Development of the Foregut Flashcards
During which process is the primitive gut tube formed?
Where does it extend from?
It is formed during embryonic folding
It extends from the oropharyngeal membrane to the cloacal membrane
During which weeks does cranio-caudual folding occur and what is the result is this?
weeks 3-4
it pushes the head and tail end in closer proximity due to the increased amount of amniotic fluid surrounding the embryo
What are the 3 parts of the primitive gut tube?
- foregut
- midgut
- hindgut
From which regions does the foregut run?
The foregut runs from the mouth to the 1st part of the duodenum
From which regions does the midgut run?
The midgut runs from the 2nd half of the duodenum to 2/3 the way along the transverse colon
From which regions does the hindgut run?
The hindgut runs from the distal 1/3 of the transverse colon to the superior 2/3 of the rectum
What is significant about the 3 regions of the primitive gut tube?
Each region is supplied by a different artery and a different branch of the autonomic nervous system
What is the midgut continuous with?
The yolk sac at the vitelline duct
What happens during lateral body wall folding?
The lateral edges come towards the midline and fuse
They fuse everywhere except the level of the midgut
What does the surface ectoderm go on to form?
The epidermis of the skin
What does the endoderm go on to form?
The epithelial lining of the primitive gut tube
What is the smooth muscle and connective tissue in the primitive gut tube derived from?
the surrounding visceral mesoderm which covers the gut tube
What do the visceral and parietal mesoderm give rise to?
the visceral and parietal peritoneum
How is the primitive gut tube suspended from the posterior abdominal wall?
Through a double layer of peritoneum
This is the dorsal mesentery
What is a mesentery?
A double fold of peritoneum that encloses an organ and connects to the body wall
What is an intraperitoneal organ?
An organ that is invested within the peritoneum
What is a retroperitoneal organ?
An organ that is not surrounded by the peritoneum
Where does the dorsal mesentery run to and from?
It runs from the lower oesophagus to the cloaca
What is the role of the dorsal mesentery?
It attaches the primitive gut tube to the posterior abdominal wall along its entire length
Where does the ventral mesentery run to and from?
It runs from the lower oesophagus to the first part of the duodenum
What is the role of the ventral mesentery?
It attaches the foregut to the anterior abdominal wall
What is the difference between the dorsal and ventral mesenteries?
The ventral mesentery is only present at the level of the foregut
What mesentery does the liver develop within?
The ventral mesentery
What are the 2 parts of the ventral mesentery?
How is it split?
- lesser omentum
- falciform ligament
It is split by the developing liver
Where does the lesser omentum run to and from?
It is the region between the stomach and the liver
Where does the falciform ligament run to and from?
it is the region between the liver and the anterior wall
What is the role of the mesenteries?
They act as suspension for the gut tube
They allow the passage of blood vessels, lymphatics and nerves to and from the organs
What is the name of the plexus of blood vessels in the developing embryo?
vitelline arterial plexus
What happens when the vitelline arteries undergo remodelling?
They form the vessels that will supply the gut tube
They lose their connection to the yolk sac to supply the GI tract
What are the branches of arteries that supply the GI tract?
- coeliac trunk supplies the foregut
- superior mesenteric artery supplies the midgut
- inferior mesenteric artery supplies the hindgut
What happens in week 6 of development?
Rapid proliferation of the endoderm derived epithelial lining occludes the gut tube
What happens during recanalisation?
During week 7 and 8, apoptosis of the epithelium occurs to create vacuoles in the occluded gut tube
What happens in week 9 of development?
The vacuoles coalesce to fully recanalise the gut tube
This forms the definitive gut tube lumen
Why is the process of recanalisation significant?
When the endoderm proliferates, the cells differentiate and become specialised
Apoptosis removes the cells which are not specialised
What can abnormal recanalisation of the gut tube cause?
Duplications of the gastrointestinal tract
Stenosis or atresia of the gut tube
What is the difference between stenosis and atresia of the gut tube?
How are they caused?
They are caused by incomplete recanalisation
Stenosis is narrowing of the gut tube
Atresia is blockage of the gut tube
Which areas of the GI tract are most commonly affected by abnormal recanalisation?
Why does it occur?
Most commonly affects the ileum, followed by the duodenum
Problems occur when apoptosis goes slightly wrong
What is the result of duplication of the gut tube?
What are examples?
Duplication cysts which are rare but have a high incidence of complications
e.g. bowel obstruction or intussusception
When does intusussception occur?
When the ileum telescopes and folds back on itself
This leads to a narrowed opening
Where does the foregut extend from and to?
What structure does it give rise to?
It extends from the oropharyngeal membrane to the first part of the duodenum
It gives rise to the respiratory diverticulum
How does the foregut separate from the respiratory diverticulum?
It forms a tracheoesophageal septum
This separates the pharynx and the oesophagus from the trachea
How does oesophageal atresia arise?
The tracheoesophageal septum may be displaced
This separates the proximal and distal ends of the oesophagus
The proximal part of the oesophagus ends in a blind-ended sac
Why does the foetus ingest and swallow amniotic fluid during development?
It helps the kidneys start to function as they filter the fluid
It allows the respiratory muscles to be built up
How does oesophageal atresia affect the ability of the foetus to ingest amniotic fluid?
It prevents the foetus from swallowing the amniotic fluid and returning it to the mother through placental circulation
What condition does oesophageal atresia in the foetus lead to and why?
Polyhydramnios
It is caused by an excess of amniotic fluid
During which week does the oesophagus form?
week 4
It forms caudal to the lung bud and begins as a short structure
What features does the oesophagus share with the rest of the gut tube?
- endodermal epithelial lining
2. smooth muscle layer derived from visceral mesoderm
From what is the skeletal muscle in the oesophagus derived from?
Paraxial mesoderm
During which weeks does rapid elongation of the oesophagus occur?
What is the purpose of this?
weeks 4 - 7
this allows the oesophagus to descend into the abdomen, where it resides
if it did not occur, the stomach would reside in the thorax
What happens in an adult hiatal hernia?
The stomach starts in the abdomen
An increase in abdominal pressure causes it to slide back into the thorax
What happens in a congenital hiatal hernia?
There is insufficient elongation of the oesophagus
This leads to a portion of the stomach residing supradiaphragmatically
How severe is a congenital hiatal hernia and how does it differ from an acquired hiatal hernia?
It has varying degrees of severity depending on how much of the stomach resides in the thorax
It differs from an acquired hiatal hernia as it is irreducible
When and what as does the stomach appear?
It appears in week 4 as a dilatation of the foregut
What is the stomach attached to?
It is attached to the anterior abdominal wall by the dorsal mesentery
It is attached to the liver by the lesser omentum (ventral mesentery)
When does differential growth of the stomach occur?
What is this and what does it form?
Week 5
The dorsal aspect of the stomach grows very quickly in relation to the ventral aspect
This forms the gearter curvature of the dorsal wall
What happens to the stomach during week 7-8?
It undergoes rotation around 2 axes
What is the first rotation of the stomach around the craniocaudal axis?
What does this cause?
90o clockwise rotation around the craniocaudal axis
This causes the lesser curvature to move from the ventral position to the right
How does the greater curvature change position during 90o clockwise rotation?
It moves from the dorsal position to the left
Where are the vagus nerves located before rotation of the stomach?
On the left and right sides of the gut tube
After 90o clockwise rotation of the stomach, where do the vagus nerves reside?
The left vagus trunk becomes anterior to the stomach
The right vagus nerve becomes dorsal to the stomach
What does rotation of the stomach around the ventrodorsal axis lead to?
The greater curvature faces slightly caudally and the lesser curvature faces slightly cranially
How are the lesser peritoneal sac and greater peritoneal sac formed?
As the stomach rotates around the craniocaudal axis, it creates a space behind it
This is the lesser peritoneal sac and the remaining peritoneal cavity is the greater peritoneal sac
How are the greater and lesser peritoneal sacs connected?
Through a narrow opening called the epiploic foramen (of Winslow)
What is an alternative name for the lesser peritoneal sac?
The omental bursa
How does the liver move as the stomach rotates?
As the stomach rotates, the liver will rotate with it
It resides in the right-hand side of the body, in the lesser peritoneal sac
How does the dorsal mesentery form an extension of the omental bursa?
The dorsal mesentery is attached to the greater curvature of the stomach and posterior abdominal wall
It continues to grow and folds back on itself
How does the greater omentum form?
The ventral and dorsal folds of the dorsal mesentery fuse to cover the viscera
The greater omentum is made from 4 layers of peritoneum
What does the posterior layer of the greater omentum fuse with?
the mesentery of the transverse colon
How does the duodenum lose its layer of peritoneum during rotation of the stomach?
The first part of the duodenum is attached to the liver by the lesser omentum
Rotation pushes it against the posterior abdominal wall
As this happens, it loses its layer of peritoneum
Which parts of the duodenum are retroperitoneal?
The first section is intraperitoneal
The other 3 parts of the duodenum are retroperitoneal
What is the role of the pyloric sphincter?
It dictates when the stomach can open and empty
What is pyloric stenosis and what does it affect?
Narrowing of the pyloric sphincter caused by hypertrophy of smooth muscle
This leads to a narrowing of the opening which affects gastric emptying
How many births are affected by congenital pyloric stenosis?
Does it affect more males or females?
it affects 1 in 500 births
it is more common in males and affects 5 times more males than females
What is the result of restricting gastric emptying in pyloric stenosis?
Dilation of the stomach
What are 3 noticeable signs of congenital pyloric stenosis?
- palpable pyloric mass
- projectile vomiting
- visible peristalsis
What is heterotopic gastric tissue and what is it a result of?
Inappropriate epithelial differentiation of the gut tube can result in ectopic gastric tissue
This is gastric tissue which is present outside of the stomach
What is the result of heterotopic gastric tissue?
Acid production can lead to inflammation and ulceration of the surrounding area
Damage can result in strictures due to scarring or rupture of the gut wall
What are the 2 origins of the duodenum?
The proximal half comes from the foregut
The distal half comes from the midgut
What is the boundary of the 2 origins of the duodenum?
the boundary is distal to the entrance of the common bile duct
What happens after secretions from the pancreas and the liver enter the duodenum?
After this point, the duodenum becomes midgut and is supplied by the superior mesenteric artery
During which week does the duodenum elongate?
What is the result of this?
It elongates in week 4, leading to a ventrally projecting C-shape
It is then dragged to the right by the rotating stomach
What processes occur prior to the duodenum becoming secondarily retroperitoneal?
The dorsal mesentery attached to the duodenum degenerates
The majority of the duodenum lies against the posterior abdominal wall
Why is the duodenum ‘secondarily retroperitoneal’?
It started with a mesentery that degenerated during development
Which blood vessels supply the duodenum?
The proximal half is supplied by the coeliac trunk
The distal half is supplied by the superior mesenteric artery
