Development of GI system

Question 1

Mature abdominal derivates of gut tube

Answer 1

• Foregut: from esophagus to first 1/3rd of duodenum, including liver, gallbladder, pancreas (and associated ducts)
• Midgut: last 2/3rds of duodenum to first 2/3rds of transverse colon
• Hind gut: last 1/3rd of transverse colon to proximal anal canal
• Dorsal mesentery (present at all 3 levels): greater momentum, gastrosplenic, gastrocolic, gastrophrenic, splenorenal ligaments, mesocolon and the mesentery
• Ventral mesentery (foregut only): lesser momentum (hepatogastric and hepatoduodenal ligs), falciform and coronary ligs

Question 2

Hepatic diverticulum 1

Answer 2

• Grows within ventral mesentery and is origin of liver, gallbladder, and part of pancreas (rest of pancreas from dorsal pancreatic bud, which is in dorsal mesentery)
• Ventral pancreatic bud wraps posteriorly, bringing the bile duct w/ it as duodenum rotates, to fuse w/ dorsal pancreatic bud to form pancreas, (both parts retain their own ducts to duodenum)
• The liver grows superiorly as an expansion of hepatic diverticulum, w/in the ventral mesentery, and eventually reaches the diaphragm
• The ventral mesentery is still btwn the diaphragm and liver except in one area that the liver pushed thru the ventral mesentery

Question 3

Hepatic diverticulum 2

Answer 3

• The remaining ventral mesentery on either side of the liver at this point is called the coronary ligament (point where diaphragm and liver are fused- bare area)
• Growth of the liver also separates the ventral mesentery into anterior and posterior domains
• Anterior ventral mesentery (attaches liver to body wall) is the falciform ligament
• Posterior ventral mesentery, or lesser omentum, attaches liver to duodenum and stomach (hepatogastric, hepatoduodenal)

Question 4

Foregut rotation and lesser sac formation

Answer 4

• Stomach rotates 90 degrees clockwise, bringing the left side ventrally and the right side dorsally
• This rotation forces part of the pancreas and duodenum to assume a secondarily retroperitoneal position
• As the stomach moves to the left side and the liver moves to the right, the liver forces what remains of the right coelomic cavity over to the left side, behind the stomach (lesser sac)
• The lesser omentum separates the lesser sac from the rest of the peritoneal cavity, minus epiploic foramen

Question 5

Spleen formation

Answer 5

• Formed w/in dorsal mesentery, but not budding from gut tube
• Instead spleen is a collection of mesodermal cells w/in the mesentery
• Thus is not an endodermally derived structure like the rest of the GI tract

Question 6

Midgut development

Answer 6

• At first the midgut remains open to the yolk sac
• This connection becomes constricted by the formation of the ventral body wall, and the connection condenses to the yolk stalk (which normally eventually disappears, but is the source of meckel’s diverticuli)
• The midgut loops out into the umbilical cord (physiologic herniation) so that the midgut can grow despite the liver taking up all the room in the embryo

Question 7

Midgut rotation

Answer 7

• While within the umbilical cord, the midgut rotates 90 degrees counterclockwise around the axis of the SMA
• Eventually the peritoneal cavity grows enough to accommodate the midgut, and it returns into the cavity
• During the return it rotates another 180 degrees counterclockwise
• The 270 degree rotation is what allows the SI to be coiled on itself
• As the SI returns to the peritoneal cavity the ascending and descending colon are forced posteriorly and become secondarily retroperitoneal

Question 8

Meckel’s diverticulum

Answer 8

• Remnant of the yolk stalk, which normally regresses
• The diverticulum is blind ended and can either be free or attached to the umbilicus
• Can become inflamed and rupture like appendix

Question 9

Hindgut development 1

Answer 9

• The cloaca is the terminal end of the hindgut and marks the transition from endoderm to ectoderm in the anal canal
• The proximal allantois (endodermal projection in the umbilical cord) becomes the bladder, and the distal allantois becomes the median umbilical ligament
• Allantois w/in the umbilical cord is surrounded by mesoderm from which the umbilical blood vessels form

Question 10

Hindgut development 2

Answer 10

• Mesoderm eventually separates the allantois from the rectum (urorectal septum), at which point the cloacal membrane now separates the anal canal from the rectum and the urethra from the urogenital sinus
• The perineum and urorectal septum is innervated by pudendal nerves
• The pectinate line in the fully developed body represents the site of the the cloacal membrane (and thus transition site for vascular, lymphatics, and nerves)