3. Development of the GI Part 1 Flashcards
when does the embryo fold
4th week
What is the gut tube?
- Endoderm lined tube
- Runs the length of the body
- Blind pouches at the head and tail ends
- Opening at the umbilicus
- Splanchnic mesoderm covering
Cranio-caudal and lateral folding occur simultaneously. Briefly compare what each of these processes achieve.
Laterally
• Creates ventral body wall
• Primitive gut becomes tubular
Craniocaudally
• Creates cranial & caudal pockets from yolk sac endoderm (beginning primitive gut development)
What germ layer is the gut tube lined with?
Endoderm
compare the foregut, midgut and hindgut at the beginning
- Foregut and hindgut begin as blind diverticula
* Midgut has an opening at first and is continuous with the yolk sac
What germ layers surrounds the gut tube?
Splanchnic mesoderm
What are the 3 divisions of the mesoderm?
1) Intermediate
2) Paraxial
3) Lateral plates - splanchnic + somatic
What is the difference between somatic and splanchnic
mesoderm, in terms of what structures they are associated with?
Somatic - forms dermis + body wall
Splanchnic - forms viscera
What structure is the developing midgut initially connected too?
What structure connects these together?
Yolk sac. VITELLIAN DUCT
What are the derivatives of the foregut?
Oesophagus, stomach, pancreas, liver, gall bladder and duodenum (proximal to entrance of bile duct)
What are the derivatives of the midgut?
Duodenum, jejunum, ileum, caecum, appendix, ascending colon, proximal 2/3 of transverse colon
What are the derivatives of the hindgut?
The distal 1/3 of the transverse colon, descending colon, sigmoid colon, rectum and upper anal canal. And the internal lining of the bladder and urethra
What is the arterial supply to structures of the foregut, midgut and hindgut?
From coeliac trunk, SMA and IMA respectively
What combination of embryonic layers does the Splanchnopleuric mesoderm consist of?
splanchnic mesoderm
visceral peritoneum
gut tube (endoderm)
What combination of embryonic layers does the Somaticpleuric mesoderm consist of?
somatic mesoderm
parietal peritoneum
ectoderm
What term is given to the space that will eventually form the thoracic and abdominal cavity?
Intraembryonic coelom
How is the intraembryonic coelom formed?
Formed as the embryo folds and begins as one large cavity
What divides the intraembryonic caelom?
Divided by the diaphragm into thoracic and abdominal cavities
Rupture Of which membrane creates a mouth opening in the developing embryo? At what point in development does this occur?
Buccophargyneal membrane. 4th week
Rupture Of which membrane creates an anus opening in the developing embryo? At what point in development does this occur?
Cloacal membrane. 7th week
What does the membrane lining the intraembryonic coelom become?
• One membrane lining the whole intraembryonic cavity • Specialises as the cavities specialise o pericardium o pleural membrane o Peritoneum & peritoneal cavity
what is the peritoneal membrane and cavity?
• the peritoneal membrane lines the abdominal cavity a invests the viscera
o during development it grows, changes shape & specialises
o the peritoneal “cavity” is a potential space only
o under normal conditions it should contain nothing
What are mesenteries?
Double layer of peritoneum suspending the gut tube from the abdominal wall
What is the function of mesenteries?
o allow a conduit for blood and nerve supply
o allow mobility where needed
How do mesenteries form?
Primitive gut tube suspended within the intraembryonic coelum by splanchnic mesoderm, mesentery formed by condensation of this mesoderm
What are the 2 mesenteries present in the embryo and where they found?
- Dorsal mesentery suspends the entire gut tube from the dorsal body wall
- Ventral mesentery ONLY in the region of the foregut
How does the dorsal and ventral mesentery divide the cavity in the foregut region and what do these then contribute to?
• Dorsal and ventral mesenteries in the region of the foregut divide the cavity into left and right sacs in this region only
o the left sac contributes to the greater sac
o the right sac becomes the lesser sac
Where is the lesser sac located?
posterior to the stomach
Describe how the liver, gallbladder and spleen form
Cranial portion of an anterior bud off the ventral mesentery becomes the liver
The caudal portion of it becomes the gallbladder
The spleen develops in the dorsal mesentery
Foregut rotates 90 degrees clockwise, such that the liver ends up on the right and spleen ends up on the left
What is the greater omentum formed from and what does it attach to?
Formed from the dorsal mesentery and attaches to greater curve of the stomach - folds back and attaches to transverse colon
What is the lesser omentum formed from and what does it attach to?
Formed from ventral mesentery and is attached to lesser curve of the stomach - connecting it to liver
what is the association between parietal and visceral peritoneum and pain?
parietal = somatic mesoderm - can localise pain visceral = splanchnic mesoderm - vague pain
What does the free edge of the lesser omentum conduct?
Portal triad
What connects the anterior abdominal wall to the liver?
Falciform ligament
What connects the stomach to the spleen?
gastrosplenic ligament
What connects the posterior abdominal wall to the spleen?
Spleno-renal/lienorenal ligament
What connects the greater sac to the lesser sac (of the peritoneal cavity)?
foramen of winslow/ Epiploic foramen
What connects the lesser curve of stomach to the liver?
lesser omentum
What influences position of greater/lesser sacs and how are the omenta formed?
Due to rotation of stomach during development
What is the effect of rotation of the stomach on the vagus nerves?
Left vagus comes to lie anterior to stomach, right vagus comes to lie posterior to stomach
What other consequences of rotation of stomach?
- Shifts cardia and pylorus from the midline
- Stomach comes to lie obliquely
- Contributes to moving the lesser sac behind the stomach
- Creates the greater omentum
What is the first structure seen when the abdominal wall is opened up anteriorly?
Greater omentum
What is a peritoneal reflection?
A change in direction (i.e. change from parietal to mesentery or mesentery to visceral)
What are retroperitoneal structures (with reference to mesentery)?
Were never in the peritoneal cavity and never had a mesentery
What are secondary retroperitoneal structures?
Began development invested by peritoneum and had a mesentery but with growth and development the mesentery is lost through fusion at the posterior abdominal wall. They can be mobilised
Which part of the primitive gut tube forms the foregut?
From the lung bud to the liver bud
What is the lung bud?
In 4th week, a respiratory diverticulum forms from the ventral wall of the foregut at the junction with the pharyngeal gut
What separates the foregut from the developing respiratory tract(lung bud)?
Tracheoesophageal septum
what is formed when the Tracheoesophageal septum seperates the lung bud?
trachea and oesophagus
• Respiratory primordium (ventrally)
• Oesophagus (dorsally)
o Separation of the developing GI and respiratory tracts
Why might abnormalities occur during development of the respiratory tract?
Abnormal positioning of the tracheoesophageal septum (e.g. blind ended oesophagus, with tracheoesophageal fistula)
What happens to the gut tube in week 5? When does this resolve?
Endoderm rapidly proliferates, which causes the gut tube to temporarily occlude
It recanalises in week 9
What does the liver and biliary system develop within?
Ventral mesentery
What does the pancreas develop within?
Unicate process and inferior head within the ventral mesentery. Superior head, neck and body within the dorsal mesentery
What are 2 major peritoneal reflections of the liver and what do they form from?
Lesser omentum on posterior surface and falciform ligament on anterior surface (both part of ventral mesentery?)
Describe the development of the duodenum
• Develops from caudal foregut and cranial midgut
What determines the shape of the duodenum?
Rotation of the stomach, which pushes it to the right and then against the posteior abdominal wall (becomes secondary retroperitoneal)
What are secondary retroperitoneal structures of the foregut?
Dudenum (except the duodenal cap) and pancreas
What is fusion fascia?
Fusion of the mesentery with parietal peritoneum (on posterior abdominal wall)
What creates the curves of the stomach?
Stomach enlarges greater on the posterior/dorsal aspect
decribe rotation of stomach
stomach and foregut rotates 90 degrees clockwise
