vitellointestinal duct

**Embryology:** The vitelline duct (omphalomesenteric duct) links the midgut lumen to the yolk sac through the umbilical ring. As the gut elongates and the yolk sac regresses, the duct normally involutes and obliterates by about weeks 5-9. It arises from the midgut (the superior mesenteric artery supplies this region). The vitelline vessels are associated with it: the right vitelline artery becomes the SMA, and the left one regresses.

**Failure of obliteration, by degree of persistence:** - **Meckel diverticulum** (most common, a persistent proximal portion): a true diverticulum containing all 3 layers of the bowel wall. It is on the antimesenteric border of the ileum, about 2 feet from the ileocecal valve. - **Vitelline (omphalomesenteric) fistula:** The entire duct stays patent, connecting the ileum to the umbilicus. It drains meconium or feces from the umbilicus. - **Vitelline cyst (enterocystoma):** A segment in the middle persists as a cyst, often joined to the umbilicus and ileum by fibrous cords. - **Umbilical sinus:** Only the distal part persists, with mucosa at the umbilicus. - **Fibrous band (vitelline ligament):** The duct is replaced by a cord from the ileum to the umbilicus. It can cause volvulus or internal hernia with small bowel obstruction. - **Umbilical polyp or granuloma:** Persistent ectopic mucosa at the umbilicus that doesn't resolve with silver nitrate.

**Meckel diverticulum, the "rule of 2s":** - About 2% of the population - About 2 feet from the ileocecal valve - About 2 inches long - Presents at about age 2 (most often under age 2, though it can be asymptomatic) - 2:1 male:female - 2 types of ectopic tissue are most common: gastric and pancreatic

**Clinical features:** - Painless lower GI bleeding in a child, with currant-jelly or bright red stool. Ectopic gastric mucosa secretes acid and ulcerates the adjacent ileal mucosa. - Intussusception, with the diverticulum as a lead point. - Diverticulitis, which mimics appendicitis (RLQ pain). - Obstruction from volvulus around a fibrous band.

**Diagnosis:** Technetium-99m pertechnetate scan (Meckel scan) is taken up by ectopic gastric parietal cells. Pentagastrin or H2 blockers can enhance the scan. Other options are CT, laparoscopy, and angiography for brisk bleeding.

**Treatment:** Surgical resection (diverticulectomy or segmental ileal resection) for symptomatic cases.

**Differential/high-yield contrasts:** - Urachus (allantois to bladder): a patent urachus leaks urine from the umbilicus. Persistent vitelline duct leaks bowel contents. - Omphalocele (failure of gut to return to the abdomen, covered by a membrane) vs gastroschisis (a lateral abdominal wall defect with no covering). Both are separate from vitelline duct anomalies. - Physiologic midgut herniation occurs in weeks 6-10, with rotation of 270° counterclockwise around the SMA. Malrotation can cause midgut volvulus.

**Associations:** Meckel diverticulum is associated with other congenital anomalies (e.g., umbilical anomalies, intestinal atresia, malrotation, and some cardiac defects) in some patients.