Embryology of GI Flashcards by Heo Mun Ho

What is gastrulation

Formation of trilaminar disc (Ectoderm, mesoderm, endoderm) from bilaminar disc (amniotic cavity + yolk sac)

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Ectoderm forms… (8)

Skin (epidermis), hair, nail, CNS, PNS, sensory epithelia of the eye/ear/nose, liver (except parenchyma), adrenal medulla,

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Mesoderm forms… (7)

Muscle, bones, cartilage, dermis, CVS, urogenital system (except bladder + urethra)), spleen

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Endoderm forms… (9)

Epithelial lining of renal system, GIT, RESP TRACT, glands, liver, pancreas, thyroid, parathyroid parenchyma, BLADDER + URETHRA

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3 phases of embryonic development

1st - growth
2nd - morphogenesis
3rd - differentiation

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What weeks are teratogens the most harmful

4-8

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What happens in week 1

Fertilisation –> implantation

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What happens in week 2

Bilaminar germ disc formation

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What happens in week 3

Gastrulation (trilaminar germ disc)

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What happens in week 3-8

Organogenesis

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What gene is a transcription factor for left sidedness

PTX2

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What does the lateral plate mesoderm ultimately form in the abdominal cavity

Parietal and visceral peritoneum/serosa

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What weeks do lateral folding of the embryo occur

4-8

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What weeks does the gut tube begin to form

3-4

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Gut tube is closed at its cranial and caudal ends by what membranes

Cranial - oropharyngeal

Caudal - cloacal

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Foregut structures (3)

Oesophagus, stomach, upper duodenum

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Development of oesophagus (2 points)

Develops from cranial part of gut tube
The lung bud develops on ventral wall of foregut then the tracheo-oesophageal septum divides the foregut into trachea and oesophagus

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Development of stomach (4 points)

in 4th week, foregut dilates to form stomach shape
Differential growth of the stomach wall creates greater and lesser curvatures
The still developing stomach (dilated foregut) rotates 90 degrees longitudinally clockwise so that the originally anterior border is now on the left and the posterior border on the right
Then it rotates anterio-posteriorly

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Development of lesser sac (omental bursa) (2 points)

Stomach attached to dorsal and ventral wall by mesogastrium

Rotation of the stomach longitudinally pulls the dorsal mesentery to the left creating the lesser sac behind the lesser omentum, stomach and gastrocolic ligament

Development of duodenum (3 points) - developed from what parts of the gut tube…

Formed from caudal part of foregut and cranial part of midgut
As stomach rotates, duodenum becomes c shaped and also rotates
Duodenal cap (first part) retains its dorsal mesentery but the rest of duodenum is retroperitoneal

Duodenum 2 blood supplies + why

Celiac trunk and superior mesenteric artery

Because it’s part of foregut and midgut

Development of liver and bile duct and gallbladder and cystic duct (4 points)

Liver bud appears in 3rd week - bud is an outgrowth of endoderm in the distal foregut
Liver bud grows into the septum transversum (separates pericardial cavity and yolk stalk)
Connection between the liver bud and foregut narrows to form bile duct
Ventral outgrowth from bile duct forms the gallbladder + cystic duct

Development of spleen (2 points)

End of 4th week, spleen develops in the dorsal mesogastrium as a mesenchymal condensation
5th week, this mesenchyme differentiates to form spleen

What happens to spleen in weeks 15 - 17 (2)

Acquires a lobular shape and colonised by T lymphocytes

Development of pancreas

``` 2 buds (ventral and dorsal) grow from the endodermal lining of the duodenum in week 5 As duodenum rotates, the ventral bud moves closer to the dorsal bud and comes to lie directly behind and below the dorsal bud The 2 buds fuse ```

What does the ventral bud become in formation of the pancreas

Becomes a protusion out of the organ and the inferior part of the head of the pancreas

3 foregut developmental anomalies

Oesophageal atresia and tracheo-oesophageal fistula Annular pancreas Accessory pancreatic tissue

What happens in oesophageal atresia and trachea-oesophageal fistula

Tracheo-oesophageal septum deviates incorrectly, then there is a incomplete separation of laryngo-tracheal tube

What is an annular pancreas (2 points)

Ventral bud fails to migrate around the duodenum correctly | Second part of the duodenum is surrounded by a ring of pancreatic tissue constricting it --> duodenal stenosis

What is accessory pancreatic tissue

Pancreatic tissue forming in other ares of foregut

Midgut structures (7) DJ ICAAP acronym

Distal part of duodenum, jejunum, ileum, caecum, appendix, ascending colon, proximal 2/3rds of the transverse colon

Development of midgut (5 points)

Gut tube rapidly elongates Midgut is described as the PRIMARY INTESTINAL LOOP 6th week - primary intestinal loop rotates 90 degrees anticlockwise around the axis of the superior mesenteric artery As the rotation occurs, the midgut loop herniates into the umbilical cord because not enough room to grow During 10th-11th week the herniation moves back into abdomen because the space has grown

2 key events in midgut development

90 degree anticlockwise rotation and physiological herniation

The cephalic/cranial part of primary intestinal loop forms...

distal duodenum, jejunum and proximal ileum

Caudal part of primary intestinal loop forms

distal ileum, caecum, appendix, ascending colon and the proximal 2/3rds of the transverse colon

5 midgut developmental anomalies

``` Abnormal rotation of primary intestinal loop Reversed rotation Omphalocele Gastroschisis Remnants of vitelline duct ```

What is an omphalocele (2 points)

When the herniation of the midgut loop into the umbilical cord doesn't move back into the abdominal cavity So abdominal organs (covered by amnion/peritoneum) herniate out umbilicus in the baby

What is gastroschisis + potential cause (2 points)

Herniation of intestines outside the body through a hole next to umbilicus without any amnion/peritoneum covering Maybe due to cocaine use

What is Meckel's diverticulum

A slight bulge in the small intestine - remnant of the vitelline duct

Hindgut structures (5)

Distal 1/3rd transverse colon, descending colon, sigmoid colon, rectum and upper part of the anal canal

Development of hindgut (2 points)

Terminal hindgut joins with posterior part of the cloaca (primitive anal canal) Allantois enters into the anterior part of the cloaca

What is the cloaca

endoderm lined cavity with surface ectoderm at its ventral boundary (cavity at the end of the digestive tract for the release of both excretory and genital products)

What forms the cloacal membrane

Endoderm/ectoderm boundary

4 hindgut developmental anomalies

Urorectal fistula Rectovaginal fistula Rectoanal atresia Imperforate anus

What is urorectal fistula

Incomplete separation of the hindgut from the urogenital sinus by the urorectal septum

What is rectovaginal fistula

Abnormal connection between rectum and vagina

What is imperforate anus

Failure of anal membrane to break down