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Yr 2 - GI & Metabolism > Embryology - Development of Midgut & Hindgut > Flashcards

Embryology - Development of Midgut & Hindgut Flashcards

1
Q

Where does the midgut run from/to?

A

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

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2
Q

What is the midgut continuous with?

A

The yolk sac at the vitelline duct

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3
Q

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

A

The primary intestinal loop (elongation efficient for SA)

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4
Q

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

A

Through the vitelline duct

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5
Q

What does the primary intestinal loop have?

A

Cranial and caudal limbs

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6
Q

What will the cranial limb form?

A

distal duodenum, jejunum and proximal ilieum

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7
Q

What does the caudal limb form?

A

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

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8
Q

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

A

Rapid elongation of the midgut and growth of the liver

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9
Q

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

A

It herniates into the umbilical cord

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10
Q

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

A

Rotates 90 degrees anti-clockwise

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

Jejunoileal loops form

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11
Q

During week 10, what occurs regarding the midgut?

A

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

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12
Q

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?

A

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

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13
Q

What does the caecum develop?

A

A wormlike diverticulum – vermiform appendix

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14
Q

What causes the vitelline duct to be obliterated?

A

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

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15
Q

By week 11, how much has the midgut rotated?

A
  • The midgut has completely returned to the abdomen

- 270 degrees anticlockwise

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16
Q

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

A

Descends from below the liver to the right iliac fossa

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17
Q

What does this descent of the caecum cause?

A

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

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18
Q

What causes the final arrangement of the midgut?

A

Descent of caecum

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19
Q

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

A

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

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20
Q

What does secondarily retroperitoneal mean?

A

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

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21
Q

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

A

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

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22
Q

How is the appendix suspended?

A

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

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23
Q

What position is appendix typically in?

A

Retro-caecal

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24
Q

What is Meckel’s/Ileal Diverticulum?

A
  • A remnant of the vitelline duct that creates an outpocketing of the ileal wall (most common GI malformation)
  • Usually asymptomatic
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25
Q

When can Meckel’s/Ileal Diverticulum cause problems?

A

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

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26
Q

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

A 
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
27
Q

What is Omphalocele? What is it caused by?

A

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.

28
Q

What is Omphalocele associated with?

A

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

29
Q

How can Omphalocele be diagnosed?

A

Prenatally using ultrasound

30
Q

What occurs during ‘non-rotation’ of the gut?

A

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

31
Q

What is result of ‘non-rotation’ of the gut?

A

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

Usually asymptomatic

32
Q

What occurs during ‘reversed rotation’ of the midgut?

A
  • Initial 90° anticlockwise rotation occurs normally, however, gut the rotates 180° clockwise
  • Total rotation results in 90° clockwise instead of 270° anti-clockwise
33
Q

What is the result of ‘reversed rotation’?

A
  • Gut enters abdomen in correct order except duodenum lies ventral (anterior) to transverse colon
34
Q

What is effect of duodenum lying anterior to transverse colon?

A
  • 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
35
Q

What is effect of volvulus?

A
  • Causes acute obstruction of the bowel and bilious vomiting

- May also constrict arterial supply to the gut causing ischaemia and infarction

36
Q

What does the hindgut give rise to?

A

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

37
Q

What does the distal end of the hindgut enter?

A

The dorsal part of the cloaca –> anorectal canal

38
Q

What is the cloaca?

A

a common cavity at the end of the digestive tract

39
Q

What is the ventral part of the cloaca called?

A

Urogenital sinus

40
Q

What does the ventral part of the cloaca form?

A

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

41
Q

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

A

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

42
Q

What does the urorectal septum appraoch?

A

Approaches close to the cloacal membrane (ectoderm derivative)

43
Q

How is anal canal opening created?

A

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

44
Q

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

A

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

45
Q

What does the perineal body act as?

A

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

46
Q

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

A

Endoderm

47
Q

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

A

The hindgut - endoderm

48
Q

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

A

The proctodeum (anal pit) - ectoderm

49
Q

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

A

When the cloacal (anal) membrane degenerates

50
Q

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

A

Different epithelial linings, lymphatic drainage and blood supply

51
Q

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

A

The pectinate line

52
Q

What are congenital rectourethral / rectovaginal fistulas caused by?

A

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

53
Q

What are rectourethral / rectovaginal fistulas?

A

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

54
Q

What is imperforate anus / anal atresia?

A

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

55
Q

How can imperforate anus / anal atresia be treated?

A

Immediate surgery to allow evacuation of faeces

Good long term prognosis

56
Q

What is the GIT innervated by?

A

The enteric nervous system (a division of the ANS)

57
Q

What are the 2 enteric plexi?

A
  1. Myenteric (Auerbach’s) plexus

2. Submucosal (Meissner’s) plexus

58
Q

Where is Myenteric (Auerbach’s) plexus?

A

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

59
Q

Where is Submucosal (Meissner’s) plexus?

A

between the circular muscle and mucosa and regulates secretion

60
Q

What is the enteric nervous system derived from?

A

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

61
Q

What is Hirschsprung Disease / Congenital Aganglionic Megacolon?

A
  • 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
62
Q

What is Hirschsprung Disease / Congenital Aganglionic Megacolon associated with?

A

Genetic condition most commonly associated with trisomy 21

63
Q

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

A

remove bowel and anastomose the remaining healthy bowel with anus

Yr 2 - GI & Metabolism (51 decks)

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