Embryology of GI tract

Question 1

How is gut tube formed

Answer 1

Ventral body wall closure and embryo folding

Endoderm is incorporated more and more into the body of the embryo

Question 2

With what does the midgut communicate with

Answer 2

Midgut communicates with the yolk sac via the vitelline duct and will eventually become long and narrow

Question 3

What forms the inner lining of the GI tract and associate organs

Answer 3

Endoderm

Question 4

What parts of the mesoderm form the rest of the GI tract

Answer 4

Somites
Intermediate mesoderm
Lateral plate mesoderm

Question 5

What does the lateral plate mesoderm form in terms of GI system

Answer 5

Parietal mesoderm - parietal serosa of the peritoneum

Visceral mesoderm - visceral serosa of the peritoneum
Layers of the GI tract wall except inner epithelial lining

Question 6

What does the intermediate mesoderm form

Answer 6

Kidneys and gonads

Question 7

What does the para-axial mesoderm form

Answer 7

Somites, which go on to form dorsal dermatomes, myotomes, sclerotomes

Question 8

What is the cranial end of gut tube closed by

Answer 8

oropharyngeal membrane

Question 9

What is the oropharyngeal membrane

Answer 9

Separates the primitive oral cavity (formed from ectoderm) from the pharyngeal part of the foregut (formed from endoderm)
Endoderm and ectoderm are in direct contact with each other
By 4th week, membrane ruptures, allowing communication between oral cavity and gut tube

Question 10

What is the cloacal membrane

Answer 10

Separates the upper part of anal canal (hindgut), derived from endoderm, from the lower anal canal formed from ectoderm (called proctodeum)​
The endoderm and ectoderm here are in direct contact with each other​
Breaks down in 7th week to create opening of the anus​

Question 11

Why is allantois important

Answer 11

With folding it gets incorporated into the body of the embryo and will form the urogenital sinus
Distal part remains in the connecting stalk
By week 5 - yolk sac, connecting sac and allantois are restricted to the umbilical region

Question 12

Explain what the pharyngeal pouches form

Answer 12

Epithelial lining of tympanic cavity and auditory tube (pharyngeal pouch 1)​

Part of tonsils (pharyngeal pouch 2) ​

Parathyroid glands (pharyngeal pouches 3 & 4)​

Part of thymus gland (pharyngeal pouch 3)​

Question 13

What is the mesentery derived from and were do the extend from in an embryo

Answer 13

Derived from mesoderm
Dorsal mesentery - extends from terminal part of the oesophagus down to the cloacal region of the hindgut
Ventral mesentery - exists from the terminal oesophagus, stomach and upper part of duodenum only

Question 14

How does oesophagus form

Answer 14

Respiratory diverticulum on the ventral wall of foregut
Tracheoesophageal septum partitions this region of foregut into trachea ventrally and oesophagus dorsally
Rapidly lengthens with descent of heart and lungs
Muscular coat of oesophagus is derived from visceral mesoderm

Question 15

What is oesophageal atresia

Answer 15

Oesophagus ends as a blind ended sac

Question 16

What is tracheoesophageal fistula

Answer 16

Distal part is connected to the trachea

Question 17

How does the stomach form

Answer 17

Dilation of foregut

Oesophageal lengthening is important for positioning the stomach in the abdomen

Question 18

What is diaphragmatic hernias

Answer 18

Failure of the oesophagus to lengthen can cause diaphragmatic hernias

Question 19

Explain the longitudinal rotation of the stomach

Answer 19

90 degrees clockwise

During this rotation, posterior wall grows faster forming the greater and lesser curvatures

Question 20

Explain the anteroposterior rotation of stomach

Answer 20

the cranial and caudal ends of the stomach were originally in midline ​
rotates so that the caudal/pyloric part moves right and upwards, and the cephalic or cardiac portion moves left and slightly downward​
​

Question 21

How is the lesser omentum derived

Answer 21

Liver cords grow into the septum transversum to form the liver
This causes the ventral mesentery to become the falciform ligament and lesser omentum

Question 22

How does greater omentum form

Answer 22

Dorsal mesentery bulges inferiorly from greater curvature of the stomach​
Grows downwards, forming a double-layered sac extending over the transverse colon and small intestine like an apron

Question 23

How is the duodenum formed

Answer 23

Distal end of foregut and proximal part of midgut = duodenum ​
Stomach rotation with rapid growth of the pancreas causes the duodenum to swing from its midline position to the right side and take on a C-shape

Question 24

How does liver form

Answer 24

Liver primordium appears as an outgrowth of the endoderm epithelium at the distal end of the foregut​
= hepatic diverticulum or liver bud​
Rapidly proliferating cells that penetrate the septum transversum

Question 25

How does bile duct form

Answer 25

the connection between the hepatic diverticulum and the foregut (specifically the duodenum) narrows to become the bile duct​

Question 26

Why does bile duct travel to the posterior part of duodenum

Answer 26

Due to the rotation and movement of the duodenum, the bile duct entrance shifts from its initial anterior position to a posterior one, hence why the bile duct travels behind the proximal part of the duodenum​

Question 27

What are some gall bladder abnormalities

Answer 27

Can get duplication of the gallbladder
Can get accessory pancreatic ducts
Can get abnormalities that involve ducts not forming a lumen

Question 28

How is the pancreas formed

Answer 28

Formed by 2 buds originating from the endoderm of the duodenum​
Dorsal pancreatic bud in dorsal mesentery​
Ventral pancreatic bud is close to bile duct​
​As the duodenum rotates right and becomes C-shaped, the ventral bud moves dorsally
Ventral bud becomes to lie immediately below and behind the dorsal bud​
Ventral bud = uncinate process
Dorsal bud = head, neck, body, tail

Question 29

Where does main pancreatic duct form from

Answer 29

Distal part of the dorsal pancreatic duct​
Entire ventral pancreatic duct merge to form the main pancreatic duct ​
(=major duodenal papilla)​

Question 30

What does the proximal part of dorsal pancreatic duct form?

Answer 30

Accessory pancreatic duct

=minor duodenal papilla