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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

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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
What is the process of lung formation?
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)
26
What is the process of stomach formation?
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
27
What is the cardiac incisure?
The point at which lesser curvature of the stomach meets the pyloric antrum of the stomach
28
What is the septum transversum?
A thick mass of cranial mesenchyme that gives rise to thoracic diaphragm and ventral mesentery of foregut
29
What causes the rotation of the stomach? Significance?
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)
30
Where is the greater curvature of the stomach located at first?
Dorsal
31
Where is the lesser curvature of the stomach located at first?
Ventral
32
What is the epiploic foramen?
AKA omental foramen Foramen of Winslow Passage between the greater sac (general cavity of abdomen) and the lesser sac
33
What is the lesser sac?
The cavity in the abdomen formed by the lesser and greater omentum
34
What are the secondarily retroperitoneal structures?
Structures that were initially suspended from the dorsal mesentery but got attached to the body wall i. duodenum ii. pancreas iii. colon
35
What structures retroperitoneal?
1. kidney | 2. bladder
36
How does the liver form?
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
37
What do liver cords give rise to?
1. hepatocytes 2. bile canaliculi 3. hepatic ducts Comes from hepatic diverticulum that appears on ventral side of duodenum
38
What connects the liver to the ventral body wall?
Falciform ligament | Comes from ventral mesentery that holds liver to ventral body wall
39
What is the region of the ventral mesentery connecting the liver to the stomach?
Lesser omentum
40
What is the lesser omentum?
The region of the ventral mesentery connecting the liver to the stomach
41
What happens to the lesser omentum when stomach rotates?
It is repositioned from a sagittal to a coronal plane | Reduces connection between greater and lesser peritoneal sacs to epiploic foramen
42
How is the gallbladder and cystic duct formed?
Cystic diverticulum forms as a second endodermal thickening on ventral side of duodenum Caudal to hepatic diverticulum Gives rise to gallbladder + cystic duct
43
What is the cystic duct?
The duct that joins the gall bladder to the common bile duct
44
How does the pancreas form?
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
45
When does the ileum get distinguished from the colon?
Week 5
46
How does the ileum get distinguished from the colon?
By the presence of a CECAL primordium at the junction between the two
47
What is the cecum?
The junction between the ileum and the ascending colon
48
What is the primary intestinal loop?
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
49
What is attached to turn of the primary intestinal loop (most ventral extreme)?
The vitelline duct
50
Why does the primary intestinal loop herniate into the umbilicus?
Because of the continued elongation of midgut and growth of abdominal organs
51
How does the cranial and caudal ends of the primary intestinal loop rotate?
Counter-clockwise A total of 270 degrees 90 degrees by week 8 Another 180 degrees by week 10
52
When does the primary intestinal loop rotate by 180 after it is already rotated by 90?
Because it is retracted into the abdominal cavity
53
What happens from week 8 to week 10?
Midgut differentiation Lengthening and folding of jejunum and ileum Formation of vermiform appendix
54
When are the small and large intestines attached in final positions?
Week 11
55
What happens after large intestine returns to the abdominal cavity (retraction of the primary intestinal loop)?
The dorsal mesenteries retracts and the cecum and ascending colon get attached to the dorsal body wall, making them secodnarily retroperitoneal
56
What is the descending colon suspended by?
Dorsal body wall after dorsal mesentery retracts | Secondarily retroperitoneal
57
What is the sigmoid colon suspended by?
Dorsal mesentery | Peritoneal
58
What does the distal end of the hindgut form?
Cloaca | An expansion that is sealed by cloaal membrane
59
What is the cardiac incisure?
The point at which lesser curvature of the stomach meets the pyloric antrum of the stomach
60
What is the septum transversum?
A thick mass of cranial mesenchyme that gives rise to thoracic diaphragm and ventral mesentery of foregut
61
What causes the rotation of the stomach? Significance?
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
Where is the greater curvature of the stomach located at first?
Dorsal
63
Where is the lesser curvature of the stomach located at first?
Ventral
64
What is the epiploic foramen?
AKA omental foramen Foramen of Winslow Passage between the greater sac (general cavity of abdomen) and the lesser sac
65
What is the lesser sac?
The cavity in the abdomen formed by the lesser and greater omentum
66
What are the secondarily retroperitoneal structures?
Structures that were initially suspended from the dorsal mesentery but got attached to the body wall i. duodenum ii. pancreas iii. colon
67
What structures retroperitoneal?
1. kidney | 2. bladder
68
How does the liver form?
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
What do liver cords give rise to?
1. hepatocytes 2. bile canaliculi 3. hepatic ducts Comes from hepatic diverticulum that appears on ventral side of duodenum
70
What connects the liver to the ventral body wall?
Falciform ligament | Comes from ventral mesentery that holds liver to ventral body wall
71
What is the region of the ventral mesentery connecting the liver to the stomach?
Lesser omentum
72
What is the lesser omentum?
The region of the ventral mesentery connecting the liver to the stomach
73
What happens to the lesser omentum when stomach rotates?
It is repositioned from a sagittal to a coronal plane | Reduces connection between greater and lesser peritoneal sacs to epiploic foramen
74
How is the gallbladder and cystic duct formed?
Cystic diverticulum forms as a second endodermal thickening on ventral side of duodenum Caudal to hepatic diverticulum Gives rise to gallbladder + cystic duct
75
What is the cystic duct?
The duct that joins the gall bladder to the common bile duct
76
How does the pancreas form?
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
When does the ileum get distinguished from the colon?
Week 5
78
How does the ileum get distinguished from the colon?
By the presence of a CECAL primordium at the junction between the two
79
What is the cecum?
The junction between the ileum and the ascending colon
80
What is the primary intestinal loop?
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
What is attached to turn of the primary intestinal loop (most ventral extreme)?
The vitelline duct
82
Why does the primary intestinal loop herniate into the umbilicus?
Because of the continued elongation of midgut and growth of abdominal organs
83
How does the cranial and caudal ends of the primary intestinal loop rotate?
Counter-clockwise A total of 270 degrees 90 degrees by week 8 Another 180 degrees by week 10
84
When does the primary intestinal loop rotate by 180 after it is already rotated by 90?
Because it is retracted into the abdominal cavity
85
What happens from week 8 to week 10?
Midgut differentiation Lengthening and folding of jejunum and ileum Formation of vermiform appendix
86
When are the small and large intestines attached in final positions?
Week 11
87
What happens after large intestine returns to the abdominal cavity (retraction of the primary intestinal loop)?
The dorsal mesenteries retracts and the cecum and ascending colon get attached to the dorsal body wall, making them secodnarily retroperitoneal
88
What is the descending colon suspended by?
Dorsal body wall after dorsal mesentery retracts | Secondarily retroperitoneal
89
What is the sigmoid colon suspended by?
Dorsal mesentery | Peritoneal
90
What does the distal end of the hindgut form?
Cloaca | An expansion that is sealed by cloaal membrane
91
What is the allantois?
A small diverticulum | Extends from cloaca into the connecting stalk (future umbilicus)
92
What happens to the cloaca?
Divides into posterior rectum and anterior primitive urogenital sinus Urorectal septum is formed Week 4-6
93
What forms the urorectal septum?
3 folds of tissue Superior fold = Tourneux fold Inferior folds = Rathke folds
94
What is the perineum?
The zone of fusion between urogenital membrane and anal membrane
95
What is the proctodeum?
AKA anal pit Formed by mesenchyme surrounding anal membrane that proliferates into a raised ectodermal structure Invaginates into anorectal canal in week 8
96
What is the pectinate line?
The line between the inferior 1/3 of anorectal canal and superior 2/3
97
What is the embryologic derivation of anorectal canal?
Inferior 1/3 = ectodermal Superior 2/3 = endodermal Because of proctodeum (which is ectodermal)
98
What are the three esophageal anomalies that happen as a result of embryological malformation?
1. Esophageal atresia 2. Esophageal stenosis 3. Short esophagus
99
What are the two ways esophageal atresia can occur?
1. tracheoesophageal fistula | 2. failure of recanalization (associated with anorectal atresia)
100
What is a fistula?
An abnormal CONNECTION between an organ, vessel or intestine and another structure
101
What are most cases of esophageal atresia associated with?
Tracheoesophageal fistula (abnormal connection) 85% When the septum separating the trachea and esophagus is displaced caudally
102
What are the sequelae to tracheoesophageal fistula that causes esophageal atresia?
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
What is polyhydraminos?
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
What are the key characteristics of esophageal stenosis?
Narrowing of the esophagus Due to incomplete recanalization Occurs anywhere along esophagus
105
What are the key characteristics of a short esophagus?
A failure of esophagus to elongate because stomach is displaced into the thoracic cavity Called congenital hiatal hernia
106
What is a congenital hiatal hernia?
When the esophagus fails to elongate and stomach goes through diaphragm
107
What are the three embryological anomalies of the stomach?
1. Congenital hypertrophic Pyloric Stenosis 2. Duodenal Stenosis 3. Duodenal Atresia
108
What are the key characteristics of Congenital Hypertrophyic Pyloric stenosis?
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
What is the pylorus?
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
What are the key characteristics of duodenal stenosis?
Stomach anomaly Narrowing of dudodenal lumen due to incomplete RECANALIZATION Characterized by expulsion of stomach contents that contain BILE
111
What are the key characteristics of duodenal atresia?
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
What are the anomalies of the liver and cystic ducts?
1. Duct anomalies (variations in the shape of hepatic, cystic and bile ducts) 2. Extrahepatic Biliary atresia
113
What are the key characteristics of extrahepatic biliary atresia?
``` 1/10,000 births Obstruction of bile duct Failure to canalize OR Liver infection in the fetus Clinical sympoms = JAUNDICE ```
114
What are the anomalies of the pancreas?
1. Accessory Pancreatic tissue | 2. Anular Pancreas
115
What are the key characteristics of Accessory Pancreatic Tissue?
Inappropriate interaction of pancreatic mesenchyme with nonpancreatic endodermal epithelium Results in formation of pancreatic tissue in stomach, duodenum and ileal (Meckel’s) diverticulum
116
What are the key characteristics of Anular pancreas?
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
What are the anomalies of the midgut?
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
What is the most common anomaly of the digestive tract?
Ileal (Meckel’s) diverticulum When a remnant of yolk stalk remains as an outpouching of the ileum Asymptomatic
119
What is an omphalocele?
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
What is gastroschisis?
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
What are the key characteristics of nonrotation?
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
What are the key characteristics of mixed rotation?
When midgut fails to complete final 90 degrees of rotation Cecum becomes inferior to stomach Can cause duodenal obstruction
123
What are the key characteristics of stenosis and atresia of midgut?
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
What causes intestinal duplication?
Failure of recanalization | Formation of two parallel lumens
125
What are the anomalies of the hindgut?
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
What are the key characteristics of congenital megacolon?
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
What are the key characteristics of anorectal agenesis
``` 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
What is meconium?
Dark green substance forming the first FECES of a newborn infant
129
What is the Golosow and Grobstein experiment of 1962?
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
What is the mesenchyme inducing mechanism for hindgut and midgut?
Both have SHH expressed in gut endoderm SHH leads to expression of Hox genes in mesenchyme Hox genes then leads to differentiation
131
What is situs inversus?
When there is a reversal in left-right symmetry (or asymmetry in the case since organs are asymmetrical to begin with)
132
What is the significance of Nodal?
TGFbeta gene | Necessary for left-right patterning and morphogenesis of visceral organs
133
What is significant about Kartagener’s Syndrome?
``` Situs inversus (reversal of asymmetry) Failure of dynein function Ciliary defect ```
134
What is the siginificance of Pdx1?
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
Pdx1
Essential for pancreas growth Located in duodenal regions where pancreatic ventral/dorsal buds develop MOUSE
136
What is IPF1?
Analog to pdx1 HUMANS If you are missing this, you are APANCREATIC and you will get type 2 diabetes (type 1?)

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