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GI Week 1 > Lecture 1: Development of the Alimentary and Paraalimentary Tract > Flashcards

Lecture 1: Development of the Alimentary and Paraalimentary Tract Flashcards

1
Q

What is a stomodeum?

A

A depression between the brain and the pericardium in an embryo
Precursor of the MOUTH and the anterior lobe of the pituitary gland

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2
Q

Where does the foregut (cranial end) of the gut tube terminate?

A

At the buccopharyngeal membrane

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3
Q

Where does the hindgut (caudal end) terminate?

A

Cloacal membrane

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4
Q

What is the proctodeum?

A

Back ectodermal part of alimentary canal
Forms the lower part of the anal canal, below the pectinate line
Lined by stratified squamous non-keratonized and stratified squamous keratinized epithelium
Hilton’s white line
Source: http://en.wikipedia.org/wiki/Proctodeum

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5
Q

Where is the epithelium of the alimentary tract derived from?

A

The ectoderm of the stomodeum and proctodeum respectively

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6
Q

What is the alimentary canal?

A

The tubular passage that extends from mouth to anus
Functions in digestion and absorption of food and elimination of residual waste and includes the mouth, pharynx, esophagus, stomach, small intestine and large intestine

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7
Q

When does the vitelline duct appear? Significance?

A

26 days postfertilization (DPF)

Significance is that this is when the midgut region of the gut tube is no longer open to the yolk sac

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8
Q

What is atresia?

A

Congenital absence of closure of a normal body opening or tubular structure

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9
Q

What is recanalization?

A

Formation of new canals or paths, especially blood vessels through an obstruction such as a clot
Spontaneous restoration of the lumen of an occluded duct or tube

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10
Q

When does occlusion of the gut tube occur? When does it recanalize?

A

During week 6
Endodermal epithelium proliferates to occlude the gut tube
Recanalizes by week 8

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11
Q

When does the endodermal lining of the gut lumen differentiate into definitive mucosal epithelium?

A

Week 9

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12
Q

What happens when there is a defect in recanalization?

A

Stenosis

Atresia at various positions of the gut tube

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13
Q

What are the derivatives of the foregut?

A
  1. Pharynx
  2. Thoracic esophagus
  3. Abdominal esophagus
  4. Stomach
  5. Superior duodenum
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14
Q

What are the derivatives of the midgut?

A
  1. Inferior duodenum
  2. Jejunnum
  3. Ileum
  4. Cecum
  5. Appendix
  6. Ascending colon
  7. Transverse colon (right 2/3)
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15
Q

What are the derivatives of the hindgut?

A
  1. Transverse colon (left 1/3)
  2. Descending colon
  3. Sigmoid colon
  4. Rectum
    Urogenital sinus and derivatives??
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16
Q

How do you tell what comes from foregut, midgut and hindgut?

A

Remember that the foregut and midgut is separated by superior and inferior duodenum, respectively
After that it is pretty easy, since the watershed area (2/3 of your way into the transverse colon) is the exact position where mid/hind gut are differentiated

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17
Q

What artery supplies the abdominal foregut?

A

CELIAC artery

i. abdominal esophagus
ii. stomach
iii. superior duodenum
iv. and its derivatives
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18
Q

What artery supplies the midgut?

A

SUPERIOR mesenteric artery

i. inferior duodenum	
ii. small intestines
iii. ascending colon
iv. 2/3 of transverse colon
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19
Q

What artery supplies the hindgut?

A

INFERIOR mesenteric artery

i. 1/3 of transverse colon	
ii. descending colon
iii. sigmoid colon
iv. rectum
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20
Q

What is the thoracic foregut supplied by?

A

Pharynx and thoracic esophagus are supplied by aortic branches

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21
Q

What is a diverticulum?

A

An abnormal sac or pouch formed at a weak point in the wall of the alimentary tract

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22
Q

What is the respiratory diverticulum? When does it form?

A

The “lung bud”
Forms at 22 dpf
Ventral outpouching of the thoracic esophagus

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23
Q

When does the respiratory diverticulum bifurcate into the left and right bronchial buds?

A

26-28 dpf

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24
Q

What are the bronchial buds?

A

The rudiments of the two lungs

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25
Q

What is the process of lung formation?

A
  1. Resiratory diverticulum (22dpf)
  2. Bronchial buds (26-28 dpf)
  3. Secondary bronchial buds (5th week)
  4. Terminal bronchioles (Week 16)
  5. Respiratory bronchioles (Week 28)
  6. Terminal sacs aka primitive alveoli (Week 36)
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26
Q

What is the process of stomach formation?

A
  1. Week 4: straight tube connected to dorsal body by DORSAL MESENTERY
    1a: dorsal wall of stomach grows faster than ventral wall to result in greater and lesser curvature
  2. Week 5: caudal portion of septum transversum thins to become the ventral mesentery connecting stomach to ventral body wall
  3. 7-8 weeks: differential thinning of right side of dorsal mesentery results in 90 degree rotation of stomach around craniocaudal axis
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27
Q

What is the cardiac incisure?

A

The point at which lesser curvature of the stomach meets the pyloric antrum of the stomach

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28
Q

What is the septum transversum?

A

A thick mass of cranial mesenchyme that gives rise to thoracic diaphragm and ventral mesentery of foregut

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29
Q

What causes the rotation of the stomach? Significance?

A

A differential thinning of the right side of the dorsal mesentery
90 degree rotation around craniocaudal axis
Rotation causes duodenum to adhere to the dorsal body wall
Forms the lesser sac of the peritoneal cavity, dorsal to the stomach
Rotation is CLOCKWISE (if you are looking down at the rotation or if you are standing superiorly over the rotation)

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30
Q

Where is the greater curvature of the stomach located at first?

A

Dorsal

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31
Q

Where is the lesser curvature of the stomach located at first?

A

Ventral

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32
Q

What is the epiploic foramen?

A

AKA omental foramen
Foramen of Winslow
Passage between the greater sac (general cavity of abdomen) and the lesser sac

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33
Q

What is the lesser sac?

A

The cavity in the abdomen formed by the lesser and greater omentum

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34
Q

What are the secondarily retroperitoneal structures?

A

Structures that were initially suspended from the dorsal mesentery but got attached to the body wall

i. duodenum
ii. pancreas
iii. colon

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35
Q

What structures retroperitoneal?

A
  1. kidney

2. bladder

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36
Q

How does the liver form?

A

Day 22, hepatic plate appears on ventral side of duodenum
Cells proliferate a few days later (Day 25) to form the hepatic diverticulum
Grows into ventral mesentery
Diverticulum gives rise to liver cords

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37
Q

What do liver cords give rise to?

A
  1. hepatocytes
  2. bile canaliculi
  3. hepatic ducts
    Comes from hepatic diverticulum that appears on ventral side of duodenum
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38
Q

What connects the liver to the ventral body wall?

A

Falciform ligament

Comes from ventral mesentery that holds liver to ventral body wall

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39
Q

What is the region of the ventral mesentery connecting the liver to the stomach?

A

Lesser omentum

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40
Q

What is the lesser omentum?

A

The region of the ventral mesentery connecting the liver to the stomach

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41
Q

What happens to the lesser omentum when stomach rotates?

A

It is repositioned from a sagittal to a coronal plane

Reduces connection between greater and lesser peritoneal sacs to epiploic foramen

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42
Q

How is the gallbladder and cystic duct formed?

A

Cystic diverticulum forms as a second endodermal thickening on ventral side of duodenum
Caudal to hepatic diverticulum
Gives rise to gallbladder + cystic duct

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43
Q

What is the cystic duct?

A

The duct that joins the gall bladder to the common bile duct

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44
Q

How does the pancreas form?

A

Day 26 DORSAL pancreatic bud forms on duodenum OPPOSITE of the hepatic diverticulum
A VENTRAL pancreatic bud grows into the ventral mesentery (inferior to liver) by day 32
Main duct of ventral bud connects to the bile duct
Week 5: bile duct and ventral pancreatic bud migrate around the duodenum to the dorsal mesentery
Week 6: pancreatic buds fuse to form the definitive pancreas
After fusion, duct from dorsal bud degenerates to leaveventral bud as only attachment (via main pancreatic duct) to the duodenum
Fuses to the body wall to become secondarily retroperitoneal

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45
Q

When does the ileum get distinguished from the colon?

A

Week 5

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46
Q

How does the ileum get distinguished from the colon?

A

By the presence of a CECAL primordium at the junction between the two

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47
Q

What is the cecum?

A

The junction between the ileum and the ascending colon

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48
Q

What is the primary intestinal loop?

A

A dorso ventral hairpin fold
Caused by the fact that the ileum lengthens more rapidly than abdominal cavity
Cranial end = ileum
Caudal end = ascending and transverse colon

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49
Q

What is attached to turn of the primary intestinal loop (most ventral extreme)?

A

The vitelline duct

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50
Q

Why does the primary intestinal loop herniate into the umbilicus?

A

Because of the continued elongation of midgut and growth of abdominal organs

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51
Q

How does the cranial and caudal ends of the primary intestinal loop rotate?

A

Counter-clockwise
A total of 270 degrees
90 degrees by week 8
Another 180 degrees by week 10

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52
Q

When does the primary intestinal loop rotate by 180 after it is already rotated by 90?

A

Because it is retracted into the abdominal cavity

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53
Q

What happens from week 8 to week 10?

A

Midgut differentiation
Lengthening and folding of jejunum and ileum
Formation of vermiform appendix

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54
Q

When are the small and large intestines attached in final positions?

A

Week 11

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55
Q

What happens after large intestine returns to the abdominal cavity (retraction of the primary intestinal loop)?

A

The dorsal mesenteries retracts and the cecum and ascending colon get attached to the dorsal body wall, making them secodnarily retroperitoneal

56
Q

What is the descending colon suspended by?

A

Dorsal body wall after dorsal mesentery retracts

Secondarily retroperitoneal

57
Q

What is the sigmoid colon suspended by?

A

Dorsal mesentery

Peritoneal

58
Q

What does the distal end of the hindgut form?

A

Cloaca

An expansion that is sealed by cloaal membrane

59
Q

What is the cardiac incisure?

A

The point at which lesser curvature of the stomach meets the pyloric antrum of the stomach

60
Q

What is the septum transversum?

A

A thick mass of cranial mesenchyme that gives rise to thoracic diaphragm and ventral mesentery of foregut

61
Q

What causes the rotation of the stomach? Significance?

A

A differential thinning of the right side of the dorsal mesentery
90 degree rotation around craniocaudal axis
Rotation causes duodenum to adhere to the dorsal body wall
Forms the lesser sac of the peritoneal cavity, dorsal to the stomach
Rotation is CLOCKWISE (if you are looking down at the rotation or if you are standing superiorly over the rotation)

62
Q

Where is the greater curvature of the stomach located at first?

A

Dorsal

63
Q

Where is the lesser curvature of the stomach located at first?

A

Ventral

64
Q

What is the epiploic foramen?

A

AKA omental foramen
Foramen of Winslow
Passage between the greater sac (general cavity of abdomen) and the lesser sac

65
Q

What is the lesser sac?

A

The cavity in the abdomen formed by the lesser and greater omentum

66
Q

What are the secondarily retroperitoneal structures?

A

Structures that were initially suspended from the dorsal mesentery but got attached to the body wall

i. duodenum
ii. pancreas
iii. colon

67
Q

What structures retroperitoneal?

A
  1. kidney

2. bladder

68
Q

How does the liver form?

A

Day 22, hepatic plate appears on ventral side of duodenum
Cells proliferate a few days later (Day 25) to form the hepatic diverticulum
Grows into ventral mesentery
Diverticulum gives rise to liver cords

69
Q

What do liver cords give rise to?

A
  1. hepatocytes
  2. bile canaliculi
  3. hepatic ducts
    Comes from hepatic diverticulum that appears on ventral side of duodenum
70
Q

What connects the liver to the ventral body wall?

A

Falciform ligament

Comes from ventral mesentery that holds liver to ventral body wall

71
Q

What is the region of the ventral mesentery connecting the liver to the stomach?

A

Lesser omentum

72
Q

What is the lesser omentum?

A

The region of the ventral mesentery connecting the liver to the stomach

73
Q

What happens to the lesser omentum when stomach rotates?

A

It is repositioned from a sagittal to a coronal plane

Reduces connection between greater and lesser peritoneal sacs to epiploic foramen

74
Q

How is the gallbladder and cystic duct formed?

A

Cystic diverticulum forms as a second endodermal thickening on ventral side of duodenum
Caudal to hepatic diverticulum
Gives rise to gallbladder + cystic duct

75
Q

What is the cystic duct?

A

The duct that joins the gall bladder to the common bile duct

76
Q

How does the pancreas form?

A

Day 26 DORSAL pancreatic bud forms on duodenum OPPOSITE of the hepatic diverticulum
A VENTRAL pancreatic bud grows into the ventral mesentery (inferior to liver) by day 32
Main duct of ventral bud connects to the bile duct
Week 5: bile duct and ventral pancreatic bud migrate around the duodenum to the dorsal mesentery
Week 6: pancreatic buds fuse to form the definitive pancreas
After fusion, duct from dorsal bud degenerates to leaveventral bud as only attachment (via main pancreatic duct) to the duodenum
Fuses to the body wall to become secondarily retroperitoneal

77
Q

When does the ileum get distinguished from the colon?

A

Week 5

78
Q

How does the ileum get distinguished from the colon?

A

By the presence of a CECAL primordium at the junction between the two

79
Q

What is the cecum?

A

The junction between the ileum and the ascending colon

80
Q

What is the primary intestinal loop?

A

A dorso ventral hairpin fold
Caused by the fact that the ileum lengthens more rapidly than abdominal cavity
Cranial end = ileum
Caudal end = ascending and transverse colon

81
Q

What is attached to turn of the primary intestinal loop (most ventral extreme)?

A

The vitelline duct

82
Q

Why does the primary intestinal loop herniate into the umbilicus?

A

Because of the continued elongation of midgut and growth of abdominal organs

83
Q

How does the cranial and caudal ends of the primary intestinal loop rotate?

A

Counter-clockwise
A total of 270 degrees
90 degrees by week 8
Another 180 degrees by week 10

84
Q

When does the primary intestinal loop rotate by 180 after it is already rotated by 90?

A

Because it is retracted into the abdominal cavity

85
Q

What happens from week 8 to week 10?

A

Midgut differentiation
Lengthening and folding of jejunum and ileum
Formation of vermiform appendix

86
Q

When are the small and large intestines attached in final positions?

A

Week 11

87
Q

What happens after large intestine returns to the abdominal cavity (retraction of the primary intestinal loop)?

A

The dorsal mesenteries retracts and the cecum and ascending colon get attached to the dorsal body wall, making them secodnarily retroperitoneal

88
Q

What is the descending colon suspended by?

A

Dorsal body wall after dorsal mesentery retracts

Secondarily retroperitoneal

89
Q

What is the sigmoid colon suspended by?

A

Dorsal mesentery

Peritoneal

90
Q

What does the distal end of the hindgut form?

A

Cloaca

An expansion that is sealed by cloaal membrane

91
Q

What is the allantois?

A

A small diverticulum

Extends from cloaca into the connecting stalk (future umbilicus)

92
Q

What happens to the cloaca?

A

Divides into posterior rectum and anterior primitive urogenital sinus
Urorectal septum is formed
Week 4-6

93
Q

What forms the urorectal septum?

A

3 folds of tissue
Superior fold = Tourneux fold
Inferior folds = Rathke folds

94
Q

What is the perineum?

A

The zone of fusion between urogenital membrane and anal membrane

95
Q

What is the proctodeum?

A

AKA anal pit
Formed by mesenchyme surrounding anal membrane that proliferates into a raised ectodermal structure
Invaginates into anorectal canal in week 8

96
Q

What is the pectinate line?

A

The line between the inferior 1/3 of anorectal canal and superior 2/3

97
Q

What is the embryologic derivation of anorectal canal?

A

Inferior 1/3 = ectodermal
Superior 2/3 = endodermal
Because of proctodeum (which is ectodermal)

98
Q

What are the three esophageal anomalies that happen as a result of embryological malformation?

A
  1. Esophageal atresia
  2. Esophageal stenosis
  3. Short esophagus
99
Q

What are the two ways esophageal atresia can occur?

A
  1. tracheoesophageal fistula

2. failure of recanalization (associated with anorectal atresia)

100
Q

What is a fistula?

A

An abnormal CONNECTION between an organ, vessel or intestine and another structure

101
Q

What are most cases of esophageal atresia associated with?

A

Tracheoesophageal fistula (abnormal connection)
85%
When the septum separating the trachea and esophagus is displaced caudally

102
Q

What are the sequelae to tracheoesophageal fistula that causes esophageal atresia?

A

A fetus cannot swallow and dispose of amniotic fluid
Accumulation of amniotic fluid
Polyhdramnios
In a newborn, the first swallow is normal but the fluid is suddenly expelled and respiratory distress occurs

103
Q

What is polyhydraminos?

A

When there is an accumulation of amniotic fluid
This can happen with a baby with esophageal atresia due to tracheoesophageal fistula
-Cannot swallow and dispose of amniotic fluid

104
Q

What are the key characteristics of esophageal stenosis?

A

Narrowing of the esophagus
Due to incomplete recanalization
Occurs anywhere along esophagus

105
Q

What are the key characteristics of a short esophagus?

A

A failure of esophagus to elongate because stomach is displaced into the thoracic cavity
Called congenital hiatal hernia

106
Q

What is a congenital hiatal hernia?

A

When the esophagus fails to elongate and stomach goes through diaphragm

107
Q

What are the three embryological anomalies of the stomach?

A
  1. Congenital hypertrophic Pyloric Stenosis
  2. Duodenal Stenosis
  3. Duodenal Atresia
108
Q

What are the key characteristics of Congenital Hypertrophyic Pyloric stenosis?

A

Stomach anomaly
Involves a thickening of the pylorus, resulting in severe stenosis, obstruction to food passage
Newborns with this condition display a distended stomach and PROJECTILE VOMITING

109
Q

What is the pylorus?

A

The region of the stomach that connects to the duodenum
Divided into two parts
i. pyloric antrum (connects to body of stomach)
ii. pyloric canal (connects to duodenum)

110
Q

What are the key characteristics of duodenal stenosis?

A

Stomach anomaly
Narrowing of dudodenal lumen due to incomplete RECANALIZATION
Characterized by expulsion of stomach contents that contain BILE

111
Q

What are the key characteristics of duodenal atresia?

A

Stomach anomaly
COMPLETE blockage of duodenum occurs in 30% of Down’s infants and 20% of premature births
Failure of recanalization
Vomiting begins within few hours and expels bile

112
Q

What are the anomalies of the liver and cystic ducts?

A
  1. Duct anomalies (variations in the shape of hepatic, cystic and bile ducts)
  2. Extrahepatic Biliary atresia
113
Q

What are the key characteristics of extrahepatic biliary atresia?

A 
1/10,000 births
Obstruction of bile duct
Failure to canalize 
OR
Liver infection in the fetus
Clinical sympoms = JAUNDICE
114
Q

What are the anomalies of the pancreas?

A
  1. Accessory Pancreatic tissue

2. Anular Pancreas

115
Q

What are the key characteristics of Accessory Pancreatic Tissue?

A

Inappropriate interaction of pancreatic mesenchyme with nonpancreatic endodermal epithelium
Results in formation of pancreatic tissue in stomach, duodenum and ileal (Meckel’s) diverticulum

116
Q

What are the key characteristics of Anular pancreas?

A

When ventral pancreatic bud forms as two attached buds
When two ventral buds rotate in opposite direction to fuse with dorsal bud, a thin band of pancreatic tissue surround duodenum
Band can cause duodenal stenosis or atresia after birth
Due to inflammation and malignant disease later in life as well

117
Q

What are the anomalies of the midgut?

A
  1. congenital omphalocele (intestinal or hepatic herniation into umbilical cord)
    • failure of midgut to return to abdominal cavity by week 10
  2. Umbilical hernia
    • umbilicus doesn’t close making it easier to herniate after week 10
  3. Gastroschisis
  4. Nonrotation
  5. Mixed rotation
  6. Reversed rotation
    • reverse of second midgut rotation
    • can cause obstruction of transverse colon
  7. Subhepatic cecum and appendix
    • adherence of cecum to liver
  8. Mobile cecum
    • causes variations in position of the appendix
    • due to incomplete fixation of ascending colon
  9. Midgut volvulus
  10. Stenosis and atresia
  11. Ileal (Meckel’s) Diverticulum
118
Q

What is the most common anomaly of the digestive tract?

A

Ileal (Meckel’s) diverticulum
When a remnant of yolk stalk remains as an outpouching of the ileum
Asymptomatic

119
Q

What is an omphalocele?

A

A birth defect in which infant’s intestine or other abdominal organs stick out of the belly button
A type of hernia
So you can see intestines through belly button

120
Q

What is gastroschisis?

A

A defect in ventral abdominal wall that results in extrusion of midgut structures WITHOUT involvement of the umbilical cord
Due to lateral folding of embryo during week four

121
Q

What are the key characteristics of nonrotation?

A

Called left-sided colon
Quite commom and asymptomatic if volvulus doesn’t occur
Happens when second midgut rotation (180 degrees) fails to occur
Large intestine on left
Small intestine on right

122
Q

What are the key characteristics of mixed rotation?

A

When midgut fails to complete final 90 degrees of rotation
Cecum becomes inferior to stomach
Can cause duodenal obstruction

123
Q

What are the key characteristics of stenosis and atresia of midgut?

A

Failure of recanalization
Necrosis and conversion to a fibrous cord due to lack of recanalization
Can also form two parallel lumens known as intestinal duplication

124
Q

What causes intestinal duplication?

A

Failure of recanalization

Formation of two parallel lumens

125
Q

What are the anomalies of the hindgut?

A
  1. Congenital megacolon (Hirchsprung’s disease)
  2. Imperforate anus and anorectal anomalies
  3. Anorectal agenesis
  4. Anal stenosis
  5. Membranous atresia
    • anus is in normal position but sealed by membrane
  6. Rectal atresia
    • anal canal and rectum are present but don’t connect
    • due to recanalization or defective blood supply
126
Q

What are the key characteristics of congenital megacolon?

A

Hirschsprung’s disease
Results as a failure of neural crest cells to migrate normally
No neural crest cells = no enteric ganglion cells
No peristalsis in colon = dilation
HINDGUT

127
Q

What are the key characteristics of anorectal agenesis

A 
Rectum ends before reaching anal pit
Fistula usually connects rectum to bladder, urethra or vagina
Caused by defect in urogenital septum
Characterized by
	i. meconium in urine (males)
	ii. meconium in vagina
128
Q

What is meconium?

A

Dark green substance forming the first FECES of a newborn infant

129
Q

What is the Golosow and Grobstein experiment of 1962?

A

Pancreas development DEPENDS on the surround mesenchyme
If mesenchyme was there, pancreatic buds grew
Lack of mesenchyme = no pancreatic buds
This means that mesenchyme released regulatory signals
Mesenchym around midgut = midgut formation
Mesenchyme around foregut = foregut formation

130
Q

What is the mesenchyme inducing mechanism for hindgut and midgut?

A

Both have SHH expressed in gut endoderm
SHH leads to expression of Hox genes in mesenchyme
Hox genes then leads to differentiation

131
Q

What is situs inversus?

A

When there is a reversal in left-right symmetry (or asymmetry in the case since organs are asymmetrical to begin with)

132
Q

What is the significance of Nodal?

A

TGFbeta gene

Necessary for left-right patterning and morphogenesis of visceral organs

133
Q

What is significant about Kartagener’s Syndrome?

A 
Situs inversus (reversal of asymmetry)
Failure of dynein function
Ciliary defect
134
Q

What is the siginificance of Pdx1?

A

Essential for pancreas development
Stands for pancreatic and duodenal homeobox
Expressed in dorsal and ventral bud regions of duodenum
SEEN ONLY in the MOUSE
Growth initiated but doesn’t progress

135
Q

Pdx1

A

Essential for pancreas growth
Located in duodenal regions where pancreatic ventral/dorsal buds develop
MOUSE

136
Q

What is IPF1?

A

Analog to pdx1
HUMANS
If you are missing this, you are APANCREATIC and you will get type 2 diabetes (type 1?)

GI Week 1 (6 decks)

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