GI Embryology (Trelease)

Question 1

4 portions of primitive gut

Answer 1

Pharyngeal gut

Foregut

Midgut

Hindgut

(all form from folding endoderm-lined yolk sac)

Question 2

What layer gives rise to the epithelium of the GI tract and the parenchyma of the GI glands and organs?

Answer 2

Endoderm

Question 3

What layer gives rise to the muscle and connective tissue of the GI tract?

Answer 3

Splanchnic mesoderm

Question 4

Pharyngeal gut (or pharynx)

Answer 4

From buccopharyngeal membrane (at opening of oral cavity) to tracheobroncheal diverticulum (site of larynx and lung buds)

Question 5

Foregut

Answer 5

Caudal to pharyngeal tube, extends to liver bud

Supplied by celiac artery

Question 6

Midgut

Answer 6

From liver bud to end of transverse colon

Supplied by superior mesenteric artery

Question 7

Hindgut

Answer 7

From left 1/3 of transverse colon to cloacal membrane

Supplied by inferior mesenteric artery

Question 8

What layer forms the body wall?

Answer 8

Somatic (parietal) mesoderm

Body wall closes around gut to form mesothelium-lined abdominal cavity

Question 9

What structures does the mesothelial peritoneum cover?

Answer 9

Inner walls of abdomen

Surfaces of gut and glands

Mesenteries of gut

Question 10

What do the primitive dorsal and ventral mesentaries do?

Answer 10

Attach portions of gut to body wall

Question 11

Dorsal mesentary

Answer 11

Caudal foregut

Midgut

Much of hindgut

Question 12

Ventral mesentary

Answer 12

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

Question 13

What structures does foregut include?

Answer 13

Esophagus

Stomach

Duodenum

Liver

Gallbladder

Pancreas

Question 14

Development of esophagus

Answer 14

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

Question 15

Esophagus and lung bud developmental abnormalities

Answer 15

Atresia (incomplete development of esophagus)

Tracheoesophageal fistulas

Stenoses

Question 16

Development of stomach

Answer 16

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

Question 17

What happens to the vagus nerve as the stomach develops?

Answer 17

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)

Question 18

As the stomach grows, what happens to the dorsal mesogastrium?

Answer 18

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)

Question 19

When the greater omentum is forming, what other ligaments form?

Answer 19

Lienorenal ligament (attaches spleen to posterior wall), which is a remnant of dorsal mesogastrium

Gastrolienal ligament is between spleen and stomach

Question 20

Development of duodenum

Answer 20

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

Question 21

Development of liver and gall bladder

Answer 21

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

Question 22

Development of liver sinusoids and parenchyma

Answer 22

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

Question 23

What ends up being ventral mesentary and how?

Answer 23

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)

Question 24

Liver and gall bladder developmental abnormalities

Answer 24

Variations in lobulation and ducts (asymptomatic)

Biliary duct atresias and hypoplasia (serious)

Question 25

Development of pancreas

Answer 25

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

Question 26

Pancreas developmental abnormalities

Answer 26

Annular pancreas

Accessory ectopic pancreas in foregut or cephalic midgut

Question 27

What does the midgut look like at week 5?

Answer 27

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)

Question 28

How does midgut form?

Answer 28

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

Question 29

What happens to mesentary proper while midgut is developing?

Answer 29

Primary mesentary twists around superior mesenteric axis as jejunum and ileum lengthen

Question 30

Midgut developmental abnormalities

Answer 30

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

Question 31

What are the midgut structures?

Answer 31

Last 1/2 (parts 3 and 4) of duodenum

Jejunum

Ileum

Cecum

Appendix

Ascending colon

Proximal 2/3 of transverse colon

Question 32

What are the hindgut structures?

Answer 32

Distal 1/3 of transverse colon

Descending colon

Sigmoid colon

Upper anal canal

Question 33

What else does the hindgut form that we don’t normally think of?

Answer 33

Hindgut endoderm also forms lining of bladder and urethra

Question 34

What does the posterior cloaca (primitive anorectal canal) communicate with?

Answer 34

Last (terminal) part of the early hindgut

Question 35

What does the anterior cloaca (primitive UG sinus) communicate with?

Answer 35

Allantois (small remaining diverticulum or yolk sac in umbilical stalk)

Question 36

What is the urorectal septum?

Answer 36

Partitions UG sinus from anorectal canal

Question 37

What is the cloacal membrane?

Answer 37

Separates anorectal canal from exterior

Cloacal membrane later breaks down to form anal orifice

Question 38

Hindgut developmental abnormalities

Answer 38

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)