Embryology 2. Development of the Midgut and Hindgut

Question 1

What are the derivatives of the midgut?

Answer 1

Small intestine (including most of the duodenum, post bile duct entry), caecum and appendix, ascending colon, proximal 2/3 of transverse colon.

Question 2

What causes the primary intestinal loop to form?

Answer 2

The midgut elongates enormously but runs out of space due to the large liver, so the midgut loops.

Question 3

What does the primary intestinal loop form around?

Answer 3

Superior mesenteric artery.

Question 4

What is the primary intestinal loop connected to the yolk sac by?

Answer 4

The vitelline duct.

Question 5

What are the limbs of the midgut primary intestinal loop?

Answer 5

Cranial and caudal limbs.

Question 6

What are the derivatives of the cranial limb of the primary intestinal loop?

Answer 6

Distal duodenum, jejunum, proximal ileum.

Question 7

What are the derivatives of the caudal limb of the primary intestinal loop?

Answer 7

Distal ileum, cecum, appendix, ascending colon, proximal 2/3 of transverse colon.

Question 8

Why is physiological herniation in the 6th week of development necessary?

Answer 8

The abdominal cavity is too small to accommodate both the growing primary loop and the liver, so the intestines herniate into the proximal umbilical cord.

Question 9

What does the primary loop herniate into in physiological herniation?

Answer 9

The proximal umbilical cord, alongside umbilical vessels.

Question 10

What is the first rotation of the midgut loop?

Answer 10

It rotates around the axis (superior mesenteric artery) in an anticlockwise direction.

Question 11

What is the result of the first midgut loop rotation?

Answer 11

The cranial limb moves to the back and the caudal limb to the front.

Question 12

What is the second rotation of the midgut loop?

Answer 12

It turns 90 degrees anticlockwise twice.

Question 13

What is the overall rotation of the midgut loop?

Answer 13

270 degrees in an anticlockwise direction.

Question 14

Where does the cranial limb end up as a result of rotation?

Answer 14

The left hand side.

Question 15

What happens to the caecum on returning to the abdomen?

Answer 15

It descends into the right lower quadrant.

Question 16

What is the incomplete rotation?

Answer 16

The midgut only makes one 90 degree rotation so there is a left sided colon.

Question 17

What is reversed rotation?

Answer 17

The midgut makes one 90 degree clockwise so the transverse colon passes posterior to the duodenum.

Question 18

What is the result of reversed rotation?

Answer 18

The duodenum can wrap around the transverse colon and occlude it. Hypermobile guts may result.

Question 19

What is a volvulus?

Answer 19

A bowel obstruction where a loop of bowel has abnormally twisted in on itself.

Question 20

What is volvulus formation more likely with?

Answer 20

Hypermobile guts.

Question 21

What can volvulus formation lead to?

Answer 21

Strangulation and ischaemia.

Question 22

What are the derivatives of the hindgut?

Answer 22

The distal 1/3 of the transverse colon, descending colon, rectum, superior part of the anal canal, and epithelium of the urinary bladder.

Question 23

What happens at 6 weeks development to the hindgut?

Answer 23

It ends in the cloaca and is separated from the outside by the cloacal membrane.

Question 24

What is cloacal partitioning?

Answer 24

A wedge of mesoderm grows down into the cloaca, dividing it in to the urogenital sinus anteriorly and the anorectal canal posteriorly.

Question 25

What are the embryonic derivations of the anal canal?

Answer 25

Superior part from hindgut. Inferior part from the endoderm.

Question 26

What are the superior and inferior parts of the anal canal separated by?

Answer 26

The pectinate line.

Question 27

What is the blood supply to the anal canal?

Answer 27

Above pectinate line - inferior mesenteric artery, below - pudendal artery.

Question 28

What is the innervation of the anal canal?

Answer 28

Above pectinate line - S2/3/4 pelvic parasympathetic, below - S2/3/4 pudendal nerve.

Question 29

What is the epithelia of the anal canal?

Answer 29

Above Pectinate line - columnar, below - stratified squamous.

Question 30

What is the lymph drainage of the anal canal?

Answer 30

Above Pectinate line - internal iliac nodes, below - superficial inguinal nodes.

Question 31

What are the sensations of the anal canal?

Answer 31

Above the Pectinate line - only stretch, below - temperature, touch, pain due to somatic innervation.

Question 32

What is Meckel's diverticulum?

Answer 32

The most common GI abnormality. It is a cul-de-sac in the ileum.

Question 33

What is the rule of 2s for Meckel's diverticulum?

Answer 33

2% of the population affected, 2 feet from the ileocaecal valve, 2 inches long, detected in under 2s, 2:1 male:female.

Question 34

What can a complication of Meckel's diverticulum be?

Answer 34

It can contain ectopic gastric or pancreatic tissue which will secrete enzymes and acids, causing ulceration.

Question 35

What is a vitelline cycst?

Answer 35

The vitelline duct forms fibrous strands at either end.

Question 36

What is a vitelline fistula?

Answer 36

A direct communication between the umbilicus and the intestinal tract.

Question 37

What is the result of a vitelline fistula?

Answer 37

Faecal matter coming out of the umbilicus.

Question 38

Why is recanalisation of the primitive gut tube needed in some gut structures?

Answer 38

The cell growth becomes so rapid that the lumen is partially or completely obliterated.

Question 39

What is the aim of recanalisation?

Answer 39

To restore the lumen.

Question 40

What results from failure to recanalise?

Answer 40

Atresia - complete loss of the lumen, or stenosis - narrowing of the lumen.

Question 41

Where does most atresia/ stenosis occur in the GI tract?

Answer 41

In the duodenum.

Question 42

What are the causes of atresias in the duodenum?

Answer 42

Upper duodenum from recanalisation failure, in the lower duodenum from vascular accident.

Question 43

What is a vascular accident causing atresia?

Answer 43

Loss of blood supply means part of the gut dies.

Question 44

What is pyloric stenosis?

Answer 44

A common abnormality of the stomach in infants. Narrowing of the exit from the stomach causing projectile vomiting.

Question 45

What causes pyloric stenosis?

Answer 45

Hypertrophy of the circular muscle in the region of the pyloric sphincter.

Question 46

What is gastroschisis?

Answer 46

The failure of closure of the abdominal wall during folding of the embryo, which leaves the gut tube and its derivative outside the body cavity.

Question 47

What is a complication of gastroschisis?

Answer 47

The gut tube and derivatives have no covering as they herniate through the abdominal wall directly into the amniotic cavity.

Question 48

What is omphalocoele?

Answer 48

The persistence of physiological herniation. Part of the gut tube fails to return to the abdominal cavity following its normal herniation into the umbilical cord.

Question 49

What is imperforate anus?

Answer 49

Failure of the anal membrane to rupture.

Question 50

What is anal/anorectal agenesis?

Answer 50

Failure of development of the anus or anus and rectum.

Question 51

What is a hindgut fistula?

Answer 51

Abnormal connection within the hindgut.

Question 52

What are the mesenteries retained by?

Answer 52

Jejunum, ileum, appendix, transverse colon, sigmoid colon.

Question 53

What are the structures of the midgut and hindgut with fused mesenteries?

Answer 53

Duodenum, ascending colon, descending colon, and rectum.