Development of the GI Tract Flashcards
1
Q
What is the significance of gut tube formation?
A
Organs and glands are produced by budding from the gut tube.
2
Q
Describe primary germ formation.
A
Formed during gastrulation.
- At the beginning of the third week, embryo has implanted into uterine wall and appears as a flat disc made up of 2 layers.
3
Q
What two layers does the embryo consist of?
A
Epiblast and hypoblast
4
Q
What happens to epiblast cells in the mid-line of the embryo?
A
Ingress, starting from the caudal end - visible as the primitive streak.
5
Q
What do ingressing cells differentiate into?
A
Mesoderm
6
Q
What does the mesoderm give rise to?
A
- Surrounding muscles
- Connective tissue
- Blood vessels
7
Q
What does the epiblast give rise to?
A
Ectoderm
8
Q
What is the importance of the neural crest?
A
Provides innervation to the gut - enteric NS
9
Q
What does the hypoblast give rise to?
A
Endoderm
10
Q
What does the endoderm differentiate to form?
A
- Epithelium
- Gut tube
- Glands
11
Q
Briefly describe the directions of the formation of the gut tube.
A
Formed by the folding of sheets of cells in two directions:
- folding towards the midline along the cranial-caudal axis
- folding towards the yolk sac at the cranial and caudal ends
12
Q
The gut tube doesn’t form as a tube the first time round. Describe this phenomenon.
A
- Cells will grow to the point where there’s no lumen
- Gut needs rearrangement (scaffolding).
- Recanalisation occurs - cells die forming a diameter within the tube
13
Q
What can occur if the middle part of the gut does not recanalise?
A
- Duodenal atresia (if short term)
- Jejunal atresia (if long term)
14
Q
How can duodenal atresia affect new borns?
A
- Projectile vomiting
15
Q
What are the subdivisions of the gut?
A
FOREGUT
MIDGUT
HINDGUT
16
Q
Outline the parts of the foregut
A
- Pharynx
- Oesophagus
- Stomach
- Cranial half of duodenum
- Ampulla of Vater
17
Q
Outline the parts of the midgut
A
- Caudal duodenum (From duodenal papilla )
- Jejunum
- Ileum
- Caecum
- Appendix
- Ascending colon
- Proximal 2/3 of transverse colon
18
Q
Outline the parts of the hindgut
A
- Distal 1/3 of transverse colon
- Descending colon
- Rectum
19
Q
What are the blood supplies to the different parts of the gut?
A
FOREGUT - coeliac artery
MIDGUT - superior mesenteric artery
HINDGUT - inferior mesenteric artery
20
Q
Where do the sympathetic ganglia develop?
A
Next to the major branches of the aorta
21
Q
What do post-ganglionic sympathetic axons innervate?
A
- The same tissues that the arteries supply with blood
22
Q
What is the innervation of the gut?
A
FOREGUT - celiac ganglion
MIDGUT - superior mesenteric ganglion
HINDGUT - inferior mesenteric ganglion
23
Q
Describe the rotation of the stomach.
A
- Tube begins to dilate, forming an enlarged lumen.
- Makes a 90° turn about the craniocaudal axis, so the dorsal border grows more rapidly than the ventral border, establishing the greater curvature of the stomach.
- Anterior part becomes the right, and the posterior becomes the left.
24
Q
What genes determine the position (of the structures) along the cranio-caudal axis?
A
Homeotic (HOX) genes.
25
What fixes the mature gut in place?
Fusion of the mesentries with the posterior abdominal wall
26
Describe pyloric stenosis.
- Gastric outlet obstruction caused by smooth muscle hypertrophy (ie. the smooth muscle forming the pyloric sphincter overgrows).
27
How can pyloric stenosis be spotted in newborns?
Will start projectile vomiting shortly after feeding
28
Describe the formation of the liver, budding from the gut tube. PART 1
- Hepatic diverticulum grows into the mesenchyme of the septum transversum.
- Cords of the hepatic endoderm, bile drainage ducts, and blood vessels proliferate, arranged as sinusoids.
- Liver exceeds the size of the septum transversum, and expands into the ventral mesentery.
29
Describe the formation of the liver, budding from the gut tube. PART 2
The remaining ventral mesentery gives rise to:
- falciform ligament between the liver and the body wall
- lesser omentum between the liver and stomach
30
Describe the formation of the pancreas, budding from the gut tube.
TWO PANCREATIC BUDS
- dorsal, from the duodenal endoderm (induced by the notochord)
- central, from the hepatic diverticulum (induced by the hepatic mesoderm)
As the duodenum rotates, the dorsal and ventral buds fuse.
31
Describe annular pancreas
- Develops from bi-lobed ventral bud that migrates in separate directions, coming around and fusing to constrict the duodenum.
32
Describe the development of the intestines.
- Mesentery and the gut grow at different rates, leading to folding of gut.
- Abdomen is too small to accommodate this, so the gut herniates into the umbilical stalk at 6/7 weeks.
- By 10 weeks, the abdomen is bigger, and the intestines return.
33
Describe an umbilical hernia.
- Intestines return normally
- Rectus abdominis fails to fuse around the umbilicus, leading to gut covered in skin.
34
Describe an omphalocele.
- Failure of the intestinal loops to return to the abdomen - ends up as a hernia covered in amnion.
- Associated with maternal obesity, alcohol/ tobacco
35
Describe gastroschisis.
- Defect in the abdominal wall, a failure of the ventral body wall to fuse.
- No covering of the gut as it sits outside the body.
- Association with young maternal age, low maternal BMI, and recreational drugs (especially cocaine).
36
Describe Hirschsprung’s disease. PART 1
- Person has an aganglionic megacolon.
- Primarily affects the hindgut.
- Absence of parasympathetic ganglia, caused by a lack of neural crest cells.
37
Describe Hirschsprung’s disease. PART 2
- Dilation of sections of the colon, with a lack of tone and peristalsis, leading to profound constipation.
- Because of the failure of the development of enteric ganglia, there is a section of the gut that is completely absent of ganglia.
38
In anal development, what is the cloaca?
- Transient common end of the digestive and urogenital systems, including the base of the allantois (urogenital sinus).
- Covered by the cloacal (proctodeal) membrane over the ectoderm depression, the proctodeum.
39
What are the cloaca split by?
- Split by the urorectal septum - gives rise to the urogenital membrane and anal membrane (that perforate at 7-8 weeks).
40
What could cause an imperforate anus?
- Persistence of the anal membrane
- Atresia of the anal canal, rectum or both
41
When does GI tract development occur?
During 3rd to 12th weeks embryonic age
42
What are the regionalised changes in the gut tube caused by?
Rotation, swelling, elongation
43
What is the purpose of gastrulation?
Generates the three primary germ layers
44
Give an overview of formation of the gut tube. PART 1
- Embryo initially a solid flat disk attached to the hemispherical yolk sac
- Part of the yolk sac cavity is enclosed within the embryo by pinching-off the yolk sac to form a yolk stalk and balloon-like yolk sac
45
Give an overview of formation of the gut tube. PART 2
- Cranial and caudal intestinal portals extend the tube towards the mouth and anus, delimited by the prochordal and cloacal plates
46
What is the primary gut tube made out of?
- Sheet of endoderm, which makes the epithelia and glands
- Surrounding mesoderm, which makes muscle and connective tissue (including mesentery)
47
What is the gut surrounded by?
Plexus of blood vessels, joining vitelline vessels to aorta.
48
What does the plexus resolve to form?
Arteries that supply the GI tract from the aorta
49
How many arterial branches lead to the thoracic oesophagus?
Where do these branches derive from?
5 branches
- derive from descending aorta
50
What does the dorsal wall of the stomach attach to and form?
Attached by dorsal mesogastrium - will form greater omentum (from greater curvature)
51
What does the ventral wall of the stomach attach to and form?
Attached by ventral mesentery, which includes the liver - will form lesser omentum (from lesser curvature)
52
Describe the mesenteric attachments from the stomach to the liver
Mesenteric arteries - arteries of the small intestine
- Connects the liver and the stomach
53
Describe the foregut development
- As the stomach rotates, the dorsal mesogastrium is drawn with it
- Mesogastrium encloses the omental bursa (lesser sac)
- Folded mesogastrium grows to form the greater omentum, the folds fusing to obliterate the bursa
54
What is peritoneum?
Serous double membrane lining the cavity of the abdomen and covering the abdominal organs.
55
What occurs as a result of pyloric stenosis?
- Pyloric channel elongation
- Length >16mm, diameter >14mm due to hypertrophy
56
What occurs in persistence of the yolk duct?
- Yolk duct attached to ileum, near ileo-cecal junction - apex of midgut loop (about 2 feet from ileocaecal junction)
57
What occurs in Meckel's diverticulum?
- May contain ectopic gastric cells: ulceration and lower GI bleeding
- Can be connected to umbilicus by ligament
- Gut rotation causes volvulus
- Can form umbilical fistula
58
What does the buccopharyngeal membrane form?
Mouth and throat
59
What is the septum tranversum?
Area where the diaphragm is
60
What does the somatic mesoderm form?
Muscles
61
What does the splanchnic mesoderm form?
Autonomic NS
62
What is the mesentery and what does it do?
- Folded membrane that anchors the GI tract to the posterior abdominal wall.
- Prevents the organs falling due to gravity.
63
How does the stomach arise?
Expansion and rotation
64
What is the stomach initially?
- concave ventral
- convex dorsal
65
What is the longest nerve in the body?
Sciatic nerve
66
What does the sciatic nerve follow?
Most of the oesophagus
67
What are the two types of peritoneum?
- Parietal and visceral peritoneum
68
What does the falciform ligament do?
Keeps the liver in place
69
What does the omentum do?
Protects the bowel from getting inflamed
70
What are retroperitoneal structures?
- oesophagus
- rectum
- kidney
71
What is the epiploic foramen?
Passage between the greater and lesser sac
72
What does the pylorus do?
Controls what can enter the duodenum
73
How are the intestines attached?
Attached throughout length by dorsal mesentery
