Week 4 - Development of mid/hindgut

Question 1

Where is the midgut?

Answer 1

-From 2nd part of duodenum after bile duct enters to proximal 2/3 of transverse colon

Question 2

Describe the midgut loop structure

Answer 2

• Has SMA at axis dividing into cranial and caudal limbs

- connected to yolk sac via vitelline duct

Question 3

What is physiological herniation and why does it occur?

Answer 3

• During 6th week growth of loop so rapid and simultaneous to liver that the abdominal cavity is too small
• Intestines herniate into the umbilical cord

Question 4

Explain midgut rotation during physiological herniation

Answer 4

• 3x90 degree anticlockwise turns whilst herniating
• One when it is herniating, one whilst its herniated and one on the return
• Rotation 1 causes limbs to be side by side
• Rotation 2 causes cranial limb on bottom and to return first
• Rotation 3 causes limbs to be side by side and pulls remaining gut tube into body
• Achieves cranial limb returning first and being pushed to the left as others return -> correct anatomical position

Question 5

How does the cecum come to lie in its correct anatomical position?

Answer 5

-The cecal bud descends from the transverse colon after physiological herniation

Question 6

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

Answer 6

• Distal duodenum
• Jejunum
• Proximal ileum

Question 7

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

Answer 7

• Distal ileum
• Cecum
• appendix
• ascending colon
• Proximal 2/3 tranverse colon

Question 8

What are 2 possible outcomes of malrotation?

Answer 8

• Incomplete rotation -> midgut loop makes one turn only -> left sided colon as cranial limb not pulled under caudal limb
• Reversed rotation -> one clockwise turn ->transverse colon passes posterior to duodenum

Question 9

What is volvulus?

Answer 9

-Twisting of intestines due to mobile mesentery which can cause strangulation and ischaemia

Question 10

Describe possible remnants of the yolk stalk (vitelline duct)

Answer 10

• Vitelline cyst as midportion on duct remains patent and fluid filled -> bowel abnormally tethered to anterior abdominal wall
• Vitelline fistula -> Patent duct resulting in spillage of intestinal contents out of umbilicus
• Meckel’s diverticulum ->proximal segment is patent

Question 11

Describe the rule of 2s and meckels diverticulum

Answer 11

• 2% of pop
• 2 feet from ileocecal valve
• 2 inches long
• Usually detected in under 2s
• 2:1 male:female
• 2 tissue types (ectopic gastric and pancreatic)

Question 12

Describe recanalisation of gut tube. Give examples of structures to which this happens

Answer 12

• Cell growth becomes so rapid the lumen is obliterated
• Recanalisation restores the lumen
• Oesophagus, bile duct, small intestine

Question 13

What is the result of incomplete recanalisation?

Answer 13

-Atresia or stenosis

Question 14

What is the cause of pyloric stenosis? How does it commonly present?

Answer 14

• Hypertrophy of the circular muscle in the region of pyloric sphincter
• Presents with characteristic projectile vomiting

Question 15

Which part of the intestines is atresia and stenosis most common?

Answer 15

-Duodenum>jejunum=ileum>colon

Question 16

What are the two main causes of duodenum atresia?

Answer 16

• Recanalisation failure (upper)

- Vascular accident (lower)

Question 17

What is gastroschisis?

Answer 17

• Failure of closure of abdominal wall during folding

- Leaves gut tubes and derivatives outside body cavity

Question 18

What is omphalocoele? How does it differ from umbilical hernia?

Answer 18

• Exomphalos
• Persistance of physiological herniation
• Differs from umbilical hernia as hernias have covering of skin and subcut ie physiological herniation had completed
• in exomphalos covered in peritoneu and amnion

Question 19

Where is the hindgut?

Answer 19

• Distal 1/3 of transverse colon to pectinate line of anus

- Also includes epithelium of urinary bladder

Question 20

What is the physiological difference superior and inferior to the pectinate line?

Answer 20

-Different arterial supply, venous and lymphatic drainage and innervation

Question 21

How do the differences superior and inferior to the pectinate lines occur?

Answer 21

-The anorectal canal is formed by the cloaca and the cloacal membrane. The cloaca is endoderm whilst the cloacal membrane is ectoderm. These embryonic tissues have different innervation and supplies

Question 22

Describe the epithelia, innervation and supply superior and inferior of pectinate line

Answer 22

• Superior -> IMA, Visceral innervtion, columnar epithelium

- Inferior -> Pudendal artery, somatic innervation, stratified epithelia

Question 23

What significance does the differences in innervation above and below the pectinate line have?

Answer 23

• Above the pectinate line only sensation possible is stretch
• Below the pectinate line the tissue is temperature, touch and pain sensitive

Question 24

Explain imperforate anus

Answer 24

-Failure of the anal membrane to rupture

Question 25

Which structures of the mid and hind gut loose their mesentery and become retroperitoneal?

Answer 25

- duodenum - Ascending and descending colon - Rectum

Question 26

Where is the gastrolienal ligament?

Answer 26

-Between stomach and spleen

Question 27

What is the blood supply to midgut? hindgut?

Answer 27

- Midgut -> SMA | - Hindgut -> IMA

Question 28

Describe the partitioning of the cloaca

Answer 28

- Urorectal septum grows down towards cloacal membrane - Separates gut tube into urogenital sinus anteriorly and anorectal canal posteriorly - Perineal body is where urorectal septum touches cloacal membrane

Question 29

What is the midgut loop?

Answer 29

-The primitive gut tube extends and elongates at the same time as the liver is enlarging, forming a loop as it is in a contained cavity-> makes the most of space