GI Embryology & Anatomy

Gut divisions and blood supply

Three gut tubes, three arteries. The primitive gut is divided into foregut, midgut, and hindgut, and each segment keeps the artery that fed it in the embryo. Each artery leaves the aorta at a characteristic vertebral level, and parasympathetic supply follows the same division.

Watershed zones. Where two arterial territories meet, the bowel depends on thin collateral connections and infarcts first during hypoperfusion. The two colonic watersheds are the splenic flexure (SMA/IMA border, Griffiths point) and the rectosigmoid junction (lower edge of the IMA territory where it borders the internal iliac supply, Sudeck point). These are the classic sites of ischemic colitis, which typically follows hypotension and presents with crampy pain and bloody diarrhea.

Midgut rotation and congenital anomalies

Normal rotation. The midgut grows faster than the abdominal cavity, so it temporarily herniates into the umbilical cord and rotates around the axis of the SMA before returning.

Malrotation. Incomplete rotation leaves the bowel abnormally fixed on a narrow mesenteric base. Ladd bands (fibrous peritoneal bands from a malpositioned cecum) can cross and obstruct the duodenum, and the narrow base allows midgut volvulus: bilious vomiting in a neonate with a corkscrew duodenum on upper GI series. Twisting the SMA threatens the entire midgut, making this a surgical emergency.

Esophageal atresia with distal TEF. The most common tracheoesophageal anomaly (~85%), from faulty division of the foregut by the tracheoesophageal septum. A blind proximal esophageal pouch sits above a fistula connecting the distal esophagus to the trachea. Clues: maternal polyhydramnios (the fetus cannot swallow amniotic fluid), drooling, choking and cyanosis with the first feeds, an NG tube that coils in the pouch, and air in the stomach (entering through the fistula). It is part of the VACTERL association. Pure atresia gives a gasless abdomen; an isolated H-type fistula causes recurrent aspiration pneumonia.

Double bubble. A distended stomach plus proximal duodenum on X-ray. Causes: duodenal atresia (failure of the duodenum to recanalize; associated with Down syndrome and polyhydramnios) and annular pancreas (the ventral pancreatic bud encircles the duodenum). Pancreas divisum is a different defect: the ventral and dorsal pancreatic ducts fail to fuse.

Ventral wall defects. Both raise maternal serum AFP, but only omphalocele is membrane-covered.

Retroperitoneum and biliary anatomy

SAD PUCKER. Retroperitoneal structures: Suprarenal glands, Aorta and IVC, Duodenum (2nd–4th parts), Pancreas (except the tail), Ureters, Colon (ascending and descending), Kidneys, Esophagus (lower thoracic), Rectum. The first part of the duodenum, the transverse and sigmoid colon, and the stomach are intraperitoneal. Retroperitoneal injuries can bleed or leak without early peritoneal signs.

Biliary tree. The right and left hepatic ducts join to form the common hepatic duct. The cystic duct from the gallbladder joins it to form the common bile duct, which meets the pancreatic duct at the ampulla of Vater, guarded by the sphincter of Oddi, in the second part of the duodenum. A stone impacted at the ampulla can therefore obstruct both bile and pancreatic outflow.

Calot triangle. The cystohepatic triangle is bounded by the cystic duct, common hepatic duct, and inferior liver edge, and contains the cystic artery, which surgeons identify and clip during cholecystectomy.

Portosystemic anastomoses

Three back roads. In portal hypertension (usually cirrhosis), portal blood detours into systemic veins wherever the two circulations meet:

• Esophagus: left gastric (portal) → esophageal veins draining to the azygos (systemic) → esophageal varices.
• Umbilicus: paraumbilical (portal) → superficial epigastric (systemic) → caput medusae.
• Rectum: superior rectal (portal, via the IMV) → middle and inferior rectal (systemic) → anorectal varices.

Varices kill. Ruptured esophageal varices cause hematemesis and are a leading cause of death in cirrhosis. Management includes octreotide (splanchnic vasoconstriction), endoscopic banding, and TIPS, which shunts portal blood directly into a hepatic vein; by bypassing the liver, TIPS raises the risk of hepatic encephalopathy.

Anorectal varices are not hemorrhoids. Internal hemorrhoids are dilated vascular cushions above the pectinate line that drain through the superior rectal vein (to the IMV and portal system), whereas external hemorrhoids drain through the inferior rectal vein (systemic). Portal hypertension produces anorectal varices at the superior–middle/inferior rectal anastomosis.

The pectinate line

Where endoderm meets ectoderm. The pectinate (dentate) line marks the junction of hindgut endoderm and ectoderm, so the epithelium, blood supply, lymphatics, innervation, and tumor type all change across it. The practical payoffs are pain (internal hemorrhoids are painless; external hemorrhoids and fissures hurt) and nodal spread.

Anal fissures sit below the pectinate line, usually in the posterior midline where perfusion is poorest, and cause severe pain with defecation and bright red blood.