TBL19 - Differentiation of the Midgut and Hindgut

Question 1

What do branches of the celiac artery, SMA, and IMA supply respectively? What does rapid elongation of the midgut during development result in? What occupies the long axis of the loop?

Answer 1

1) Branches of the celiac artery, SMA, and IMA supply the foregut, midgut, and hindgut derivatives, respectively
2) Rapid elongation during midgut development results in formation of the intestinal loop
3) The SMA occupies the long axis of the loop

Question 2

What is the intestinal loop attached to at its apex within the connecting stalk? What does elongation of the loop and expansion of the liver cords temporarily do?

Answer 2

1) Within the connecting stalk, the intestinal loop is attached at its apex by the vitelline duct to the yolk sac
2) Elongation of the loop and expansion of the liver cords temporarily reduces the capacity of the abdominal cavity; thus, the loop enters the stalk and later returns to the abdominal cavity

Question 3

How does an omphalocele form and what are the consequences?

Answer 3

1) Omphalocele involves herniation of abdominal viscera through an enlarged umbilical ring
2) The origin of the defect is a failure of the bowel to return to the body cavity from its physiological herniation during the 6th to 10th weeks
3) It is associated with a high rate of mortality and severe malformations, such as cardiac anomalies and neural tube defects

Question 4

What forms an ileal (Meckel) diverticulum?

Answer 4

In 2% to 4% of people, a small portion of the vitelline duct persists, forming an outpocketing of the ileum, Meckel’s diverticulum or ileal diverticulum

Question 5

In what direction does the intestinal loop rotate during its herniation and what does it rotate around? What does the transverse colon pass during this rotation?

Answer 5

1) The intestinal loop makes a counterclockwise rotation around the SMA during its herniation
2) During the rotation, the transverse colon passes anterior to the duodenum

Question 6

What determines the final deposition of the midgut and hindgut derivatives in the abdominal cavity?

Answer 6

Completion of the counterclockwise rotation determines the final deposition of the midgut and hindgut derivatives in the abdominal cavity

Question 7

What is the distal duodenum derived from? What other structures that continue after the distal duodenum are also derived from this same region? What parts of the intestinal tract are derived from the hindgut?

Answer 7

1) The distal duodenum is midgut-derived
2) The jejunum, ileum, cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon are midgut derivatives
3) The distal third of the transverse colon, descending colon, sigmoid colon, rectum, and upper anal canal are hindgut derivatives

Question 8

Why are most gut atresias and stenoses caused by vascular “accidents”? Why is proximal duodenal atresia an exception?

Answer 8

1) Atresias in the upper (proximal) duodenum are probably due to a lack of recanalization
2) From the distal portion of the duodenum caudally, however, stenoses and atresias were thought to be caused by vascular “accidents” that resulted in compromised blood flow and tissue necrosis in a section of the gut resulting in the defects
3) It was suggested that these accidents could be caused by malrotation, volvulus, gastroschisis, omphalocele, and other factors

Question 9

How does Hirschsprung disease cause congenital megacolon and where does it most commonly occur?

Answer 9

1) Congenital megacolon is due to an absence of parasympathetic ganglia in the bowel wall (aganglionic megacolon or Hirschsprung disease)
2) These ganglia are derived from neural crest cells that migrate from the neural folds to the wall of the bowel
3) In most cases, the rectum is involved, and in 80%, the defect extends to the midpoint of the sigmoid colon

Question 10

What is the allantois and where does it extend to? What does the proximal part of the allantois form? What is the urorectal septum derived from and what does it divide the endoderm-lined cloaca into?

Answer 10

1) The allantois, a slender endodermal diverticulum of the distal hindgut, extends into the connecting stalk
2) The proximal part of the allantois forms the cloaca
3) The mesoderm-derived urorectal septum divides the endoderm-lined cloaca into urogenital and anorectal portions (to be studied later)