected bowel and anastamose the remaining healthy bowel with anus.