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B - Embryology for EoY4 > Development of the Midgut and Hindgut > Flashcards

Development of the Midgut and Hindgut Flashcards

1
Q

What does the midgut give rise to?

A
  • Small intestine, including most of the duodenum post bile duct entry
  • Caecum and appendix
  • Ascending colon
  • Proximal 2/3 or transverse colon
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2
Q

Draw a diagram illustrating the position of the midgut in an embryo

A
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3
Q

What causes the midgut to make a loop?

A

It elongates enormously, and quickly runs out of space due to the large size of the developing liver

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4
Q

What does the loop made by the midgut have at its axis?

A

The superior mesenteric artery

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5
Q

What is the loop made by the midgut connected to?

A

The yolk sac

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6
Q

How is the midgut loop connected to the yolk sac?

A

By the vitelline duct

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7
Q

What are the limbs of the midgut loop called?

A

Cranial and caudal limbs

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8
Q

Label this diagram

A
  • A - Cranial limb
  • B - Superior mesenteric artery at axis
  • C - Caudal limb
  • D - Vitelline duct to yolk sac
  1. SMA
  2. Cranial
  3. Caudal
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9
Q

What are the derivatives of the cranial limb?

A
  • Distal duodenum
  • Jejunum
  • Proximal ileum
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10
Q

What are the derivates of the caudal limb?

A
  • Distal ileum
  • Cecum
  • Appendix
  • Ascending colon
  • Proximal 2/3 of transverse colon
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11
Q

What happens to the primary loop during the 6th week of development?

A

It elongates very rapidly

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12
Q

What happens to the liver during the 6th week of development?

A

It grows very rapidly

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13
Q

What is the problem with the rapid growth of both the liver and the primary loop?

A

The abdominal cavity is too small to accomodate both

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14
Q

What is the result of the abdominal cavity being too small too accomodate the primary loop and the liver?

A

Physiological herniation

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15
Q

What is physiological herniation?

A

Where the intestines herniate into the proximal umbilical cord, alongside the umbilical vessels

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16
Q

What are the stages in rotation of the midgut loop?

A
  1. First rotation
  2. Second rotation
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17
Q

By how much is the first rotation of the midgut loop?

A

90 degrees

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18
Q

By how much is the second rotation of the midgut loop?

A

180 degrees

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19
Q

What happens in the first rotation of the midgut loop?

A

During herniation into the umbilical cord, the midgut rotates around the axis formed by the SMA in a counter clockwise direction (cranial limb moves to back, caudal to the front)

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20
Q

Draw a diagram illustrating what happens in the first rotation of the midgut loop?

A
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21
Q

Does elongation of the small intestinal lumen continue during rotation?

A

Yes

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22
Q

What is the result of the elongation of the small intestinal loop continuing during rotation?

A

The jejunum and ileum form a number of coiled loops

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23
Q

What happens to the large intestine during rotation?

A

It lengthens, but does not participate in the coiling phenomenon

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24
Q

When does the second rotation of the midgut loop occur?

A

When it returns into the abdominal cavity, around week 10

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25
Q

In what direction does the second rotation of the midgut loop occur?

A

Turns 90 degrees counter-clockwise twice

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26
Q

Draw a diagram illustrating the second rotation of the midgut loop

A
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27
Q

In total, by how much does the midgut loop rotate?

A

270 degrees counter-clockwise

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28
Q

Which limb returns to the abdomen first?

A

The cranial limb

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29
Q

In what direction does the cranial limb move on return to the abdomen?

A

To the left hand side

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30
Q

What returns to the abdomen last following rotation?

A

The cecal bud

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31
Q

What happens once the cecal bud has returned to the abdomen?

A

It descends, moving the ceacum to the right lower quadrant

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32
Q

What does the rotation of the midgut loop account for?

A

The positions of the small and large intestines, and the twisted apperances of the mesentery of the small intestine

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33
Q

Are abnormalities of rotation common?

A

Yes

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34
Q

What do abnormalities of rotation cause?

A

Abnormal positioning of the midgut derivatives, e.g. the appendix on the left

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35
Q

What happens in incomplete rotation?

A

The midgut only makes one 90 degree rotation

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36
Q

What is the consequence of incomplete rotation?

A

Left sided colon

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37
Q

Draw a diagram showing the consequence of incomplete rotation

A
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38
Q

What happens in reversed rotation?

A

The midgut makes one 90 degree rotation clockwise

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39
Q

What is the consequence of reversed rotation?

A

The transverse colon passes posterior to the duodenum, and can wrap arround and occlude

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40
Q

Draw a diagram showing the consequences of reversed rotation

A
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41
Q

What does incomplete or reversed rotation lead to?

A

Hypermobile guts

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42
Q

What is a volvulus?

A

A bowel obstruction where a loop of bowel has abnormally twisted in on itself

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43
Q

What makes a volvulus more likely?

A

Hypermobile guts

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44
Q

What can a volvulus lead to?

A
  • Strangulation
  • Ischaemia
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45
Q

What does the hindgut give rise to?

A
  • The distal 1/3 transverse colon
  • Descending colon
  • Rectum
  • Superior part of anal canal
  • Epithelium of the urinary bladder
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46
Q

Draw a diagram illustrating the position of the hindgut

A
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47
Q

What happens to the hindgut at 6 weeks?

A

It ends in the cloaca

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48
Q

What separates the cloaca from the outside?

A

The cloacal membrane

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49
Q

What does the cloaca undergo after formation?

A

An anteroposterior division

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50
Q

What happens in cloacal partitioning?

A

A wedge of mesoderm grows down into the cloaca

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51
Q

What is the cloaca divided into in partitioning?

A
  • The urogenial sinus anteriorly
  • The anorectal canal posteriorly
52
Q

Draw a labelled diagram illustrating the progression of cloacal partitioning

A
53
Q

What is the anal canal derived from?

A
  • The superior part is derived from the hindgut
  • The inferior part is derived from the endoderm
54
Q

What is the pectinate line?

A

The line at which the two parts fo the anal canal meet

55
Q

Draw a diagram illustrating the formation of the anal canal

A
56
Q

Label this diagram

A
  • A - Rectum
  • B - Pectinate line
  • C - ‘White’ line
  • D - Anal pecten
  • E - Anal aperture
57
Q

What is the blood supply for the anal canal above the pectinate line?

A

Inferior mesenteric artery

58
Q

What is the blood supply for the anal canal below the pectinate line?

A

Pudendal artery

59
Q

What is the innervation of the anal canal above the pectinate line?

A

S2/3/4 pelvic parasympathetic

60
Q

What is the epithelia in the anal canal above the pectinate line?

A

Columnar

61
Q

What is the lymph drainage of the anal canal above the pectinate line?

A

Internal iliac nodes

62
Q

What is the blood supply of the anal canal below the pectinate line?

A

Pudendal A.

63
Q

What is the innervation of the anal canal below the pectinate line?

A

S2/3/4 Pudendal N.

64
Q

What is the epithelia of the anal canal below the pectinate line?

A

Stratified squamous (non-keratinised)

65
Q

What is the lymph drainage of the anal canal below the pectinate line?

A

Superficial inguinal nodes

66
Q

Why do the two different parts of the anal canal vary in several ways?

A

Because they have different origins

67
Q

What sensation is possible above the pectinate line?

A

Stretch

68
Q

What sensation is possible below the pectinate line?

A
  • Temperature
  • Touch
  • Pain
69
Q

Why is the tissue below the pectinate line sensitive to temperature, touch, and pain?

A

Due to its somatic innervation by the Pudendal nerve (S2/3/4)

70
Q

What are some common congital defects of the GI tract?

A
  • Vitelline duct
  • Meckel’s Diverticlum
  • Vitelline cyst
  • Vitelline fistula
  • Recanalisation problems
  • Pyloric stenosis
71
Q

What is the most common GI abnormality?

A

Meckel’s Diverticulum

72
Q

What is the problem with a vitelline duct?

A

It can persist, resulting in a number of different abnormalities

73
Q

What is Meckel’s Diverticulum also known as?

A

Ilieal diverticulum

74
Q

What does Meckel’s Diverticulum follow?

A

A rule of 2’s

75
Q

What is the rule of 2’s followed by Meckel’s diverticulum?

A
  • 2% of population affected
  • 2 feet from ileocecal valve
  • 2 inches long
  • Usually detected in under 2’s
  • 2:1 male:female
76
Q

Can Meckel’s Diverticulum be asymptomatic?

A

Yes

77
Q

What does Meckel’s Diverticulum cause?

A

Ulceration

78
Q

Why does Meckel’s Diverticulum cause ulceration?

A

Because the diverticulum can contain ectopic gastric or pancreatic tissue. The ectopic tissue will secrete enzymes and acids into the tissue not protected from them, causing ulceration

79
Q

Draw a diagram illustrating Meckel’s diverticulum

A
80
Q

What is a vitelline cyst?

A

When the vitelline duct forms fibrous strands at either end

81
Q

Draw a diagram illustrating a vitelline cyst?

A
82
Q

What is a vitelline fistula?

A

When there is direct communication between the umbilicus and the intestinal tract

83
Q

What is the result of a vitelline fistula?

A

Faecal matter coming out of the umbilicus

84
Q

Draw a diagram illustrating a vitelline fistula

A
85
Q

Why is recanalisation required in the embryo?

A

The primitive gut tube is a simple tube. In some gut stuctures, cell growth becomes so rapid that the lumen is partially or completely obliterated. Recanalisation occurs to restore the lumen

86
Q

What structures can cause the obliteration of the lumen in the primitive gut tube?

A
  • Oesophagus
  • Bile duct
  • Small intestine
87
Q

What happens if recanalisation is wholly or partially unsuccessful?

A

Atresia or stenosis of the stucture can occur

88
Q

When does atresia of the gut tube structure occur?

A

When there is a complete loss of the lumen

89
Q

When does stenosis of the gut tube structures occur?

A

When there is a narrowing of the lumen

90
Q

Where does most atresia/stenosis occur?

A

In the duodenum

91
Q

What is the most likely cause of atresia/stenosis in the duodenum?

A

Incomplete canalisation, but ‘vascular accidents’ can also contribute

92
Q

What is meant by a ‘vascular accident’?

A

Where there is a loss of blood supply and that part of the gut dies

93
Q

Where does atresia occur more, the duodenum or the jejenum?

A

Duodenum

94
Q

Where does atresia/stenosis occur more, the jejenum or the ileum?

A

The same

95
Q

Where does atresia/stenesis occur more, the ileum or the colon?

A

Ileum

96
Q

What are atresias most often due to in the upper duodenum?

A

Recanalisation failure

97
Q

What are atresias most commonly due to in the lower duodenum?

A

A vascular accident

98
Q

What can cause a vascular accident in the lower duodenum?

A
  • Malrotation
  • Volvulus
  • Body wall defects
99
Q

What is pyloric stenosis?

A

A narrowing of the exit from the stomach

100
Q

Where is pyloric stenosis common?

A

In infants

101
Q

What does pyloric stenosis result in?

A

Characteristic projectile vomiting

102
Q

What causes pyloric stenosis?

A

Hypertrophy of the circular muscle in the region of the pyloric sphincter

103
Q

What are some defects of the abdominal wall?

A
  • Gastroschisis
  • Omphalocoele
104
Q

What is gastroschisis?

A

The failure of closure of the abdominal wall during folding of the embryo, leaving the gut tube and its derivatives outside the body cavity

105
Q

What happensto the gut tube/derivates in gastroschisis?

A

There is no covering over them as they herniate through the abdominal wall directly into the amniotic cavity

106
Q

What is omphalocoele?

A

The persistance of physiological herniation

107
Q

What physiological herniation persists in omphalocoele?

A

A part of the gut tube fails to return to the abdominal cavity following normal herniation into the umbilical cord

108
Q

Is there a covering in omphalocoele?

A

Yes

109
Q

Why is there a covering in omphalocoele?

A

Since the umbilical cord is covered by a reflection of the amnion, this epithelial layer covers the defect

110
Q

What are some hindgut abnormalities?

A
  • Imperforate anus
  • Anal/anorectal agenesis
  • Hindgut fistulae
111
Q

What is an imperforate anus?

A

Failure of the anal membrane to rupture

112
Q

Draw a diagram of an imperforate anus

A
113
Q

What is anal/anorectal agenesis?

A

Failure of development

114
Q

Draw a diagram illustrating anal/anorectal agenesis?

A
115
Q

What is a hindgut fistulae?

A

An abnormal connection within the hindgut

116
Q

Draw a diagram illustrating hindgut fistulae

A
117
Q

What are the mesenteries retained by?

A
  • Jejenum
  • Ileum
  • Appendix
  • Transverse colon
    Sigmoid colon
118
Q

What structures of the midgut/hindgut are fused with mesenteries?

A
  • Duodenum
  • Ascending colon
  • Descending colon
  • Rectum (no peritoneal covering in distal 1/3)
119
Q

What happens, regarding gut development, in week 3?

A

Tubular gut begins to form

120
Q

What happens, regarding gut development, in week 4?

A
  • Primordia of liver, pancreas, and trachea
  • Buccopharyngeal membrane ruptures
121
Q

What happens, regarding gut development, in week 5?

A
  • Expansion and early rotation of the stomach
  • First intestinal loop appears
  • Caecum and bile duct develop
122
Q

What happens, regarding gut development, in week 6?

A
  • Increased liver growth
  • Herniation of intestinal loop
  • Appendix develops
  • Cloacal partioning begins - urorectal septum appears
123
Q

What happens, regarding gut development, in week 7?

A
  • Pancreatic buds fuse
  • Clocal partioning complete
  • Rupture of clocal membrane
124
Q

What happens, regarding gut development, in week 8?

A
  • Counterclockwise rotation of herniated loop
  • Recanalisation
125
Q

What happens, regarding gut development, in week 10?

A
  • Return of herniated loop
  • Adult disposition achieved

B - Embryology for EoY4 (7 decks)

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