Gastrointestinal Development
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Direct answer
The primitive gut tube forms in the fourth week as the embryo folds and encloses yolk sac endoderm, and is divided by arterial supply into foregut (coeliac trunk), midgut (superior mesenteric artery) and hindgut (inferior mesenteric artery). The midgut lengthens so fast that it herniates into the cord in week six, returning and rotating 270 degrees anticlockwise by week ten. Failure of these steps explains malrotation, volvulus, omphalocele and the Meckel diverticulum.
What you must remember
- Foregut derivatives: pharynx, oesophagus, stomach, duodenum to the major duodenal papilla, liver and biliary apparatus, pancreas and the respiratory diverticulum.
- Stomach: a fusiform dilatation in week four rotating 90 degrees clockwise so the greater curvature faces left; its dorsal mesogastrium becomes the greater omentum and lesser sac.
- Duodenum: becomes C-shaped; its lumen recanalises after a transient plug — failure gives duodenal atresia with a double-bubble radiograph, strongly associated with Down syndrome.
- Midgut loop: herniates into the extraembryonic coelom at week six, rotates 270 degrees anticlockwise around the superior mesenteric artery while returning in week ten, placing the caecum in the right iliac fossa.
- Meckel diverticulum: a persistent vitellointestinal duct following the rule of twos — about two per cent of people, two feet from the ileocaecal junction, two inches long, with two ectopic tissues, gastric and pancreatic.
- Hindgut: runs from the distal third of the transverse colon to the upper anal canal; the urorectal septum divides the cloaca into urogenital sinus and anorectal canal, the membranes perforating in week eight.
- Anal canal: the upper two-thirds are hindgut endoderm drained by the superior rectal vein, the lower third proctodeal ectoderm drained by the inferior rectal vein — the pectinate line is the embryological junction.
Common confusion
Omphalocele and gastroschisis are forever interchanged: an omphalocele is a midline defect with the bowel covered by peritoneum and amnion and the cord at its apex, from failure of midgut return; gastroschisis is a paramedian, usually right-sided defect with no covering membrane. Students also swap rotation directions — the stomach rotates clockwise, the midgut anticlockwise — and pair oesophageal atresia, not duodenal, with polyhydramnios and Down syndrome.
Exam-focused takeaway
Theory questions ask for gut divisions with their arteries, rotations of the stomach and midgut loop, and the embryology of Meckel diverticulum, malrotation and imperforate anus. In the viva, contrast the anal canal above and below the pectinate line by origin, supply, lymphatics and nerves. Practicals include specimens of malrotation and Meckel diverticulum.
Frequently asked questions
What supplies the foregut, midgut and hindgut?
The coeliac trunk to the distal duodenum, the superior mesenteric artery to the distal third of the transverse colon, and the inferior mesenteric artery to the upper anal canal.
Why does the midgut loop rotate?
Rapid lengthening forces physiological herniation into the umbilical cord; during return the 270-degree anticlockwise rotation around the superior mesenteric artery packs the small bowel and fixes the caecum in the right iliac fossa.
What is the rule of twos for Meckel diverticulum?
About two per cent prevalence, two feet from the ileocaecal junction, two inches long, twice as common in males, and two ectopic tissues — gastric mucosa and pancreatic tissue — a cause of painless lower gastrointestinal bleeding.
How do omphalocele and gastroschisis differ?
Omphalocele is a covered midline defect at the umbilicus from failure of gut return; gastroschisis is an uncovered paramedian defect, usually on the right, not involving the cord insertion.
What marks the junction of endodermal and ectodermal anal canal?
The pectinate line: above it the mucosa is hindgut endoderm with visceral innervation and portal drainage; below it is proctodeal ectoderm with somatic innervation and systemic drainage.