Session 3: Development of the Midgut and Hindgut

Question 1

What is the primitive gut formed from?

Answer 1

The definitive yolk sac during folding.

Question 2

What can be found in the primitive gut, cranially and caudally respectively?

Answer 2

Buccopharyngeal membrane cranially Cloacal membrane caudally

Question 3

What will the midgut form into?

Answer 3

Most of the duodenum (post-junction) Jejunum Ileum Caecum & appendix Ascending colon Proximal 2/3 of the transverse colon

Question 4

What happens to the midgut at the 5th week at the start of development of the final midgut?

Answer 4

It elongates enormously to form a loop called the primary intestinal loop.

Question 5

What is the consequence of the rapid growth of the primary intestinal loop during 5th week and beginning of 6th week?

Answer 5

It will run out of space in the abdominal cavity to grow since the liver is also growing at a fast rate at this time. The growth of the abdominal cavity cannot keep up to the pace of the growth of the primary intestinal loop and the liver. This makes the primary intestinal loop to herniate into the umbilical cord.

Question 6

The primary intestinal loop has something as its axis which it has looped ‘around’. What? What is the primary intestinal loop anchored to?

Answer 6

The superior mesenteric artery. To the yolk sac by the vitelline duct

Question 7

The superior mesenteric artery becomes an important landmark in the embryo as it divides the primitive intestinal loop into two parts. Which?

Answer 7

A cranial limb superior to the SMA A caudal limb inferior to the SMA

Question 8

What will the cranial limb give rise to?

Answer 8

The distal duodenum Jejunum And majority of the ileum

Question 9

What will the caudal limb give rise to?

Answer 9

Very distal part of the ileum Caecum & appendix Ascending colon Proximal 2/3 of the transverse colon

Question 10

What happens to the primary intestinal loop as it houses inside of the umbilical cord in early 8th week?

Answer 10

It will rotate upon the axis of the SMA 90 degrees counterclockwise.

Question 11

What is the consequence of this rotation?

Answer 11

This means that the cranial limb will be located to the right of SMA Caudal limb will be located to the left of SMA.

Question 12

From week 8 to week 10 the primary intestinal loop will continue to elongate in the umbilical cord as well as grow an important part of itself. What?

Answer 12

The caecal bud.

Question 13

What will the caecal bud eventually form?

Answer 13

The caecum.

Question 14

What happens to the primary intestinal loop in the 10th week of development?

Answer 14

The abdominal cavity has grown enough to accommodate the primary intestinal loop once again. This means that the primary intestinal loop begins its retraction out of the umbilical cord into the abdominal cavity once again.

Question 15

What happens during the retraction of the primary intestinal loop?

Answer 15

It will rotate 180 degrees once again counterclockwise upon the axis of the SMA.

Question 16

What is the consequence of this rotation?

Answer 16

Everything will fall into place and the duodenum will lie posterior to the transverse colon.

Question 17

Which limb will be the first one to retract into the abdominal cavity? Why?

Answer 17

The cranial limb. Because the caudal limb has the caecal bud which acts like a not preventing it from retracting first.

Question 18

What is the consequence of the order of retraction of the primary intestinal loop?

Answer 18

That the cranial limb will be located to the left in the abdominal cavity whereas the caudal will be to the left.

Question 19

The primary intestinal loop has now fully retracted back into the abdominal cavity. Where can the caecal bud and the ascending colon be found?

Answer 19

By the liver in the upper right quadrant.

Question 20

What happens to the ascending colon and caecal bud during week 11?

Answer 20

The ascending colon starts to elongate and this causes the descension of the caecal bud into the lower right quadrant. The caecal bud will now form the caecum and the appendix.

Question 21

Everything has now fallen into place, what is the final step to form the final product of the intestines?

Answer 21

The ascending colon and descending colon will have their mesentery fuse with the posterior abdominal wall and become 2nd retroperitoneal.

Question 22

Give examples of abnormalities of development of the midgut.

Answer 22

Omphalocoele Gastroschisis Meckel’s (Ileal) Diverticulum Vitelline cyst Vitelline fistula Incomplete rotation/Non-rotation Reversed rotation Recanalisation

Question 23

Explain incomplete rotation of the midgut.

Answer 23

This is when the primary intestinal loop only rotates once and that is during the herniation into the umbilical cord. There is no rotation as the primary intestinal loop retracts into the abdominal cavity once again.

Question 24

What is the consequence of an incomplete rotation of the midgut?

Answer 24

A left-sided colon

Question 25

Explain reversed rotation of the midgut.

Answer 25

When the midgut loop makes on 90 degree rotation clockwise instead. (Possibly due to the 90 degree rotation counterclockwise on herniation and then 180 degree clockwise upon retraction)

Question 26

Consequence of reversed rotation of the midgut.

Answer 26

The transverse colon passes posterior to the duodenum.

Question 27

Explain the complications of malrotation of the midgut.

Answer 27

Volvulus are likely to form which is when the intestine twists on its own axis causing strangulation and possible ischaemia.

It is likely to happen in non-rotation as the duodenum becomes dilated.

Also has a tendency to happen in reversed rotation as the SMA starts to compress the transverse colon.

Question 28

Give three types of remnants of the yolk sac.

Answer 28

Vitelline cyst

Vitelline fistula

Meckel’s diverticulum

Question 29

Explain vitelline cyst.

Answer 29

When the vitelline duct forms a fibrous strand it can also form a cyst. This is usually not harmful.

Question 30

Explain vitelline fistula.

Answer 30

This is a direct communication between the umbilicus and the intestinal tract. This is potentially harmful and means that faecal matter can leak through the umbilicus.

Question 31

Explain Meckel’s diverticulum.

Answer 31

When there is a small bulge in the intestinal tract with or without a remnant of the vitelline ligament.

Usually you go by there rules of 2 to explain Meckel’s diverticulum.

2% of the population has it.

It is usually 2 feet from the ileocaecal valve and usually detected in under 2 years of age.

2:1 ratio male:female

Question 32

Complications of Meckel’s diverticulum.

Answer 32

Can come to contain ectopic gastric or pancreatic tissue and therefore become inflamed.

The reason for this is unknown.

Question 33

Explain recanalisation as a consequence of the development of the midgut.

Explain its complications.

Answer 33

Since the primitive gut is a simple tube the cell growth can become so rapid that the lumen may become partially or completely obliterated i.e. it fills up.

If this happens it usually happens to the oesophagus, bile duct or small intestines.

In order to sort out a partially or completely obliterated part of the midgut recanalisation will occur to restore the lumen.

If it occurs it might not work at all or only work partially. THis can cause atresia or stenosis of the midgut.

Question 34

Where in the midgut does atresia/stenosis most commonly occur?

Answer 34

In the duodenum as incomplete canalisation.

Question 35

Explain pyloric stenosis.

Answer 35

Hypertrophy of the circular muscle in the region of the pyloric sphincter. This is not a recanalisation failure.

This is a common abnormality in the stomach of infants.

Question 36

Common presentation of pyloric stenosis in infants.

Answer 36

Narrowing exit leads to projectile vomiting.

Will commonly be palpable due to its size.

Question 37

Briefly explain gastroschisis.

Answer 37

Failure of closure of the abdominal wall during folding of the embryo. This leaves the gut tube and its derivatives outside the body cavity.

The gut content will not be enclosed in any peritoneum at all. Not even visceral peritoneum.

Question 38

Briefly explain omphalocoele.

Answer 38

When the primitive intestinal loop fails to retract back into the abdominal cavity.

This means that the gut will be covered in peritoneum, however it differs from umbilical hernias as umbilical hernias will also be covered in skin and subcutaneous tissue.

Question 39

What does the hindgut give rise to?

Answer 39

Distal 1/3 of transverse colon

Descending colon

Rectum

Superior part of anal canal

Epithelium of urinary bladder

Question 40

The anal canal is divided by an anatomical landmark into a superior and inferior part.

What is dividing it?

Answer 40

The pectinate line.

Question 41

Why is the pectinate line important?

Answer 41

Because it indicates the differences in arterial supply, venous, lymphatic drainage and innervation.

Question 42

Blood supply, innervation, type of epithelium and lymphatic drainage of the anal canal above the pectinate line.

Answer 42

Inferior mesenteric artery

Innervated by the S2, S3 and S4 pelvic parasympathetic

Lined with columnar epithelium

Lymph drainage via internal iliac nodes.

Question 43

Blood supply, innervation, type of epithelium and lymphatic drainage ofthe anal canal inferior to the pectinate line.

Answer 43

Pudendal artery

Innervated by the S2, S3, S4 pudendal nerve

Lined with stratified epithelium

Lymphatic drainage via superficial inguinal nodes

Question 44

Why is it important to note the differences of the anal canal above and below the pectinate line?

Answer 44

Because the superior part will only have visceral innervation meaning the only sensation will be stretching.

Below the pectinate line there will be somatic innervation and pain, temperature and touch will be sensed.

Question 45

Give examples of hindgut abnormalities due to its development.

Answer 45

Imperforate anus

Anal and anorectal agenesis

Hindgut fistulae

Question 46

Structures of the midgut and hindgut which still have their mesenteries.

Answer 46

Jejunum

Ileum

Appendix

Transverse colon

Sigmoid colon

Question 47

Structures of the midgut and hindgut with fused mesenteries to the posterior abdominal wall.

Answer 47

Duodenum

Ascending colon

Descending colon

Rectum