Embryology - Development of Midgut & Hindgut

Question 1

Where does the midgut run from/to?

Answer 1

2nd half of duodenum to 2/3 along transverse colon

Question 2

What is the midgut continuous with?

Answer 2

The yolk sac at the vitelline duct

Question 3

During week 5, the midgut and associated dorsal mesentery undergo rapid elongation. What does this form?

Answer 3

The primary intestinal loop (elongation efficient for SA)

Question 4

How does the primary intestinal loop communicate with the yolk sac?

Answer 4

Through the vitelline duct

Question 5

What does the primary intestinal loop have?

Answer 5

Cranial and caudal limbs

Question 6

What will the cranial limb form?

Answer 6

distal duodenum, jejunum and proximal ilieum

Question 7

What does the caudal limb form?

Answer 7

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

Question 8

During week 6, what occurs regarding the midgut and the liver?

Answer 8

Rapid elongation of the midgut and growth of the liver

Question 9

There is not enough room in the abdomen, so what happens to the 1ary intestinal loop?

Answer 9

It herniates into the umbilical cord

Question 10

As herniation occurs, how does the midgut rotate? What effect does this have on the cranial and caudal limb?

Answer 10

Rotates 90 degrees anti-clockwise

Brings the cranial limb to the right and caudal limb to the left

Jejunoileal loops form

Question 11

During week 10, what occurs regarding the midgut?

Answer 11

Returns to the abdomen and rotates a further 180 degrees anti-clockwise

Question 12

What is the effect of the rotation of the midgut 180 degrees anti-clockwise in week 10 on the proximal jejunal loops and the caecum?

Answer 12

Brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver

Question 13

What does the caecum develop?

Answer 13

A wormlike diverticulum – vermiform appendix

Question 14

What causes the vitelline duct to be obliterated?

Answer 14

Rotation of midgut a further 180 degrees anti-clockwise as it moves back to the abdomen

Question 15

By week 11, how much has the midgut rotated?

Answer 15

• The midgut has completely returned to the abdomen

- 270 degrees anticlockwise

Question 16

Once the midgut has returned to the abdomen, where does the caecum move?

Answer 16

Descends from below the liver to the right iliac fossa

Question 17

What does this descent of the caecum cause?

Answer 17

Pulls the ascending and transverse colon into place resulting in the final arrangement of the midgut

Question 18

What causes the final arrangement of the midgut?

Answer 18

Descent of caecum

Question 19

Once the final arrangement of the midgut has occurred, what regarding the dorsal mesentery of the ascending and descending colon?

Answer 19

The dorsal mesentery of the ascending and descending colons now shortens and degenerates pulling them against the posterior abdominal wall – secondarily retroperitoneal

Question 20

What does secondarily retroperitoneal mean?

Answer 20

Starts off tethered to mesentery but then gets pushed against posterior wall and loses mesentery

Question 21

What effect does the descent of the caecum have on the appendix?

Answer 21

Causes the appendix to be located in the retro-caecal position in the majority of individuals

Question 22

How is the appendix suspended?

Answer 22

Suspended by a mesentery so is relatively mobile (position variable from person to person)

Question 23

What position is appendix typically in?

Answer 23

Retro-caecal

Question 24

What is Meckel’s/Ileal Diverticulum?

Answer 24

• A remnant of the vitelline duct that creates an outpocketing of the ileal wall (most common GI malformation)
• Usually asymptomatic

Question 25

When can Meckel's/Ileal Diverticulum cause problems?

Answer 25

May contain ectopic pancreatic or gastric tissue (so secretes acid), causing inflammation, ulceration and bleeding

Question 26

What are the 'rules of 2's in Meckel's/Ileal Diverticulum?

Answer 26

``` Affects 2% population 2 times more common in males 2 feet (50 cm) from ileocaecal junction 2 inches (3-6 cm) long Symptomatic in 2% cases ```

Question 27

What is Omphalocele? What is it caused by?

Answer 27

Infant's intestines, liver, or other organs stick outside of the belly through the belly button. The organs are covered in a thin, nearly transparent sac that hardly ever is open or broken. Cause: Problem with herniation as don't develop enough space (abdominal viscera herniating into base of umbilicus). Failure of midgut to return to return to abdomen in week 10.

Question 28

What is Omphalocele associated with?

Answer 28

Associated with an increased risk of mortality and other malformation e.g. cardiac or neural tube defects

Question 29

How can Omphalocele be diagnosed?

Answer 29

Prenatally using ultrasound

Question 30

What occurs during 'non-rotation' of the gut?

Answer 30

Gut undergoes initial 90° anti-clockwise rotation but fails to rotate a further 180° when the gut is retracted

Question 31

What is result of 'non-rotation' of the gut?

Answer 31

Results in small intestine on the right side and large intestine on the left (left sided colon) Usually asymptomatic

Question 32

What occurs during 'reversed rotation' of the midgut?

Answer 32

- Initial 90° anticlockwise rotation occurs normally, however, gut the rotates 180° clockwise - Total rotation results in 90° clockwise instead of 270° anti-clockwise

Question 33

What is the result of 'reversed rotation'?

Answer 33

- Gut enters abdomen in correct order except duodenum lies ventral (anterior) to transverse colon

Question 34

What is effect of duodenum lying anterior to transverse colon?

Answer 34

- Doesn't get pushed against posterior wall so hasn't lost mesentery (isn't retroperitoneal but remains suspended by dorsal mesentery) - Is more mobile - Can lead to volvulus (twisting) of the midgut

Question 35

What is effect of volvulus?

Answer 35

- Causes acute obstruction of the bowel and bilious vomiting | - May also constrict arterial supply to the gut causing ischaemia and infarction

Question 36

What does the hindgut give rise to?

Answer 36

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

Question 37

What does the distal end of the hindgut enter?

Answer 37

The dorsal part of the cloaca --> anorectal canal

Question 38

What is the cloaca?

Answer 38

a common cavity at the end of the digestive tract

Question 39

What is the ventral part of the cloaca called?

Answer 39

Urogenital sinus

Question 40

What does the ventral part of the cloaca form?

Answer 40

The bladder, pelvic urethra, penile urethra (males) and caudal part of the vagina (females)

Question 41

During weeks 4-6, how is the urogenital sinus and anorectal canal separated?

Answer 41

A layer of mesoderm extends caudally to separate them --> urorectal septum

Question 42

What does the urorectal septum appraoch?

Answer 42

Approaches close to the cloacal membrane (ectoderm derivative)

Question 43

How is anal canal opening created?

Answer 43

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

Question 44

What lies between the anal opening and a ventral opening for the urogenital sinus?

Answer 44

The tip of the urorectal septum --> forms the perineal body

Question 45

What does the perineal body act as?

Answer 45

It acts as a point of attachment for muscle fibres from the pelvic floor and the perineum itself

Question 46

Where does the inner epithelial lining of the gut tube come from?

Answer 46

Endoderm

Question 47

What is the upper 2/3 of the anal canal derived from?

Answer 47

The hindgut - endoderm

Question 48

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

Answer 48

The proctodeum (anal pit) - ectoderm

Question 49

When do the upper 2/3 (endoderm) and lower 1/3 (ectoderm) become continuous?

Answer 49

When the cloacal (anal) membrane degenerates

Question 50

Why is it important that the upper 2/3 and lower 1/3 of the anal canal have different origins?

Answer 50

Different epithelial linings, lymphatic drainage and blood supply

Question 51

What is the junction between endoderm and ectoderm in the anal canal marked in adults by?

Answer 51

The pectinate line

Question 52

What are congenital rectourethral / rectovaginal fistulas caused by?

Answer 52

Abnormal cloaca e.g. too small or failure of urorectal septum to extend caudally

Question 53

What are rectourethral / rectovaginal fistulas?

Answer 53

Abnormal connections between rectum and urethra (men) or rectum and vagina (women) Opening of hindgut is shifted ventrally to the urethra in males and the vagina in females

Question 54

What is imperforate anus / anal atresia?

Answer 54

Failure of anal membrane to degenerate (blind ended sac instead)

Question 55

How can imperforate anus / anal atresia be treated?

Answer 55

Immediate surgery to allow evacuation of faeces Good long term prognosis

Question 56

What is the GIT innervated by?

Answer 56

The enteric nervous system (a division of the ANS)

Question 57

What are the 2 enteric plexi?

Answer 57

1. Myenteric (Auerbach's) plexus | 2. Submucosal (Meissner's) plexus

Question 58

Where is Myenteric (Auerbach's) plexus?

Answer 58

Between the circular and longitudinal muscle layers co-ordinates muscle contraction

Question 59

Where is Submucosal (Meissner's) plexus?

Answer 59

between the circular muscle and mucosa and regulates secretion

Question 60

What is the enteric nervous system derived from?

Answer 60

Neural crest cells (ectoderm origin) that migrate from the neural tube to GIT

Question 61

What is Hirschsprung Disease / Congenital Aganglionic Megacolon?

Answer 61

- Failure of neural crest cells to migrate to bowel - Absence of enteric ganglia leads to bowel obstruction due to lack of peristalsis (due to lack of nerve supply) - This causes dilation of the aganglionic part of the bowel (bacteria respires as food builds up) – usually rectum and sigmoid colon

Question 62

What is Hirschsprung Disease / Congenital Aganglionic Megacolon associated with?

Answer 62

Genetic condition most commonly associated with trisomy 21

Question 63

What is the effective treatment for Hirschsprung Disease / Congenital Aganglionic Megacolon?

Answer 63

remove bowel and anastomose the remaining healthy bowel with anus