7. Gastrointestinal - Development of Peritoneal cavity and Foregut Flashcards
When does the formation of the primitive gut tube start?
Week 3, folding starts in week 4
How is the primitive gut tube formed?
- Folding of the embryo creates a primitive gut tube
- It is lined with endoderm
- It is divisible into three regions: foregut, midgut, hindgut
- All parts of the adult GI tract arise from one of these regions
Note: the images are in both sagittal and transverse planes so you can see the difference
What happens when the embryo is folded 1. laterally and 2. craniocaudally?
- creates ventral body wall (anterior body wall). primitive gut becomes tubular
- creates cranial and caudal pockets from yolk sac endoderm
What is the gut tube?
- Endoderm lined tube
- Runs the length of the body (See sagittal plane)
- There are blind pouches at the head and tail ends (See sagittal plane) and an opening at the umbilicus (that’s the yolk sac protruding out)
- External lining of the tube = splanchnic mesoderm (red part covering the gut tube)
Label the foregut, mid gut and hindgut on a sagittal section of the embryo
- Midgut has an opening at first and is continuous with the yolk sac
- Note: these embryonic divisions have implications for blood supply and lymphatic drainage in the adult
Each embryonic segment receives blood supply from a distinct branch of the abdominal aorta. What are they?
Note: those structures that develop close to the junction between foregut and midgut will have a mixed blood supply e.g.
- duodenum: prox to bile duct = gastroduodenal artery and superior pancreaticoduodenal artery (CT). distal = inferior pancreaticoduodenal artery (SMA)
- pancreas: head = superior pancreaticoduodenal artery (CT) AND inferior pancreaticoduodenal artery (SMA)
What parts of the GI system are derived from the: Foregut, Midgut and Hindgut?
Explain what the mesoderm splits into after folding and the relevance of this
- The mesoderm splits into two layers: somatic and splanchnic
- Somatic layer develops into the abdominal wall
- Splanchnic layer develops into the smooth muscles of the gut wall
What is the coelomic cavity, how is it formed and what is it a primordium of?
- The coelomic cavity is formed by the space inbetween the splanchnic and somatic mesoderm
- Later divided by the future diaphragm into abdominal and thoracic cavities
- Cavity is the forerunner of the pleural cavity and peritoneal cavity
How is the peritoneal cavity developed?
- The peritoneal membrane lines the abdominal cavity and invests the viscera (during development it grows, changes shape and specialises)
- The peritoneal cavity is a potential space only: under normal conditions it should contain nothing
What are mesenteries? What is their functional relevance?
- Double layer of peritoneum suspending the gut tube from the abdominal wall
- Allow a conduit for blood and nerve supply, allow mobiltiy where needed
How is the mesentery formed?
- The developing gut is attached to the roof of the abdominal cavity by a fold of mesoderm known as the dorsal mesentery
- Additionally the foregut (ONLY) is attached to the floor by a ventral mesentery
- The doral and ventral mesenteries become the various peritoneal folds and reflections that suspend the gut and give passage to vessels and nerves in the adult
What are the omenta?
- Specialised region of the peritoneum
Where are the greater and lesser omentums derived from?
- The greater omentum: dorsal mesentery, first structure seen when the abdominal cavity is opened anteriorly
- The lesser omentum: ventral mesentery: free edge conducts the portal triad
How are the greater and lesser peritoneal sacs formed and what is their functional relevance?
- Dorsal and ventral mesenteries divide the cavity into left and right sacs
- Left sac contributes to the greater sac
- Right sac becomes the lesser sac (comes to lie behind the stomach)
What influences the position of the greater and lesser sacs, and how are the omenta formed?
- Rotation of the stomach during development
- The stomach is fixed in place (by dorsal and ventral mesenteries as well as prox and distal GI system), as it rotates, everything around it must move too
Where does the stomach evelop its characteristic shape?
- dorsal border develops faster giving it its characteristic shape (greater curve)
What are peritoneal reflections?
- A change in direction (from parietal peritoneum to mesentery, from mesentery to visceral peritoneum, from visceral peritneum etc…)
What are structures that are not suspended in the abdominal cavity called?
- Retroperitoneal
- i.e. were never in the peritoneal cavity and never had a mesentery
- OR secondaryily retropertioneal
- which means they began developement invested by peritoneum, had a mesentery but with growth and development, mesentery is lost through fusion with posterior abdominal wall
Looking at the development of the foregut, what two structures are formed from it, and how?
- Lung and bronchial tree as well as the oesophagus
- 4th week - respiratory diverticulum forms in the ventral wall of foregut, oesophagus dorsally
- Tracheoesophageal septum develops separating the respiratory diverticulum (future trachea) and oesophagus
What are the consequences of an abnormal positioning of the tracheoesophageal septum?
- oesophageal abnormalities
- 1/3000 live birth incidence
- inc. tracheoesophageal fistulae, proximal blind end oesophagus
What foregut derived glands are formed in the ventral and dorsal mesenteries respectively?
- Liver
- Biliary system
- Part of pancreas (uncinate process and inferior head)
- Pancreas (superior head, neck, body, tail)
How does the liver form?
- Earliest GI associated gland
- Develops from the hepatic bud within the ventral mesentery
What are the liver’s peritoneal reflections?
- Lesser omentum: connects liver to stomach
- Falciform ligament: connects liver to anterior abdominal wall
- Bare area: no visceral pertioneum here due to liver and diaphragm growing and developing close together
Explain the development of the duodenum
- Develops from both the foregut and midgut
- Duodenum grows rapidly and as the stomach rotates, the duodenum takes a C shape, pancreas sits inside this.
- As it grows, it thickens a lot, 5th-6th week, lumen is obliterated
- end of embryonic period (8th week), recanalisation must occur to restore lumen
- We get fusion fascia as mesentery fuses with the posterior abdo wall and duodenum becomes secondarily retropertoneal
What are two secondarily retroperitoneal structures of the foregut?
- Duodenum (duodenal cap)
- Pancreas
