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

Development of the Midgut and Hindgut Flashcards

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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
What is the position of the appendix in the majority (64%) of individuals?
The appendix is in the retrocaecal position This is where the appendix is located just behind the caecum
26
Why is the appendix relatively mobile?
It is suspended by a mesentery Mesenteries dictate how mobile an organ is, and organs with longer mesenteries tend to be more mobile
27
Why are the symptoms and site of pain in appendicitis different in different people?
It is due to the position of the appendix being variable
28
What Meckel's/Ileal diverticulum?
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
29
What are the usual symptoms of Meckel's diverticulum?
It is usually asymptomatic unless there is heterotopic tissue present (usually ectopic pancreatic or gastric tissue)
30
What is ectopic tissue refer to?
Certain types of tissue being located in areas where they are not usually found
31
What does ectopic tissue usually cause?
Inflammation, ulceration and bleeding
32
What are the 'rule of 2s' that characterise Meckel's diverticulum?
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
33
What causes omphalocele?
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
What is omphalocele associated with?
An increased risk of mortality and other malformations e.g. association with cardiac and neural tube defects
35
How can omphalocele be diagnosed prenatally?
ultrasound
36
How does the severity of omphalocele vary?
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
What is the difference between omphalocele and gastroschisis?
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
What happens in non-rotation of the midgut?
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
What is non-rotation of the midgut usually referred to as and why?
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
What are the symptoms of left-sided colon?
It is usually asymptomatic
41
What happens in reversed rotation of the midgut?
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
How does the arrangement of the gut change in reversed rotation of the midgut?
The majority of the gut enters the abdomen in the correct order, except for the duodenum
43
How does the position and properties of the duodenum change in reversed rotation of the midgut?
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
What is caused by abnormal rotation of the midgut?
Parts of the GI tract that would normally be retroperitoneal become intraperitoneal This is because they remain suspended by the dorsal mesentery
45
What is twisting of the midgut known as? Why does this happen after abnormal rotation of the midgut?
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
What symptoms and consequences can volvulus cause?
1. acute obstruction of the bowel 2. bilious vomiting 3. constriction of the arterial supply to the gut
47
What is bilious vomiting and how is it brought about?
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
What is the result of constricting the arterial supply to the gut?
Tissue ischaemia and infarction
49
What structures does the hindgut give rise to?
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
What does the distal end of the hindgut enter? What forms here?
It enters the dorsal part of the cloaca This is where the anorectal canal will form
51
What is the cloaca?
The region where excretions from the future bladder and gut tube empty in to
52
What is the ventral part of the cloaca?
Urogenital sinus This goes on to form the bladder, pelvic urethra, penile urethra/vagina
53
What forms relating to the hindgut during weeks 4-6?
Urorectal septum a layer of mesoderm extends caudally to separate the urogenital sinus and the anorectal canal
54
What happens relating to the hindgut in week 7 of development?
the urorectal septum approaches close to the cloacal membrane In week 7, the cloacal membrane ruptures
55
What is the result of the cloacal membrane rupturing?
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
What is the upper 2/3 of the anal canal derived from? | cranial part
The hindgut It is derived from the endoderm
57
What is the lower 1/3 of the anal canal? What is is derived from? (caudal part)
Anal pit It is derived from proctodaeum which is derived from the ectoderm
58
What separates the cranial and caudal parts of the anus?
The cloacal (anal) membrane When the membrane degenerates, they become continuous
59
What is the result of the cranial and caudal parts of the anal canal being derived from different cells?
They have different epithelial linings, nerve and blood supply and lymphatic drainage
60
In an adult, what is the junction between endoderm and ectoderm derivatives marked by?
It is marked by the pectinate line This is the region where the cloacal membrane ruptures
61
Why is the pectinate line important clinically?
It delineates whether haemorrhoids are internal or external
62
What happens if septation of the cloaca goes slightly wrong?
It results in abnormal connections (fistulas) These may be rectourethral or rectovaginal
63
Why might an abnormal cloaca lead to fistulas?
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
What happens in males due to abnormal septation of the cloaca?
The opening of the hindgut is shifted ventrally to the urethra The connection between the anus and the urethra is a urorectal fistula
65
What happens in females due to abnormal septation of the cloaca?
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
What causes an imperforate anus?
The anal membrane fails to degenerate This means there is no way of evacuating the anus
67
What is the long-term prognosis in majority of cases of imperforate anus?
There is a good long term prognosis as long as immediate surgery is performed to allow evacuation of faeces
68
What is the nervous supply of the GI tract?
The enteric nervous system This is a division of the autonomic nervous system
69
What are the 2 different enteric plexi?
1. myenteric plexus | 2. submucosal plexus
70
Where is the myenteric plexus found and what does it do?
It is between the circular and longitudinal muscle layers It coordinates muscle contraction
71
What is the other name for the myenteric plexus?
Aucherbach's plexus
72
Where is the submucosal plexus found and what does it do?
It is between the circular muscle and the mucosa It regulates secretion
73
What is the other name of the submucosal plexus?
Meissner's plexus
74
What is the enteric nervous system derived from?
neural crest cells that migrate from the neural tube to the GI tract The neural crest cells are of ectoderm origin
75
What is the other name for Hirschsprung disease and what is it caused by?
Congenital aganglionic megacolon It is caused by a failure of the neural crest cells to migrate to the bowel
76
Why does absence of enteric ganglia lead to bowel obstruction?
There is a lack of peristalsis There is no nervous system so intestinal contents cannot be squeezed through the gut
77
What is the major consequence of Hirschsprung disease?
Dilation of the aganglionic part of the bowel This is usually the rectum or sigmoid colon
78
How often does someone with Hirschsprung disease empty their bowels? What is the consequence of this?
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
what is the only effective treatment for Hirschsprung disease?
removing the affected bowel which does not have a nervous system the remaining healthy bowel is anastomosed with the anus
80
What does the severity of Hirschsprung disease depend on?
How much of the gut tube lacks enteric ganglia

BS - Gastrointestinal System pt.1 (22 decks)

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