Development of the GIT - Foregut Flashcards
1
Q
When is the primitive gut tube formed?
How can it be divided into 3 parts and what are they continuous with?
A
- the primitive gut tube is formed during cephalocaudal and lateral embryonic folding
- it is derived from endoderm
- it extends from the oropharyngeal membrane to the cloacal membrane and can be divided into 3 parts:
- foregut - from mouth to 1st half of duodenum
- midgut - from 2nd half of duodenum to 2/3 along transverse colon
- hindgut - distal 1/3 of transverse colon to superior 2/3 of rectum
- the foregut and hindgut form a blind-ended tube, but the midgut is continous with the yolk sac via the vitelline duct
2
Q
What are the endodermal and mesodermal derivatives of the primitive gut tube?
How is this suspended from the abdominal wall?
A
- the epithelial lining is derived from the endoderm
- the smooth muscle and connective tissue is derived from the surrounding visceral mesoderm
- the visceral and parietal mesoderm give rise to the visceral and parietal peritoneum
- the primitive gut tube is suspended from the posterior abdominal wall by the dorsal mesentery
3
Q
What are mesenteries?
What is the difference between an intraperitoneal and retroperitoneal organ?
A
- the visceral and parietal peritoneum are continuous with each other
- a mesentery is a double layer of peritoneum that encloses an organ and connects it to the body wall
- organs that are suspended by a mesentery and enclosed in peritoneum are intraperitoneal
- organs that are covered by peritoneum on their anterior surface only (not fully surrounded) are retroperioneal
4
Q
What are peritoneal ligaments?
What is the significance of these and mesenteries?
A
- peritoneal ligaments are double layers of peritoneum (mesenteries) that pass from one organ to another or from an organ to the body wall
- ligaments and mesenteries provide pathways for vessels, nerves and lymphatics to and from abdominal viscera
5
Q
Where is the dorsal mesentery located and what are its derivatives?
A
- the dorsal mesentery connects the caudal foregut, midgut and a major part of the hindgut to the posterior abdominal wall
- it extends from the lower end of the oesophagus to the cloacal region of the hindgut
- in the region of the stomach, it forms the greater omentum (dorsal mesogastrium)
- in the region of the duodenum, it forms the dorsal mesoduodenum
- in the region of the colon, it forms the dorsal mesocolon
- the dorsal mesentery of the jejunal and ileal loops forms the mesentery proper
6
Q
Where is the ventral mesentery located?
How can it be divided?
A
- the ventral mesentery runs from the lower oesophagus** to the **1st part of the duodenum
- it is derived from the septum transversum
- growth of the liver into the septum transversum divides the ventral mesentery into the lesser omentum** and the **falciform ligament
- the lesser omentum extends from the lower oesophagus, stomach and upper duodenum to the liver
- the falciform ligament extends from the liver to the ventral body wall
7
Q
Where are the arteries of the GIT derived from?
What are the 3 main arteries supplying the GIT?
A
- the vitelline arteries undergo remodelling and lose their connection to the yolk sac to supply the GIT
- there are 3 arteries that supply the GIT
- the foregut is supplied by the coeliac trunk
- the midgut is supplied by the superior mesenteric artery
- the hindgut is supplied by the inferior mesenteric artery
8
Q
How is the definitive gut lumen formed?
By which week has this occurred?
A
- in week 6, there is proliferation of the endoderm-derived epithelial lining to occlude the gut tube
- apoptosis of the epithelium occurs over the next 2 weeks
- this process of recanalisation produces vacuoles
- the vacuoles coalesce to fully recanalise the gut tube by week 9
- during this process, the epithelial lining undergoes further differentiation
9
Q
What can happen as a result of abnormal recanalisation?
Which part of the GIT is most commonly affected?
A
- abnormal recanalisation can produce duplications of the GIT
- incomplete recanalisation can cause stenosis (narrowing) or atresia (blockage) of the gut tube
- the ileum is most commonly affected, followed by the duodenum
10
Q
What are the possible consequences of duplication of the gut tube?
A
- duplication of the gut tube produces “duplication cysts”
- these are most commonly present in the small intestine (particularly ileum) but can occur anywhere
- symptoms depend on the location:
- duplications higher up can cause difficulty breathing due to airway compression
- lower duplications can cause abdominal pain, GI bleeding, vomiting, bowel obstruction and palpable mass
- whilst duplication cysts are rare, they have a high incidence of complications including bowel obstruction and intususception
11
Q
Where does the foregut extend to and from?
What appears during week 4 of development?
A
- it extends from the oropharyngeal membrane to the first part of the duodenum
- in week 4, the respiratory diverticulum (lung bud) appears from the ventral wall of the foregut
- the tracheooesophageal septum gradually partitions the diverticulum from the dorsal part of the foregut
- this divides the foregut into a ventral portion (respiratory primordium) and a dorsal portion (oesophagus)
12
Q
What happens in oesophageal atresia and why does this occur?
A
- displacement of the tracheoesophageal septum leads to separation of the proximal and distal ends of the oesophagus
- this prevents the foetus from swallowing amniotic fluid and returning it to the mother through the placental circulation
- polyhydramnios results from accumulation of excess fluid in the amniotic sac
- surgical repair results in 85% survival rate
13
Q
In which week does the oesophagus form and how does it develop following this?
What layers is it composed of?
A
- the oesophagus forms in week 4 caudal to the lung bud
- it is initially very short, but rapidly lengthens with descent of the heart and lungs
- like the rest of the gut tube, the oesophagus has an endodermal epithelial lining and smooth muscle layer derived from visceral mesoderm
- it also has some skeletal muscle derived from paraxial mesoderm
14
Q
How can a congenital hiatal hernia result from problems with development of the oesophagus?
A
- the oesophagus rapidly lengthens in weeks 4-7 as the stomach descends to the abdomen
- if the oesophagus fails to lengthen sufficiently, the stomach is pulled up into the oesophageal hiatus through the diaphragm
- this results in a congenital hiatal hernia as part of the stomach is positioned supradiaphragmatically
- this differs from an acquired hiatal hernia as it is irreducible
15
Q
When does the stomach first appear?
What drives its positional changes?
A
- it appears in week 4 as a dilation of the foregut
- its appearance and position change greatly as a result of changes in position of surrounding organs and different rates of growth in various regions of its wall
- it is suspended in the abdomen by the dorsal and ventral mesenteries
16
Q
What is the initial rotation performed by the stomach?
What 2 prominent features are formed during this rotation?
A
- the stomach rotates 90 degrees around its longitudinal axis
- this causes the left side (innervated by left X) to face anteriorly and the right side (innervated by right X) to face posteriorly
- during this rotation, the original posterior wall grows faster than the anterior portion, forming the greater and lesser curvatures
- this differential growth happens in week 5
17
Q
How does the stomach first rotate during weeks 7-8?
A
- the stomach undergoes rotation around 2 axes
- 90o clockwise rotation around the craniocaudal axis causes the lesser curvature to move from the ventral position to the right
- the greater curvature moves from the dorsal position to the left
- the vagus nerves are initially located on the left and right sides of the gut tube, but are rotated so the left vagus trunk becomes anterior and right becomes posterior
18
Q
What is the second rotation performed by the stomach in weeks 7-8?
A
- there is some rotation around the ventrodorsal axis
- the greater curvature faces slightly caudally
- the lesser curvature faces slightly cranially
19
Q
What is the lesser peritoneal sac (omental bursa) and how is it formed?
A
- the stomach is attached to the dorsal body wall by the dorsal mesogastrium and to the ventral body wall by the ventral mesogastrium
- its rotation and disproportionate growth alter the position of these mesenteries
- rotation around the craniocaudal axis pulls the dorsal mesogastrium to the left and creates a space behind the stomach - the omental bursa
20
Q
What is the greater peritoneal sac?
How is it connected to the lesser peritoneal sac (omental bursa)?
A
- the remaining peritoneal cavity, excluding the omental bursa, is the greater sac
- the epiploic foramen (of Winslow) is the narrow opening that connects the greater and lesser sacs
21
Q
When does the spleen primordium appear and where?
How does development lead to it being connected to the stomach?
A
- spleen primordium appears as a mesodermal proliferation between the 2 leaves of dorsal mesogastrium in week 5
- the dorsal mesogastrium continues to lengthen and the portion between the spleen and dorsal midline swings to the left and fuses with peritoneum of the posterior abdominal wall
- the posterior leaf of dorsal mesogastrium and the peritoneum along the line of fusion degenerate
- the spleen (intraperitoneal) is connected to the body wall (in the region of the left kidney) by the lienorenal ligament
- it is connected to the stomach by the gastrolienal ligament
22
Q
Why is the pancreas described as being “secondarily retroperitoneal”?
How does it develop as the stomach rotates?
A
- lengthening and fusion of the dorsal mesogastrium to the posterior body wall determine the final position of the pancreas
- initially, it grows into the dorsal mesoduodenum
- eventually, its tail extends into the dorsal mesogastrium
- this portion of dorsal mesogastrium fuses with the posterior body wall, meaning the tail of the pancreas lies against this region
- after degeneration of the posterior leaf of dorsal mesogastrium and peritoneum along the line of fusion, the tail of the pancreas is covered by peritoneum on its anterior surface only
- it is secondarily retroperitoneal as it was originally covered by peritoneum, but it fused with the posterior body wall later to become retroperitoneal
23
Q
What is the greater omentum and how is it formed?
A
- the dorsal mesogastrium bulges down as a result of rotation of the stomach about its anteroposterior axis
- the dorsal mesentery attached to the greater curvature and posterior abdominal wall continues to grow down
- it reflects back on itself to form an extension of the lesser sac (omental bursa)
- it forms a double-layered sac extending over the transverse colon and small intestinal loops like an apron
- this apron is the greater omentum
24
Q
What happens to the 2 layers that make up the greater omentum?
A
- the ventral and dorsal folds fuse before birth to form a single sheet hanging from the greater curvature of the stomach
- the posterior layer of the greater omentum also fuses with the mesentery of the transverse colon
25
Q
How can the ventral mesogastrium be divided and why does this division occur?
A
it can be divided into the lesser omentum** and the **falciform ligament
- the ventral mesogastrium is derived from the septum transversum
- when the liver grows into the septum transversum, it thins to form:
- peritoneum of the liver
- falciform ligament - from the liver to the ventral body wall
- lesser omentum - from the stomach / upper duodenum to the liver
27
Q
What happens in congenital pyloric stenosis?
What signs are present and who is more likely to be affected?
A
- occurs when there is narrowing of the pyloric sphincter due to hypertrophy of smooth muscle in this region
- it is more common in males with a ratio of 5:1
- extreme narrowing of the pyloric lumen obstructs the passage of food and restricts gastric emptying
- this results in dilation of the stomach and severe projectile vomiting
- other signs include visible peristalsis and a palpable pyloric mass
28
Q
What is meant by heterotopic gastric tissue?
What are the risks associated with this?
A
- inappropriate epithelial differentiation of the gut tube can result in ectopic gastric tissue
- due to acid production, this can cause inflammation and ulceration in the surrounding area
- damage can result in strictures due to scarring or rupture of the gut wall
29
Q
What are the 2 origins of the duodenum and where does the boundary between them lie?
A
- the proximal half is formed by the terminal part of the foregut
- the distal half is formed by the cephalic part of the midgut
- the junction between the 2 parts is distal to the entrance of the common bile duct
30
Q
How does the duodenum assume its position during development?
A
- the duodenum elongates in week 4 as the stomach rotates, leading to formation of a C-shaped loop
- rotation of the stomach and rapid growth of the head of the pancreas cause the duodenum to swing from its initial midline position to the right
- the duodenum and head of the pancreas lie against the posterior abdomminal wall
- the dorsal mesoduodenum degenerates, causing the duodenum and head of the pancreas to become fixed in a (secondarily) retroperitoneal position
- the dorsal mesoduodenum disappears entirely except for in the region of the pylorus of the stomach (duodenal cap)
- the duodenal cap retains its mesentery and is intraperitoneal
