GI embryology 1 Flashcards
When does embryonic folding occur?
4th week
What happens during embryonic folding?
Laterally - creates ventral body wall and primitive gut tube becomes tubular.
Cradiocaudally - creates cranial and caudal pockets from yolk sac endoderm (beginning primitive gut development)
What is the gut tube?
Endoderm lined tube Runs the length of the body Blind pouches at the head and tail (no opening here) Opening at the umbilicus Splanchnic mesoderm covering
What are the embryonic divisions of the gut?
Foregut - begins as diverticula
Midgut - opening at first and continuous with yolk sac
Hindgut - begins as diverticula
What is derived from the foregut?
Oesophagus
Stomach
Pancreas, liver and gall bladder
Duodenum (proximal to entrance of bile duct)
What is derived from the midgut?
Duodenum (distal to entrance of bile duct) Jejunum Ileum Cecum Ascending colon Proximal 2/3 transverse colon
What structures have mixed blood supply?
Duodenum and Pancreas
This is because they develop close to the junction between the foregut and midgut.
What is the intraembryonic coelom?
A single cavity that is later subdivided by the future diaphragm into abdominal and thoracic cavities. This is formed as the embryo folds.
There is one membrane lining the whole cavity which eventually specialises to form pericardium and pleural membrane as the cavity specialises.
What are mesenteries?
A double layer of peritoneum suspending the gut tube from the abdominal wall.
Why do we have mesenteries?
Allows a conduit (a channel) for blood and nerve supply
Allows mobility where needed
How do mesenteries develop?
From condensation of the splanchnic mesoderm that surrounds the new gut.
Where are the mesenteries?
Dorsal - whole length of the gut tube from the dorsal body wall.
Ventral - just the foregut
Why do we have greater and lesser peritoneal sacs?
Because the dorsal and ventral mesenteries in the region of the foregut divide the cavity into left and right sacs in this region only.
What are omenta?
Specialised regions of peritoneum.
What is a peritoneal reflection?
A change in direction…
- From parietal peritoneum to mesentery
- From mesentery to visceral peritoneum
- From visceral peritoneum to…
What are retroperitoneal structures?
These are structures that were never in the peritoneal cavity and never had a mesentery
What are secondarily retroperotoneal structures?
Began in peritoneum and had a mesentery BUT with successive growth and development, the mesentery is lost through fusion at posterior abdominal wall.
What do the liver glands and billiard system develop from?
Ventral mesentary
How is the shape and position of the duodenum determined?
-Shape determined by rotation of the stomach. This pushes the duodenum to the right then against posterior abdominal wall. This makes it secondarily retroperitoneal.
What is fusion fascia?
When two layers of membrane fuse together.
What are the secondary retroperitoneal structures of the foregut?
Duodenum
Pancreas
What is derived from the hindgut?
Distal 1/3 transverse colon Descending colon Sigmoid colon Rectum Upper anal canal Internal lining of bladder and urethra
How is blood supply to each of the structures of GI system decided?
Arterial supply reflects the embryonic development. Each embryonic segment (foregut, midgut, hindgut) receives blood supply from a distinct branch of the abdominal aorta.
What forms the greater omentum?
Formed from the dorsal mesentery. It is the first structure seen when the abdominal cavity is opened anteriorly.
