Embryology of the GI Tract

Question 1

What is gastrulation?

Answer 1

Formation of the trilaminar disc:

Ectoderm, endoderm & mesoderm

Question 2

What occurs in week 1?

Answer 2

Fertilisation and implantation

Question 3

What occurs in week 2?

Answer 3

Bilaminar disc formation

Question 4

What occurs in week 2-3?

Answer 4

Establishment of body axes (anterior-posterior/left-right) due to gene expression

Question 5

What occurs in week 3?

Answer 5

Gastrulation: bilaminar disc –> trilaminar disc

Question 6

What occurs in week 3-8?

Answer 6

Organogenesis (formation of organs)

Question 7

In what week does the primordial gut tube form and from what layer?

Answer 7

4th week and from the endoderm lining the yolk sac

Question 8

What two membranes close the ends of the primordial gut tube?

Answer 8

Cranial end: oropharyngeal membrane

Caudal end: cloacal membrane

Question 9

Where does the oeosophagus form from?

Answer 9

Cranial end of the primitive tube. The trachea-oesophageal septum (ridge) divide the foregut into the trachea and oesophagus

Question 10

In what week does the stomach form?

Answer 10

4th week, the foregut dilates to form stomach

Question 11

What causes the stomach to form a lesser and greater curvature?

Answer 11

Differential growth rates

Question 12

What two rotations does the stomach undergo?

Answer 12

Longitudinal

Anterio-posterior

Question 13

Describe the longitudinal rotation the stomach

Answer 13

90 degree rotation so that the ventral border now lies on the left side (LC) and the dorsal border now on the right (GC) - faces different direction

Question 14

Describe anterio-posterior rotation of the stomach

Answer 14

Creates final position: tilts backwards so that LC is superior to GC

Question 15

How is the omental bursa (lesser sac) created?

Answer 15

Mesogastrium (mesentery connecting stomach to body walls) repositioned during stomach rotation.

Longitudinal rotation pulls dorsal mesentery to the LEFT to create the omental bursa (and ventral mesogastrium pulled to right)

Question 16

From which part of the gut tube is the duodenum formed from?

Answer 16

Caudal part of foregut and cranial part of the midgut

Question 17

How is the duodenum peritonised?

Answer 17

The 1st part is intraperitoneal as the duodenal cap retains its dorsal mesentery, but the rest of the duodenum is retroperitoneal as its mesentery fuses with the peritoneum of the posterior abdominal wall

Question 18

When does the liver primordial bud begin to develop and from what layer?

Answer 18

In the 3rd week as an outgrowth of endoderm in the distal end of the foregut

It grows into the septum transverse (primitive diaphragm)

Question 19

How does the bile duct, Gallbladder and cystic duct form?

Answer 19

Connection between liver bud and foregut (duodenum) narrows to form the bile duct.

Ventral outgrowth from the bile duct forms the gall badder and cystic duct

Question 20

When does the spleen begin to develop and how?

Answer 20

Mesenchymal condensation in the dorsal mesogastrium (L side of body) but develops in the 5th week when the mesenchyme differentiates

Question 21

What is mesenchyme?

Answer 21

Mesodermal embryonic tissue which develops into connective and skeletal tissues, including blood and lymph (forms spleen)

Question 22

What is the role of the spleen in weeks 14, 15-7 and 23?

Answer 22

W14: haematopoietic organ (producing blood cells)

W15-17: becomes lobular and colonised by T-lymphocytes

W23: development of B-cell precursors and spleen begins lymphoid function

Question 23

In what week does the pancreas develop and from what layer?

Answer 23

In week 5 from two buds:
Ventral (which outgrows from the bile duct
Dorsal from the duodenum

Question 24

How does the pancreas develop?

Answer 24

As duodenum rotates (due to stomach repositioning) the ventral bud moves to lie directly below the dorsal bud.

Question 25

How does the main pancreatic duct form?

Answer 25

The ventral and dorsal buds fuse, with the ventral + distal part of dorsal duct forming MPD. The proximal part of the dorsal duct forms an accessory duct.

Question 26

What structures are found in the midgut?

Answer 26

``` Distal part of duodenum Jejunum Ileum Appendix Caecum Ascending colon 2/3 transverse colon ```

Question 27

What is the primary intestinal loop?

Answer 27

Part of the midgut, after rapid elongation of the gut tube and mesentery

Question 28

What does the cephalic part of the primary intestinal loop become?

Answer 28

Distal duodenum, jejenum and proximal ileum

Question 29

What does the caudal part of the primary intestinal loop become?

Answer 29

Distal ileum, caecum, appendix, ascending colon and proximal 2/3 of transverse colon

Question 30

What two movements must the primary intestinal loop undergo to reach adult pattern?

Answer 30

Rotation and herniation

Question 31

How does rotation of the primary intestinal loop occur?

Answer 31

Rotates around the axis of the superior mesenteric artery (cranial part shifted to the right side of abdomen- goes under the caudal part) - 90 degrees anticlockwise

Question 32

In what week does rotation of the primary intestinal loop occur?

Answer 32

6th week

Question 33

Describe the process of herniation of the primary intestinal loop

Answer 33

As 90 rotation occurs, the gut tube herniates into the extra embryonic cavity in the umbilical cord Retraction occurs in the 10th week (as abdominal cavity becomes more spacious) and as the loops move back into the abdomen, there is a further 180 degree rotation anti-clockwise

Question 34

What does the hindgut consist of?

Answer 34

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

Question 35

What embryonic layer forms the urethra and bladder?

Answer 35

Endoderm of the hindgut

Question 36

What is the primitive anal canal called?

Answer 36

Cloaca

Question 37

What embryonic layers is the cloaca derived from?

Answer 37

It is an endoderm lined cavity, however the clocal membrane (at the ventral surface of cloacal on body wall) is ectoderm

Question 38

What embryonic layer separates the allantois and hindgut?

Answer 38

Mesoderm (becomes the ureorectal septum)

Question 39

What is the primitive urogenital sinus?

Answer 39

When the allantois enters into the anterior part of the cloaca

Question 40

What happens in to the clocal membrane in the 7th week?

Answer 40

Ruptures, creating an opening for the hindgut

Question 41

How is the anal canal reformed after rupturing of cloacal membrane?

Answer 41

Ectoderm of anal canal proliferation closing the caudal end - so causal anal canal is ectoderm derived with a different blood supply

Question 42

In what week does the anal canal reopen?

Answer 42

Week 9

Question 43

Name three conditions caused by abnormal development in the foregut

Answer 43

Oesophageal atresia Trachea-oesophageal fistula Annular pancreas

Question 44

What is annular pancreas?

Answer 44

Developmental condition caused by failure of ventral pancreatic bud to migrate around the duodenum correctly (could cause duodenal stenosis)

Question 45

Name three conditions cause by abnormal development of the midgut

Answer 45

Omphalocele Gastrochisis Remnant of Vitelline Duct

Question 46

What is omphalocele?

Answer 46

Herniation of abdominal viscera through enlarge umbilical ring but still covered by layer of amnion - may include liver, stomach, intestinal loops

Question 47

What is gastrochisis?

Answer 47

Herniation of abdominal content directly though the body wall into amniotic cavity (no covering) through week area in umbilicus

Question 48

What happens if there are remnants of the Vitelline duct?

Answer 48

Forms Meckel's Diverticulum: remnants of ileum goes into the umbilical cord and stays intact, so faecal matter passes though the umbilicus

Question 49

What abnormal developmental issue causes conditions in the hindgut?

Answer 49

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

Question 50

What conditions arise due to lack of urorectal septum formation?

Answer 50

Urorectal fistula Rectovaginal fistula Rectoanal atresia Imperforate anus