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BS - Gastrointestinal System pt.1 > Development of the Midgut and Hindgut > Flashcards

Development of the Midgut and Hindgut Flashcards

1
Q

Where does the midgut run to and from?

A

It runs from the 2nd part of the duodenum to 2/3 along the transverse colon

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2
Q

What is the midgut continuous with?

A

The yolk sac at the vitelline duct

this is around the level of the umbilicus

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3
Q

Where does the hindgut run to and from?

A

It runs from the final 1/3 of the transverse colon to the superior 2/3 of the rectum

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4
Q

What happens to the midgut during week 5 of development?

A

The midgut and associated dorsal mesentery undergo rapid elongation

This forms the primary intestinal loop

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5
Q

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

A

Through the vitelline duct

Both the yolk sac and the vitelline duct are encompassed in the umbilical cord

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6
Q

What will the primary intestinal loop go on to form?

A

It grows very quickly to form cranial and caudal limbs

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7
Q

What will the cranial limb of the primary intestinal loop go on to form?

A
  1. distal part of duodenum
  2. jejunum
  3. proximal ileum
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8
Q

What will the caudal limb of the primary intestinal loop go on to form?

A
  1. distal ileum
  2. caecum
  3. appendix
  4. ascending colon
  5. proximal 2/3 of transverse colon
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9
Q

What happens to the midgut during week 6 of development?

What is the consequence of this?

A

There is rapid elongation of the midgut and growth of the liver

There is not enough room in the abdomen for the rapidly extending intestinal loop

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10
Q

What happens due to there not being enough space in the abdomen for the primary intestinal loop?

A

The primary intestinal loop herniates into the umbilical cord

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11
Q

What happens as the primary intestinal loop herniates into the umbilical cord?

A

The midgut rotates around its axis and rotates 90o anti-clockwise

Jejunoileal loops form

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12
Q

What is the result of the midgut rotating 90o anti-clockwise?

A

the cranial limb is brought to the right and the caudal limb is brought to the left

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13
Q

During which week does the midgut return to the abdominal cavity?

A

For 4 weeks it develops outside the abdominal cavity

It returns to the abdominal cavity in week 10, when there is enough space for it

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14
Q

What happens in week 10 when the midgut returns to the abdomen?

A

It rotates a further 180o anti-clockwise

In total, the midgut has rotated 270o anti-clockwise

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15
Q

What is the result of the midgut rotating 180o anti-clockwise in week 10?

A

The proximal jejunal loops are brought to the left side

The caecum ends up sitting just inferior to the liver

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16
Q

What wormlike diverticulum develops from the caecum?

A

the vermiform appendix

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17
Q

What happens to the vitelline duct as the midgut returns to the abdomen in week 10?

Why does this happen?

A

The vitelline duct is closed off and obliterated

The primary function of the yolk sac was to get nutrients, but once the gut tube has developed this is no longer needed

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18
Q

What is the position of the midgut by week 11 of development?

A

The midgut has completely returned to the abdomen and has undergone 270o anti-clockwise rotation in total

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19
Q

What happens to the caecum once the midgut has returned to the abdomen?

A

It descends from below the liver to the right iliac fossa

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20
Q

As the caecum descends, which structures does it bring with it?

A

It pulls the rest of the gut tube with it

It will pull the ascending and transverse colon into their adult anatomical positions

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21
Q

What happens to the ascending and descending colons as the caecum pulls them into position?

A

the dorsal mesentery of the ascending and descending colons shortens and degenerates

This pulls them against the posterior abdominal wall

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22
Q

Are the ascending and descending colons intraperitoneal or retroperitoneal?

A

Secondarily retroperitoneal

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23
Q

Why does the dorsal mesentery shorten as it moves?

What happens to the gut tube during this process?

A

This is due to elongation of the lumbar region

As it grows longer, it pulls the gut tube closer to the posterior wall

The dorsal mesentery decreases in length until it is lost

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24
Q

Why is the position of the appendix variable?

A

The position of the appendix is dictated by movement of caecum

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25
Q

What is the position of the appendix in the majority (64%) of individuals?

A

The appendix is in the retrocaecal position

This is where the appendix is located just behind the caecum

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26
Q

Why is the appendix relatively mobile?

A

It is suspended by a mesentery

Mesenteries dictate how mobile an organ is, and organs with longer mesenteries tend to be more mobile

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27
Q

Why are the symptoms and site of pain in appendicitis different in different people?

A

It is due to the position of the appendix being variable

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28
Q

What Meckel’s/Ileal diverticulum?

A

This is where a remnant of the vitelline duct creates an outpocketing of the ileal wall

This is due to the vitelline duct not being obliterated entirely

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29
Q

What are the usual symptoms of Meckel’s diverticulum?

A

It is usually asymptomatic unless there is heterotopic tissue present

(usually ectopic pancreatic or gastric tissue)

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30
Q

What is ectopic tissue refer to?

A

Certain types of tissue being located in areas where they are not usually found

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31
Q

What does ectopic tissue usually cause?

A

Inflammation, ulceration and bleeding

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32
Q

What are the ‘rule of 2s’ that characterise Meckel’s diverticulum?

A
  1. affects 2% of the population
  2. 2 times more common in males
  3. it is located 2 feet (50 cm) from the ileocaecal junction
  4. it is 2 inches (3-6 cm) long
  5. it is symptomatic in 2% of cases
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33
Q

What causes omphalocele?

A

Failure of the midgut to return to the abdomen in week 10 of development

The intestines do not fully develop in the abdominal cavity

34
Q

What is omphalocele associated with?

A

An increased risk of mortality and other malformations

e.g. association with cardiac and neural tube defects

35
Q

How can omphalocele be diagnosed prenatally?

A

ultrasound

36
Q

How does the severity of omphalocele vary?

A

this depends on whether other organs have also failed to return to the abdomen

e.g. liver may also herniate through due to confined abdominal space

37
Q

What is the difference between omphalocele and gastroschisis?

A

Gastroschisis - intestines develop outside intestinal cavity so have no amnion around them

Omphalocele - intestines herniate into umbilical cord first so are covered with amnion

38
Q

What happens in non-rotation of the midgut?

A

It undergoes the initial 90o anti-clockwise rotation

It fails to rotate a further 180o when the gut is retracted back into the abdomen

39
Q

What is non-rotation of the midgut usually referred to as and why?

A

It results in the small intestine on the right side and the large intestine on the left side

It is referred to as left-sided colon

40
Q

What are the symptoms of left-sided colon?

A

It is usually asymptomatic

41
Q

What happens in reversed rotation of the midgut?

A

The initial 90o anti-clockwise rotation occurs

When the gut retracts into the abdomen, it rotates 180o clockwise

The overall rotation is 90o clockwise

42
Q

How does the arrangement of the gut change in reversed rotation of the midgut?

A

The majority of the gut enters the abdomen in the correct order, except for the duodenum

43
Q

How does the position and properties of the duodenum change in reversed rotation of the midgut?

A

It is usually posterior to the colon and becomes retroperitoneal as it is pushed against the body wall

In reversed rotation, it lies anterior to the transverse colon and is intraperitoneal

44
Q

What is caused by abnormal rotation of the midgut?

A

Parts of the GI tract that would normally be retroperitoneal become intraperitoneal

This is because they remain suspended by the dorsal mesentery

45
Q

What is twisting of the midgut known as?

Why does this happen after abnormal rotation of the midgut?

A

Twisting of the midgut is volvulus

It happens due to presence of the dorsal mesentery meaning the organs are moveable and can twist on themselves

46
Q

What symptoms and consequences can volvulus cause?

A
  1. acute obstruction of the bowel
  2. bilious vomiting
  3. constriction of the arterial supply to the gut
47
Q

What is bilious vomiting and how is it brought about?

A

It occurs when bile starts to make its way back up the GI tract

Bile usually enters the 2nd part of the duodenum, but due to the volvulus, it can’t

48
Q

What is the result of constricting the arterial supply to the gut?

A

Tissue ischaemia and infarction

49
Q

What structures does the hindgut give rise to?

A
  1. the distal 1/3 of the transverse colon
  2. descending colon
  3. sigmoid colon
  4. rectum and cranial 2/3 of the anal canal
50
Q

What does the distal end of the hindgut enter?

What forms here?

A

It enters the dorsal part of the cloaca

This is where the anorectal canal will form

51
Q

What is the cloaca?

A

The region where excretions from the future bladder and gut tube empty in to

52
Q

What is the ventral part of the cloaca?

A

Urogenital sinus

This goes on to form the bladder, pelvic urethra, penile urethra/vagina

53
Q

What forms relating to the hindgut during weeks 4-6?

A

Urorectal septum

a layer of mesoderm extends caudally to separate the urogenital sinus and the anorectal canal

54
Q

What happens relating to the hindgut in week 7 of development?

A

the urorectal septum approaches close to the cloacal membrane

In week 7, the cloacal membrane ruptures

55
Q

What is the result of the cloacal membrane rupturing?

A

It creates the anal opening and a ventral opening for the urogenital sinus

The tip of the urogenital septum lies between them and forms the perineal body

56
Q

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

cranial part

A

The hindgut

It is derived from the endoderm

57
Q

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

What is is derived from?

(caudal part)

A

Anal pit

It is derived from proctodaeum which is derived from the ectoderm

58
Q

What separates the cranial and caudal parts of the anus?

A

The cloacal (anal) membrane

When the membrane degenerates, they become continuous

59
Q

What is the result of the cranial and caudal parts of the anal canal being derived from different cells?

A

They have different epithelial linings, nerve and blood supply and lymphatic drainage

60
Q

In an adult, what is the junction between endoderm and ectoderm derivatives marked by?

A

It is marked by the pectinate line

This is the region where the cloacal membrane ruptures

61
Q

Why is the pectinate line important clinically?

A

It delineates whether haemorrhoids are internal or external

62
Q

What happens if septation of the cloaca goes slightly wrong?

A

It results in abnormal connections (fistulas)

These may be rectourethral or rectovaginal

63
Q

Why might an abnormal cloaca lead to fistulas?

A

The cloaca may be too small so there is not enough space for 2 structures to form

There may be failure of the urorectal septum to extend caudally

64
Q

What happens in males due to abnormal septation of the cloaca?

A

The opening of the hindgut is shifted ventrally to the urethra

The connection between the anus and the urethra is a urorectal fistula

65
Q

What happens in females due to abnormal septation of the cloaca?

A

The opening of the hindgut is shifted ventrally to the vagina

The connection between the rectum/anus and the vagina is a rectovaginal fistula

66
Q

What causes an imperforate anus?

A

The anal membrane fails to degenerate

This means there is no way of evacuating the anus

67
Q

What is the long-term prognosis in majority of cases of imperforate anus?

A

There is a good long term prognosis as long as immediate surgery is performed to allow evacuation of faeces

68
Q

What is the nervous supply of the GI tract?

A

The enteric nervous system

This is a division of the autonomic nervous system

69
Q

What are the 2 different enteric plexi?

A
  1. myenteric plexus

2. submucosal plexus

70
Q

Where is the myenteric plexus found and what does it do?

A

It is between the circular and longitudinal muscle layers

It coordinates muscle contraction

71
Q

What is the other name for the myenteric plexus?

A

Aucherbach’s plexus

72
Q

Where is the submucosal plexus found and what does it do?

A

It is between the circular muscle and the mucosa

It regulates secretion

73
Q

What is the other name of the submucosal plexus?

A

Meissner’s plexus

74
Q

What is the enteric nervous system derived from?

A

neural crest cells that migrate from the neural tube to the GI tract

The neural crest cells are of ectoderm origin

75
Q

What is the other name for Hirschsprung disease and what is it caused by?

A

Congenital aganglionic megacolon

It is caused by a failure of the neural crest cells to migrate to the bowel

76
Q

Why does absence of enteric ganglia lead to bowel obstruction?

A

There is a lack of peristalsis

There is no nervous system so intestinal contents cannot be squeezed through the gut

77
Q

What is the major consequence of Hirschsprung disease?

A

Dilation of the aganglionic part of the bowel

This is usually the rectum or sigmoid colon

78
Q

How often does someone with Hirschsprung disease empty their bowels?

What is the consequence of this?

A

They can go weeks/months without emptying their bowels

There is a build-up of bacteria which produce gases

This leads to a distended abdomen

79
Q

what is the only effective treatment for Hirschsprung disease?

A

removing the affected bowel which does not have a nervous system

the remaining healthy bowel is anastomosed with the anus

80
Q

What does the severity of Hirschsprung disease depend on?

A

How much of the gut tube lacks enteric ganglia

BS - Gastrointestinal System pt.1 (22 decks)

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