Development of the GIT - Midgut & Hindgut Flashcards
Where does the midgut run to and from?
What is it continuous with and what is its blood supply?
- it runs from the distal 1/2 of the duodenum (immediately distal to the entrance of the bile duct) to 2/3 along the transverse colon
- the midgut is continuous with the yolk sac at the vitelline duct
- it is supplied by the superior mesenteric artery throughout its entire length
What is formed in week 5 as a result of growth of the midgut?
- the midgut and associated dorsal mesentery undergo rapid elongation to form the primary intestinal loop
- the primary intestinal loop communicates with the yolk sac via the vitelline duct
- it is divided into cranial (cephalic) and caudal limbs
What will the cranial and caudal limbs of the primary intestinal loop go on to form?
Cranial limb:
- distal part of the duodenum
- jejunum
- proximal ileum
Caudal limb:
- distal ileum
- caecum
- appendix
- ascending colon
- proximal 2/3 of the transverse colon
What is meant by physiological herniation of the midgut and why does this occur?
- during week 6, there is rapid elongation of the midgut and growth of the liver
- the abdominal cavity temporarily becomes too small to contain all of the intestinal loops
- the primary intestinal loop herniates into the umbilical cord
As herniation of the midgut occurs, how does it rotate?
- as herniation occurs, the midgut rotates 90o anti-clockwise
- this brings the cranial limb to the right and the caudal limb to the left
- jejunoileal loops form
What axis does the midgut rotate around?
What is the total amount of rotation performed and what else is occurring during this time?
- the midgut rotates around an axis formed by the superior mesenteric artery
- the overall rotation is 270o anti-clockwise
- during rotation, elongation of the jejunum and ileum to form coiled loops (jejunoileal loops) is occurring
- rotation occurs during physiological herniation (90o) and during return of the intestinal loops into the abdominal cavity (180o)
When does the midgut return to the abdomen following herniation?
- the herniated intestinal loops begin to return to the abdomen in week 10
- as this is occurring, the midgut rotates a further 180o anti-clockwise
- this brings the proximal jejunal loops to the left side and the caecum lies inferior to the liver
What is the first part of the intestine to return to the abdomen following herniation?
What forms from the caecum during this process and what is obliterated?
- the proximal portion of the jejunum is the first to re-enter the abdominal cavity and this comes to lie on the left side
- later returning loops will settle more and more to the right
- the last part of the gut to re-enter the abdomen is the caecal bud
- this is a small dilation of the caudal limb of the primary intestinal loop
- the caecal bud lies in the RUQ directly below the right lobe of the liver, before descending into the right iliac fossa
- during the descent, the distal end of the caecal bud forms a narrow diverticulum - the vermiform appendix
- the vitelline duct is obliterated during return of the midgut to the abdomen
When has the midgut completely returned to the abdomen?
How has it rotated during this time?
- the midgut has completely returned to the abdomen by week 11
- it has undergone 270o anti-clockwise rotation in total
How is the final arrangement of the midgut achieved?
- once the midgut has returned to the abdomen, the caecum descends from below the liver to the right iliac fossa
- this pulls the ascending and transverse colon into place on the right side of the abdomen, resulting in the final position of the midgut
What is the mesentery of the primary intestinal loop?
How does this change with rotation and coiling of the bowel?
- the mesentery proper undergoes profound changes with rotation and coiling of the bowel
- when the caudal limb of the loop moves to the right side of the abdomen, the dorsal mesentery twists around the origin of the superior mesenteric artery
- when the ascending and descending colon obtain their definitive positions, their mesenteries press against the posterior abdominal wall
this anchors the ascending and descending colon in a (secondarily) retroperitoneal position
What structures of the midgut maintain their mesentery?
What is the fate of the transverse mesocolon?
- Structures that maintain their free mesenteries are:
- appendix
- lower end of the caecum
- sigmoid colon
- the transverse mesocolon fuses with the posterior wall of the greater omentum, but maintains its mobility
- its line of attachment extends from the hepatic flexure of the ascending colon to the splenic flexure of the descending colon
Where does the mesentery of the jejunoileal loops run to and from?
- it is initially continuous with the mesentery of the ascending colon
- when the mesentery of the ascending mesocolon fuses with the posterior abdominal wall, the mesentery of the jejunoileal loops obtains a new line of attachment
- the new line of attachment runs from the area where the duodenum becomes intraperitoneal to the ileocaecal junction
Why does the appendix have a variable position?
- the descent of the caecum causes the appendix to be located in the retrocaecal position in the majority of individuals (posterior to the caecum or colon)
- the appendix is suspended by a mesentery and is relatively mobile
- it can project inferiorly towards the pelvic brim, affecting the symptoms and site of pain in appendicitis