Embryology 2 - Midgut and Hindgut

Question 1

What are the derivatives of the cranial limb of the midgut loop?

Answer 1

Distal duodenum, jejunum, proximal ileum

Everything else in the midgut comes from the caudal limb

Question 2

Explain what is meant by “physiological herniation”?

Answer 2

The intestines herniate out into the umbilical cord as during growth they become too large to fit inside the abdominal cavity. It is a normal physiological process and resolves later in development.

Question 3

What happens to the midgut during physiological herniation?

Answer 3

Rotates 90* anti-clockwise 3 times:
1st rotation puts them on horizontal plane
2nd rotation puts caudal limb on top of cranial
3rd rotation puts small bowel posterior to large bowel

Question 4

How is physiological herniation resolved?

Answer 4

After 3rd rotation, loop of intestine moves back into the abdomen and the caecal bud begins to descend to form the ascending colon.

Question 5

What error during rotation would result in a left-sided colon?

Answer 5

One 90* turn only

Question 6

What error during rotation would result in the transverse colon sitting posteriorly to duodenum rather than anteriorly?

Answer 6

Reversed rotation - i.e 1x 90* rotation clockwise instead of 3 anti-clockwise

Question 7

What happens if the caecal bud fails to descend?

Answer 7

Absence of ascending colon

Question 8

List 3 abnormalities that result from persistence of the Vitelline duct

Answer 8

Vitelline cyst
Vitelline fistula
Meckel’s diverticulum

Question 9

What is a Vitelline cyst?

Answer 9

Vitelline duct forms fibrous strands either side of a pouch of fluid

Question 10

What is a Vitelline fistula?

Answer 10

Direct communication between the umbilicus and intestinal tract - faecal matter passes out of umbilicus

Question 11

What is a Meckel’s diverticulum?

Answer 11

Out-pouching of the ileal wall as a result of incomplete obliteration of the Vitelline duct

Question 12

Explain the rule of 2s with regard to Meckel’s diverticulum

Answer 12

2 feet from ileocaecal junction
2 inches long
2% of population
2:1 male:female ratio
Usually presents in under 2s

Question 13

Explain the need for recanalisation during development

Answer 13

In some gut structures, cell growth becomes so rapid that the lumen becomes obliterated –> must then be recanalised

Question 14

Give 3 examples of structures that must be recanalised

Answer 14

Oesophagus
Bile duct
Small intestine

Question 15

What happens if recanalisation is unsuccessful?

Answer 15

Atresia - no orifice –> blind-ended tube

Stenosis if only partially unsuccessful

Question 16

Apart from recanalisation failure, why else might you get atresia?

Answer 16

Vascular problems, usually caused by malrotation/volvulus

Question 17

Where is atresia most likely to be due to vascular problems?

Answer 17

Lower duodenum onwards

Question 18

What is gastroschisis?

What does it result in clinically?

Answer 18

Failure of the abdominal wall to close during folding of the embryo
Results in gut tube and derivatives outside of body

Question 19

What is omphalocoele (aka exomphalos)?

Answer 19

Persistence of physiological herniation

Question 20

How does omphalocoele differ from an umbilical hernia?

Answer 20

In an umbilical hernia, the protrusion is covered with skin and subcutaneous tissues, whereas in omphalocoele, the protrusion is covered with amnion only.
Furthermore, umbilical hernias occurred after physiological herniation has successfully resolved.

Question 21

What divides the anal canal into superior and inferior?

Answer 21

The pectinate line

Question 22

Why is there a clear distinction between the regions of the anal canal above and below the pectinate line?

Answer 22

The regions are derived from different embryonic tissues

Question 23

How is the cloaca divided into the urogenital sinus and anorectal canal?

Answer 23

A wedge of mesoderm grows down into it, forming a separation –> becomes the urorectal septum

Question 24

Where is the perineal body found?

Answer 24

At the point at which the urorectal septum meets the cloacal membrane

Question 25

What must occur in order for the anal canal to fully form?

Answer 25

The cloacal membrane must rupture at the proctodeum

Question 26

What abnormality results if the cloacal membrane fails to rupture?

Answer 26

Imperforate anus

Question 27

Which midgut/hindgut structures retain their mesenteries, i.e. are intraperitoneal?

Answer 27

Jejunum/ileum
Appendix
Transverse colon
Sigmoid colon

Question 28

Which midgut/hindgut structures are secondarily retroperitoneal as they lose their mesentery?

Answer 28

Duodenum
Ascending and descending colons
Rectum

Question 29

What are the 2 derivatives of the ventral mesentery?

Answer 29

Lesser omentum

Falciform ligament

Question 30

What is the innervation of the midgut vs the hindgut?

Answer 30

Midgut - parasympathetic = vagus nerve
- sympathetic = superior mesenteric ganglion + plexus
Hindgut - parasympathetic = pelvic nerves
- sympathetic = inferior mesenteric ganglion + plexus

Question 31

Briefly describe what happens as the midgut elongates

Answer 31

Elongates enormously - runs out of space
Therefore forms a loop with 2 limbs - cranial and caudal
SMA points down axis of loop
Loop connected to yolk sac via Vitelline duct

Question 32

During resolution of physiological herniation, what re-enters the abdominal cavity first?

Answer 32

The jejunum

Question 33

Apart from recanalisation failure and vascular accident, why else may there be oesophageal atresia?

Answer 33

Tracheo-oesophageal septum may be posteriorly displaced by chance
Septum may be anteriorly displaced due to pressure on foregut dorsal wall

Question 34

What sign may be observed during pregnancy that indicates oesophageal atresia?

Answer 34

Accumulation of amniotic fluid