Week 1: Development of the GI system

Question 1

Organization of the gut in early embryo post lateral folding

Answer 1

-foregut: has ventral and dorsal mesentery. Ventral mesentery persists.
-midgut: body wall doesn’t completely enclose embryo, leaving open and continuous to yolk sac
-hindgut: dorsal mesentery only connected to posterior body wall.
Gut is endoderm derived
mesentery serves as conduit between gut and rest of body: blood, nerves, lymphatics

Question 2

Hepatic diverticulum and first rotation of the gut

Answer 2

• grows within ventral mesentery: origin of liver, gall bladder, and ventral part of pancreas
• first rotation: around duodenal axis. Ventral bud rotated and fuses with dorsal pancreatic bud. Ventral becomes head and dorsal becomes body and tail of pancreas. Bile duct dragged along.

Question 3

Development of the liver

Answer 3

• liver grows within ventral mesentery
• mesentery is reflected onto underside of diaphragm, this part becomes coronary ligament
• the mesentery between the liver and gut tube becomes the lesser omentum
• mesentery between liver and ventral body wall becomes the falciform ligament

Question 4

Foregut rotation

Answer 4

• second rotation
• 90 degree clockwise rotation around a vertical axis
• what was left becomes anterior and what was right becomes posterior
• sets liver on right side and stomach besides it to the left
• what used to be the right coelomic cavity becomes a cavity behind the stomach the lesser sac

Question 5

Development of the spleen

Answer 5

• from mesodermal cells

- within the dorsal mesentery

Question 6

Duodenal stenosis and atresia

Answer 6

• duodenal canal fills with epithelial cells during 2nd month and reanalyzed thereafter
• failure to fully open up may lead to stenosis. If it doesn’t open at all end up with atresia.
• vomiting within hours of birth, vomit contains bile because after ampulla of vater

Question 7

Mid gut development and umbilical herniation

Answer 7

• yolk sac becomes smaller and smaller and get stretched out and only a narrow yolk stalk connects the yolk sac, which ends up being eliminated from embryo
• portion of the midgut herniates outside of body wall because fetal liver is so large that there isn’t enough room for intestines to develop within

Question 8

Midgut rotation

Answer 8

• third rotation of gut
• 270 degree counterclockwise rotation around axis of superior mesenteric artery
• first 90 degrees is outside the body wall. 10th week, midgut returns into abdominal cavity and then another 180 rotation
• ascending and descending colon forced against posterior body wall when it returns into abdominal cavity, and becomes retroperitoneal
• pancreas and duodenum also become retroperitoneal

Question 9

Merkel’s diverticulum

Answer 9

• in 2% of people, a diverticulum remains of the yolk stalk, vestige of projection of the yolk stalk from ileum
• can get inflamed like the appendix

Question 10

hindgut development

Answer 10

• allantois, endodermal projection into the connecting stalk and then into umbilical cord, proximal part becomes bladder, distal becomes median umbilical ligament
• urorectal septum (mesoderm) grows toward cloacal membrane and fuses to form the perineum. Separates cloaca into the rectum/anal canal and urethra/urogenital sinus.
• ectoderm of hindgut becomes terminal 1/3 of anal canal and endoderm becomes upper 2/3rds
• separated by anal membrane, which breaks down and later is marked by pectinate line