Embryology of the GI Flashcards
What gene is a transcription factor for establishing left sidedness?
PTX2
What does the lateral plate mesoderm give rise to?
Parietal and visceral mesoderm
How does the gut tube form?
From the endoderm lining the yolk sac in the 3rd and 4th week
What membrane closes the gut tube at the cranial end?
Oropharyngeal membrane
What membrane closes the gut tube at the caudal end?
Cloacal membrane
What does the allantois form?
Urethra and bladder
What does the oesophagus form from?
Cranial part of the primitive gut tube
Where does the laryngo-tracheal diverticulum (lung bud) form from?
Ventral wall of the foregut
What divides the foregut tube into trachea and oesophagus?
Tracheo-oesophageal septum
When does the foregut dilate to form the stomach?
4th week
What does differential growth of the stomach create?
Greater and lesser curvatures
Why does the stomach rotate?
To fit in with other developing organs
Describe the rotation of the stomach
90 degrees around its own longitudinal axis
ventral border now LC
dorsal border now GC
How does the stomach reach its final adult position?
Anterio-posterior rotation
How is the stomach attached to the dorsal and ventral walls
Mesentery called mesogastrium
Which mesentery of the stomach creates the omental bursa (lesser sac)?
Dorsal mesentery
What is the duodenum formed from?
Caudal part of foregut and cranial part of midgut
What is the blood supply of the duodenum?
Coeliac trunk and superior mesenteric artery
Is the duodenum retroperitoneal or intraperitoneal?
Duodenal cap - dorsal mesentery = intraperitoneal
Rest of duodenum = retroperitoneal after fusion of dorsal mesentery with the peritoneum of the posterior abdominal wall
When does the liver bud appear?
Middle of the 3rd week
How is the liver bud formed?
Outgrowth of endoderm in the distal end of the foregut
What does the liver bud grow into?
Septum Transversum
What is the septum transversum?
Mesodermal plate between pericardial cavity and yolk stalk
How is the bile duct formed?
Narrowing of the connection between the liver bud and foregut (duodenum)
How is the gallbladder and cystic duct formed?
Ventral outgrowth from the bile duct
How is the spleen formed?
Mesenchymal condensation forms (end of 4th week) in dorsal mesogastrium near wall
Mesenchyme differentiates forming spleen (5th week)
How long does the spleen act as a haematopoietic organ?
Up to week 14
When is the spleen colonised by T-lymphocytes?
Weeks 15-17
When do the B-cell precursors arrive in the spleen and what happens after?
Week 23
Spleen starts its lymphoid function
How does the pancreas form?
2 buds grow from endodermal lining of the duodenum (week 5)
Duodenum rotates and ventral bud moves close to dorsal bud (week 6)
2 buds fuse = pancreas
What makes up the main pancreatic duct?
Ventral duct + distal part of dorsal duct
Proximal part of dorsal duct may form an accessory duct
How does oesophageal atresia and tracheo-oesophageal fistula form?
If the tracheo-oesophageal septum deviates incorrectly, then there is an incomplete separation of laryngo-tracheal tube (lung bud)
What is an annular pancreas?
If the ventral bud fails to migrate around the duodenum correct
May cause duodenal stenosis
What makes up the midgut?
Distal part of duodenum Jejunum Ileum Caecum Appendix Ascending colon Proximal 2/3rds of transverse colon
How is the midgut formed?
Rapid elongation of the gut tube + associated mesentery
‘Primary intestinal loop’
What happens to the cephalic part of the primary intestinal loop?
Becomes:
Distal duodenum
Jejunum
Proximal ileum
What happens to the caudal part of the primary intestinal loop?
Becomes: Distal ileum Caecum Appendix Ascending colon Proximal 2/3rds of the transverse colon
What must happen for the midgut to reach adult pattern?
BOTH rotation and physiological herniation
When does rotation of the primary intestinal loop occur?
6th week
Where does initial rotation of the primary intestinal loop occur?
Around the axis of the superior mesenteric artery
Describe rotation of the primary intestinal loop?
90 degrees anti-clockwise
cranial part of midgut carried to the right
What happens as rotation of the primary intestinal loop occurs?
Gut tube herniates into the extraembryonic cavity in the umbilical cord
Why does physiological herniation occur?
Allows growth of gut tube due to lack of room
Massive liver and kidneys
When does the abdominal cavity become more spacious and why?
10th week
Due to growth, regression of kidney and slower growth of liver)
What is omphalocele?
Herniation of abdominal viscera through an enlarged umbilical ring from failure of retraction of umbilical ring
Viscera covered by a layer of amnion
What is gastrochisis?
Herniation of abdominal contents directly through the body wall into the amniotic cavity through a weak area right of umbilicus
Not covered by peritoneum or amnion
Not associated with chromosomal abnormalities or other defects
What happens when remnants of vitelline duct persists?
Forms Meckel’s diverticulum
May form fistula or vitelline cyst/ligament
What makes up the hindgut?
Distal 1/3rd transverse colon Descending colon Sigmoid colon Rectum Upper part of anal canal
What does the endoderm of hindgut form?
Lining of the bladder and urethra
What part of the cloaca joins with the terminal portion of the hindgut?
Posterior
Where does the allantois enter into?
Anterior part of cloaca
What is the cloaca?
Endoderm lined cavity with surface ectoderm at its ventral boundary
Endoderm/ectoderm boundary is the cloacal membrane
What artery supplies the foregut?
Coeliac artery
What artery supplies the midgut?
Superior mesenteric artery
What artery supplies the hindgut?
Inferior mesenteric artery
What can 4 anomalies can occur when there is incomplete separation of the hindgut from the urogenital sinus by the urorectal septum
Urorectal fistula
Rectovaginal fistula
Rectoanal atresia
Imperforate anus
