GI Embryology (Trelease) Flashcards
4 portions of primitive gut
Pharyngeal gut
Foregut
Midgut
Hindgut
(all form from folding endoderm-lined yolk sac)
What layer gives rise to the epithelium of the GI tract and the parenchyma of the GI glands and organs?
Endoderm
What layer gives rise to the muscle and connective tissue of the GI tract?
Splanchnic mesoderm
Pharyngeal gut (or pharynx)
From buccopharyngeal membrane (at opening of oral cavity) to tracheobroncheal diverticulum (site of larynx and lung buds)
Foregut
Caudal to pharyngeal tube, extends to liver bud
Supplied by celiac artery
Midgut
From liver bud to end of transverse colon
Supplied by superior mesenteric artery
Hindgut
From left 1/3 of transverse colon to cloacal membrane
Supplied by inferior mesenteric artery
What layer forms the body wall?
Somatic (parietal) mesoderm
Body wall closes around gut to form mesothelium-lined abdominal cavity
What structures does the mesothelial peritoneum cover?
Inner walls of abdomen
Surfaces of gut and glands
Mesenteries of gut
What do the primitive dorsal and ventral mesentaries do?
Attach portions of gut to body wall
Dorsal mesentary
Caudal foregut
Midgut
Much of hindgut
Ventral mesentary
Derived from septum transversum (the mesodermal plate between pericardial cavity and yolk sac)
Only found on lower esophagus, stomach, upper duodenum
Divided by growing liver bud
What structures does foregut include?
Esophagus
Stomach
Duodenum
Liver
Gallbladder
Pancreas
Development of esophagus
At week 4, lung bud (respiratory diverticulum) appears attached to foregut at pharyngeal border
Tracheoesophageal septum partitions esophagus from trachea
Upper esophagus wrapped in striated muscle and lower esophagus wrapped in smooth muscle
Esophagus and lung bud developmental abnormalities
Atresia (incomplete development of esophagus)
Tracheoesophageal fistulas
Stenoses
Development of stomach
Starts in week 4 as just fusiform dilation of foregut tube standing up
Differential growth causes greater curvature dorsally
Stomach rotates 90 degrees clockwise along longitudinal axis so greater curvature on left
Then differential growth again causes clockwise rotation around anteroposterior axis
Now stomach looks as we see it–longer on bottom and slightly more mass at top left
What happens to the vagus nerve as the stomach develops?
LARP!
Left vagus comes to lie anterior to stomach (stomach snuk up behind it)
Right vagus comes to lie posterior to stomach (stomach pushes it back)
As the stomach grows, what happens to the dorsal mesogastrium?
Dorsal mesogastrium moves left, grows a TON, drapes over transverse colon and its mesocolon to become the greater omentum
Space behind stomach is created called omental bursa, or lesser peritoneal sac)
When the greater omentum is forming, what other ligaments form?
Lienorenal ligament (attaches spleen to posterior wall), which is a remnant of dorsal mesogastrium
Gastrolienal ligament is between spleen and stomach
Development of duodenum
As the stomach rotates, it causes duodenum to have a rightward C-curve around the pancreas buds
Duodenum is terminal part of foregut and first part of midgut
Duodenum and pancreas rotate around longitudinal axis, press against posterior body wall and become retroperitoneal (peritoneal layer disappears) with pancreas on the left
Note: during month 2, lumen of duodenum obliterated/filled w/proliferating cells, but then becomes recanalized (hollow) again
Development of liver and gall bladder
Liver bud begins as outgrowth of endodermal epithelium
Hepatic cells penetrate septum transversum so liver bud is growing within ventral mesogastrium
Then, connection between liver and foregut narrows to form the bile duct
Small ventral outgrowth from bile duct becomes gall bladder and cystic duct
Development of liver sinusoids and parenchyma
Epithelial liver cord cells grow between vitelline and umbilical veins to form sinusoids
Cords differentiate into parenchyma and form lining of biliary ducts
Much early function is hematopoietic, with large proliferating nests of WBCs and RBCs
Hematopoietic cells, Kupfer cells, connective tissue originate from mesoderm of septum transversum
What ends up being ventral mesentary and how?
Any mesentary that started in front of the stomach will be ventral, and both falciform ligament and lesser omentum were!
Growing liver partitions the ventral mesogastrium into the falciform ligament (liver to ventral wall) and lesser omentum (between liver and stomach/duodenum)
Liver and gall bladder developmental abnormalities
Variations in lobulation and ducts (asymptomatic)
Biliary duct atresias and hypoplasia (serious)
Development of pancreas
Pancreatic buds originate from duodenal endoderm
Dorsal pancreatic bud forms in dorsal mesentary
Ventral pancreatic bud forms close to bile duct anteriorly and moves dorsally as duodenum rotates
Ventral bud comes to lie just below dorsal bud to the left of duodenum
Ventral component form uncinate process, hooking around SMA
Distal dorsal and entire ventral ducts fuse to form main pancreatic duct
Pancreas developmental abnormalities
Annular pancreas
Accessory ectopic pancreas in foregut or cephalic midgut
What does the midgut look like at week 5?
Primary intestinal loop (simple midgut loop) is suspended from dorsal abdominal wall by single mesentery (proper)
Midgut loop communicates with yolk sac via vitelline duct (yolk stalk)
How does midgut form?
Midgut rotates counterclockwise 90 degrees around axis of SMA as it grows to 18 feet of small intestine
This rotation happens with bowel loops “herniated” in umbilical sac, but then during week 10 the bowel retracts back into abdomen for the rest of rotation to occur
270 degree counterclockwise rotation pulls large intestine around
Jejunum and ileum elongate and form coiled loops but colon lengthens without coiling
Ascending and descending colon become retroperitoneal and transverse mesocolon fuses with posterior wall of greater omentum
What happens to mesentary proper while midgut is developing?
Primary mesentary twists around superior mesenteric axis as jejunum and ileum lengthen
Midgut developmental abnormalities
If lateral plate didn’t fuse well, get anterior abdominal wall problem
Omphalocele: persistent herniation tof bowel loops through enlarged umbilical ring
Gastroschisis: herniation directly into amniotic cavity lateral to umbilicus
Abnormal rotation of midgut (partial or reversed)
Persistent vitelline duct (Meckel’s diverticulum or ileal diverticulum, vitelline cyst w/ligaments, vitelline fistula)
Duplication of intestinal loops, cysts, atresias (apple peel atresia), stenoses
What are the midgut structures?
Last 1/2 (parts 3 and 4) of duodenum
Jejunum
Ileum
Cecum
Appendix
Ascending colon
Proximal 2/3 of transverse colon
What are the hindgut structures?
Distal 1/3 of transverse colon
Descending colon
Sigmoid colon
Upper anal canal
What else does the hindgut form that we don’t normally think of?
Hindgut endoderm also forms lining of bladder and urethra
What does the posterior cloaca (primitive anorectal canal) communicate with?
Last (terminal) part of the early hindgut
What does the anterior cloaca (primitive UG sinus) communicate with?
Allantois (small remaining diverticulum or yolk sac in umbilical stalk)
What is the urorectal septum?
Partitions UG sinus from anorectal canal
What is the cloacal membrane?
Separates anorectal canal from exterior
Cloacal membrane later breaks down to form anal orifice
Hindgut developmental abnormalities
Rectoanal atresias and urorectal fistulas (partitioning of cloaca incomplete)
Congenital megacolon: aganglion megacolon (or Hirschsprung’s disease) due to lack of parasympathetic ganglia (due to failure of neural crest migration or RET gene mutation)
