Gastrointestinal Embryology and Gut Rotation

• Foregut — oesophagus to the proximal duodenum, plus liver, gallbladder, pancreas and spleen. Celiac trunk; pain referred to the epigastrium.
• Midgut — distal duodenum to the proximal two-thirds of the transverse colon. Superior mesenteric artery; pain referred periumbilically — which is why early appendicitis hurts around the umbilicus before localising.
• Hindgut — distal transverse colon to the upper anal canal. Inferior mesenteric artery; pain referred to the hypogastrium.
• Physiological herniation and rotation: the midgut herniates into the umbilical cord around week 6 and returns by week 10, rotating a total of 270° counterclockwise around the superior mesenteric artery. Failure gives malrotation with Ladd bands and a narrow mesenteric base predisposing to midgut volvulus — bilious vomiting in a neonate is this until proven otherwise.
• Abdominal wall defects: omphalocele — midline, covered by peritoneum, from failure of the gut to return, and associated with other anomalies; gastroschisis — right of the umbilicus, uncovered, generally isolated.
• Vitelline duct remnants: Meckel diverticulum, the rule of 2s, a true diverticulum that may contain ectopic gastric or pancreatic tissue and bleed.
• Tracheoesophageal fistula: most commonly oesophageal atresia with a distal fistula, presenting with polyhydramnios, choking and an inability to pass a nasogastric tube.

Tube formation

• Endoderm plus folding: the gut tube is endoderm-lined; cephalocaudal and lateral folding in weeks 3–4 pinches the dorsal yolk sac into a tube, leaving the vitelline (omphalomesenteric) duct as the temporary connection to the yolk sac. Splanchnic mesoderm supplies smooth muscle and serosa; neural crest cells migrate craniocaudally to form the myenteric and submucosal plexuses.
• Regional patterning: Sonic hedgehog from gut endoderm signals to surrounding mesoderm, which expresses a rostrocaudal HOX code (with retinoic acid gradients) that assigns foregut, midgut and hindgut identity. Arterial supply is assigned at the same time, which is why the celiac/SMA/IMA territories track the embryological segments rather than gross anatomical landmarks.
• Septation: the tracheoesophageal septum divides the ventral respiratory diverticulum from the dorsal oesophagus; the urorectal septum divides the cloaca into the urogenital sinus and anorectal canal, meeting the proctodeum at the future pectinate line — the endoderm/ectoderm junction that explains the abrupt change in blood supply, lymphatics and innervation there.

Lumen and rotation

• Recanalisation: the duodenal lumen is obliterated by proliferating epithelium in weeks 5–6 and normally recanalises by weeks 8–10. This is the step examiners test — the duodenum is the one segment whose atresia is a recanalisation failure rather than a vascular event.
• Physiological herniation: rapid midgut elongation outpaces the abdominal cavity, which is crowded by the liver and mesonephroi, so the midgut loop herniates into the umbilical cord at about week 6 and returns by week 10.
• 270° counterclockwise rotation about the SMA: 90° occurs during herniation, carrying the cranial (pre-arterial) limb to the right and the caudal (post-arterial) limb to the left; the remaining 180° occurs on return, ultimately placing the duodenojejunal flexure to the left of the midline and bringing the caecum from the right upper quadrant down to the right lower quadrant last.
• Fixation: after rotation the ascending and descending colon and the duodenum fuse to the posterior wall and become secondarily retroperitoneal, creating a broad mesenteric root from the ligament of Treitz to the ileocaecal valve. The vitelline duct normally obliterates around week 7.

Septation and recanalisation failures

• Esophageal atresia with distal tracheoesophageal fistula: faulty tracheoesophageal septation. Polyhydramnios in utero, then choking/cyanosis with feeds and a coiled nasogastric tube on radiograph; the distal fistula fills the stomach with air. Part of the VACTERL association, so examine the anus, spine, heart and radii.
• Duodenal atresia: failed recanalisation. Bilious vomiting in the first day of life with the double bubble sign and no distal gas; strongly associated with trisomy 21.
• Mimics and contrasts: annular pancreas (failure of the ventral bud to migrate/rotate normally) and a duodenal web cause incomplete duodenal obstruction — a double bubble with distal gas, as does malrotation with midgut volvulus. Jejunoileal atresia, by contrast, is an in-utero vascular accident (apple-peel deformity) producing multiple dilated proximal loops — proximal jejunal atresia classically gives a triple bubble (stomach, duodenum, proximal jejunum) — with a gasless distal abdomen.

Rotation and fixation failures

• Malrotation with midgut volvulus: incomplete rotation leaves a narrow mesenteric pedicle and fibrous Ladd bands crossing the duodenum. Bilious emesis in a previously well neonate is ischaemic bowel until excluded. Per ACR Appropriateness Criteria, the upper GI series is the diagnostic study of choice (abnormal position of the duodenojejunal junction); definitive treatment is the Ladd procedure.

Wall and duct defects

• Omphalocele: peritoneum/amnion-covered midline herniation; look for trisomy 13/18 and Beckwith-Wiedemann. Gastroschisis: uncovered bowel to the right of the cord, no sac, typically isolated but with bowel inflammation from amniotic fluid exposure.
• Vitelline duct remnants: Meckel diverticulum (painless lower GI bleeding, intussusception lead point, or Meckel diverticulitis mimicking appendicitis); complete patency gives a vitelline fistula with faeculent umbilical discharge; partial persistence gives a sinus, cyst or fibrous band causing volvulus.

Neural crest failure

• Hirschsprung disease: arrested craniocaudal neural crest migration, so the rectum is always involved; RET mutations, trisomy 21 association. Failure to pass meconium in 48 hours, explosive stool after rectal exam, transition zone on contrast enema, and suction rectal biopsy showing absent ganglion cells as the confirmatory test.

• Bilious vomiting in a neonate: assume malrotation with volvulus. If haemodynamically stable, the single best next step is an upper GI series; if peritonitic or unstable, straight to the operating room for a Ladd procedure. Do not choose ultrasound-first or observation.
• Double bubble: with no distal gas = duodenal atresia (think trisomy 21); with distal gas = malrotation/volvulus or an incomplete obstruction such as annular pancreas or duodenal web. That gas pattern is the whole question.
• The classic distractor: hypertrophic pyloric stenosis is acquired, not embryological — nonbilious projectile vomiting at 3–6 weeks, palpable olive, hypochloraemic hypokalaemic metabolic alkalosis. It is not on the rotation-anomaly differential because the vomitus is nonbilious.
• Omphalocele vs gastroschisis: covered/midline/syndromic vs uncovered/right of the cord/isolated. Both raise maternal serum AFP; gastroschisis raises it more. Beckwith-Wiedemann and trisomy 18 belong to omphalocele only.
• Meckel diverticulum: the rule of 2s, a true diverticulum (all three layers) on the antimesenteric border of the ileum, from a persistent vitelline duct. Best test is the technetium-99m pertechnetate scan, which images ectopic gastric mucosa — that same mucosa is what ulcerates and bleeds.
• Hirschsprung disease always involves the rectum because neural crest migration is craniocaudal, so a "skip lesion" of aganglionosis is a wrong answer. Diagnosis rests on suction rectal biopsy, not on the contrast enema.
• Referred pain follows the embryological segment, not the organ's final position: periumbilical pain that migrates to McBurney point is the appendix transitioning from visceral midgut afferents to somatic parietal peritoneal irritation.
• Pectinate line: above it, hindgut endoderm — visceral innervation, internal haemorrhoids, portal drainage, adenocarcinoma; below it, ectoderm — somatic pain, external haemorrhoids, systemic drainage, squamous cell carcinoma.