GI Embryology (Trelease) Flashcards by Jill Klausner

4 portions of primitive gut

Pharyngeal gut

Foregut

Midgut

Hindgut

(all form from folding endoderm-lined yolk sac)

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

Endoderm

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

Splanchnic mesoderm

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Pharyngeal gut (or pharynx)

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

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Foregut

Caudal to pharyngeal tube, extends to liver bud

Supplied by celiac artery

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Midgut

From liver bud to end of transverse colon

Supplied by superior mesenteric artery

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Hindgut

From left 1/3 of transverse colon to cloacal membrane

Supplied by inferior mesenteric artery

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What layer forms the body wall?

Somatic (parietal) mesoderm

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

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What structures does the mesothelial peritoneum cover?

Inner walls of abdomen

Surfaces of gut and glands

Mesenteries of gut

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What do the primitive dorsal and ventral mesentaries do?

Attach portions of gut to body wall

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Dorsal mesentary

Caudal foregut

Midgut

Much of hindgut

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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

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What structures does foregut include?

Esophagus

Stomach

Duodenum

Liver

Gallbladder

Pancreas

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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

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Esophagus and lung bud developmental abnormalities

Atresia (incomplete development of esophagus)

Tracheoesophageal fistulas

Stenoses

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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)