Gut development
Gut development: midgut rotation, atresia, Meckel diverticulum, Hirschsprung
Foregut, midgut and hindgut development for FCPS Part 1: midgut rotation, atresias, wall defects, Meckel diverticulum, Hirschsprung, liver and pancreas. Each neonatal surgical emergency is linked to the developmental step that failed.
Divisions of the gut tube
Body folding in week 4 pinches the endoderm-lined gut tube off the yolk sac, leaving the vitelline (vitellointestinal) duct connecting the midgut to the yolk sac. The epithelium is endoderm; muscle and connective tissue are splanchnic mesoderm; enteric neurones are neural crest.
| Part | Artery | Extent | Nerve to its pain |
|---|---|---|---|
| Foregut | Coeliac trunk | Pharynx to the second part of the duodenum (major papilla); liver, gallbladder, pancreas; spleen develops in its dorsal mesogastrium | Epigastrium |
| Midgut | Superior mesenteric | Rest of duodenum to the proximal two-thirds of transverse colon | Periumbilical |
| Hindgut | Inferior mesenteric | Distal third of transverse colon to the upper anal canal; lining of bladder and urethra via the urogenital sinus | Suprapubic |
Midgut rotation
- In week 6 the midgut loop outgrows the small abdomen (the liver is huge) and enters the umbilical cord: physiological umbilical herniation.
- It rotates 270° anticlockwise (viewed from the front) around the axis of the SMA: 90° while herniated, 180° more on return.
- It returns by week 10, jejunum first; the caecum returns last, sits under the liver and then descends to the right iliac fossa.
| Anomaly | What failed | Features |
|---|---|---|
| Malrotation | Incomplete rotation | Caecum high in the upper abdomen; Ladd bands cross and compress the duodenum; narrow mesenteric base twists: midgut volvulus with bilious vomiting in the neonate, a surgical emergency (Ladd procedure) |
| Omphalocele (exomphalos) | Herniated gut fails to return | Through the umbilical ring, covered by a sac of peritoneum and amnion; the cord inserts into the sac; frequent associated anomalies (trisomies 13 and 18, Beckwith–Wiedemann, cardiac defects) |
| Gastroschisis | Body wall closure defect | Defect to the right of a normal cord; no sac; bowel exposed to amniotic fluid becomes thickened; young mothers; other major anomalies uncommon (some intestinal atresia). Cover bowel with cling film, keep warm and replace fluids |
Atresias and stenoses
| Lesion | Mechanism | Features |
|---|---|---|
| Duodenal atresia | Failure of recanalisation after the solid epithelial stage (weeks 5–8) | Double bubble, bilious vomiting (if below the papilla), polyhydramnios; about a third have Down syndrome |
| Jejunoileal atresia | Intrauterine mesenteric vascular accident (volvulus, thrombosis) | Blind ends with a gap in the mesentery; apple-peel distal bowel coiled round a single vessel; not strongly linked with Down syndrome |
| Oesophageal atresia with TOF | Faulty division of foregut by the tracheo-oesophageal septum | Commonest type (about 85%): blind upper pouch with distal fistula; drooling, choking and regurgitation with the first feed, cyanosis, NG tube coils in the chest, gas in the stomach; polyhydramnios; VACTERL association |
| Pyloric stenosis | Postnatal muscle hypertrophy, not atresia | First-born boys, projectile non-bilious vomiting at 2–8 weeks, olive-shaped mass, hypochloraemic hypokalaemic metabolic alkalosis; Ramstedt pyloromyotomy |
Vitelline duct remnants
The vitelline duct normally obliterates by week 7–10. Persistence of part of it gives:
| Remnant | Presentation |
|---|---|
| Meckel diverticulum | Persistent proximal part on the antimesenteric border of the ileum |
| Vitelline (umbilicoileal) fistula | Faecal or mucus discharge from the umbilicus |
| Vitelline cyst or fibrous band | Small bowel volvulus or obstruction round the band |
Meckel diverticulum is the commonest congenital anomaly of the small intestine and a true diverticulum (all coats of the wall). The rule of twos: about 2% of people, about 2 feet (60 cm) from the ileocaecal valve, about 2 inches long, two types of ectopic tissue (gastric, pancreatic), often symptomatic before age 2, about twice as common in boys.
- Painless, large, dark red rectal bleeding in a young child comes from acid secreted by ectopic gastric mucosa ulcerating the adjacent ileum.
- Best test: technetium-99m pertechnetate scan, which is taken up by gastric mucosa.
- Other presentations: intussusception (red currant jelly stool), obstruction, Meckel diverticulitis mimicking appendicitis.
- Urine from the umbilicus means a patent urachus (allantois), not a vitelline remnant.
Hirschsprung disease
- Neural crest cells fail to complete their migration down the hindgut (craniocaudal, reaching the rectum by week 12), so ganglion cells are absent from both the myenteric (Auerbach) and submucosal (Meissner) plexuses of the distal bowel.
- These are parasympathetic postganglionic neurones; in the distal colon they normally receive preganglionic fibres from the pelvic splanchnic nerves (S2–S4).
- The aganglionic segment cannot relax and stays narrow; bowel above it dilates. It always involves the rectum and extends proximally a variable distance, most often to the rectosigmoid.
- Delayed meconium (beyond 48 hours), distension, bilious vomiting, enterocolitis; linked with Down syndrome and RET mutations.
- Diagnosis: rectal suction biopsy (no ganglion cells, thick acetylcholinesterase-positive nerve fibres); contrast enema shows a transition zone. Treated by resecting the aganglionic segment (pull-through).
Anorectal malformations
The urorectal septum divides the cloaca into the urogenital sinus in front and the anorectal canal behind. Faulty division gives imperforate anus, often with a fistula to the urethra, bladder, vagina or perineum.
Liver, biliary tree, pancreas and spleen
- Liver: a foregut bud grows into the septum transversum. Hepatocytes and bile duct epithelium are endoderm; Kupffer cells, stellate cells and the capsule are mesoderm. The fetal liver is the main site of haematopoiesis from about week 6 to 7 months.
- Biliary atresia: progressive obliteration of extrahepatic ducts; jaundice beyond 2 weeks with pale stools, dark urine, conjugated hyperbilirubinaemia; Kasai portoenterostomy ideally before 6–8 weeks.
- Pancreas: a dorsal and a ventral bud. The ventral bud, with the bile duct, swings behind the duodenum to fuse with the dorsal bud and forms the uncinate process and lower head; its duct forms the main pancreatic duct with the distal dorsal duct. Annular pancreas (bifid ventral bud ringing the second part of duodenum); pancreas divisum (ducts fail to fuse).
- Spleen: mesoderm in the dorsal mesogastrium, supplied by the splenic branch of the coeliac trunk.
Remember Rotation 270° anticlockwise round the SMA, home by week 10.
Exam trap Bilious vomiting in a neonate is malrotation with volvulus until proved otherwise.
Exam trap Duodenal atresia is a failure of recanalisation (Down syndrome); jejunal atresia is vascular.
How FCPS asks about this
- Origin of Meckel diverticulum: persistent vitellointestinal duct; bleeding from ectopic gastric mucosa; Tc-99m pertechnetate scan.
- Bowel to the right of a normal cord with no sac: gastroschisis, with fewer associated anomalies than omphalocele.
- Defect covered by amnion and peritoneum: omphalocele, linked to trisomies.
- Blind jejunum with mesenteric gap and apple-peel bowel: intrauterine vascular accident.
- Delayed meconium, absent ganglion cells: Hirschsprung disease from failed neural crest migration; rectum always involved; rectal suction biopsy.
- Double bubble in a baby with Down syndrome: duodenal atresia from failed recanalisation.
- Timing of physiological herniation and return: week 6 out, week 10 back.
Test yourself on this topic
5 of the 44 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
During its herniation into the umbilical cord and return to the abdomen, the midgut loop rotates in total by how much, and around which axis?
Answer: 270° anticlockwise around the SMA
The midgut loop rotates around the axis of the superior mesenteric artery. Viewed from the front, it turns 90° anticlockwise while herniated in the cord (week 6), and a further 180° anticlockwise as it returns to the abdomen (week 10), making 270° in all. This brings the duodenojejunal flexure to the left behind the SMA and the caecum to the right upper quadrant, from where it descends to the right iliac fossa.
Midgut rotation
- Axis: superior mesenteric artery
- 90° in the cord, 180° on return
- Total 270° anticlockwise
- Caecum ends in right iliac fossa
Remember 270 anticlockwise around the SMA.
Exam trap Incomplete rotation leaves the duodenojejunal flexure to the right and risks volvulus.
Why the other options are wrong
- 180° clockwise around the SMA Wrong direction and amount.
- 90° anticlockwise around the coeliac trunk The coeliac trunk supplies the foregut; it is not the axis.
- 270° clockwise around the IMA Wrong direction, and the IMA supplies the hindgut.
- 360° anticlockwise around the vitelline duct Too much rotation, and the vitelline duct is not the axis.
A 20-year-old has appendicitis with pain and tenderness under the right costal margin. At surgery the caecum and appendix lie just below the liver. What is the developmental explanation?
Answer: Caecum did not descend
After the midgut returns to the abdomen in week 10, the caecum lies in the right upper quadrant just below the liver. It then descends to the right iliac fossa as the ascending colon lengthens. If this descent fails, the caecum and appendix remain subhepatic, and appendicitis then causes pain and tenderness under the right costal margin, mimicking cholecystitis. The rotation itself is complete, so the duodenojejunal flexure is normally placed.
Subhepatic caecum
- Caecum first lies below the liver
- Then descends to right iliac fossa
- Failure: subhepatic caecum
- Appendicitis mimics cholecystitis
Remember A high caecum gives high appendix pain.
Exam trap Do not confuse failed descent with malrotation; rotation is normal here.
Why the other options are wrong
- Midgut rotated 90° clockwise Clockwise rotation gives reversed rotation, with the colon behind the SMA.
- Vitelline duct persisted Gives Meckel diverticulum or a fistula at the umbilicus.
- Dorsal mesentery failed to fuse Gives a mobile caecum and colon, not a high caecum.
- Hindgut failed to rotate The hindgut does not rotate during development.
From which germ layer do the insulin-secreting beta cells of the pancreatic islets develop?
Answer: Endoderm
Both the exocrine acini and the endocrine islets develop from the endoderm of the pancreatic buds. In the third month, groups of cells separate from the developing ducts to form the islets of Langerhans, and insulin secretion begins by about the fifth month. The connective tissue and vessels of the pancreas come from splanchnic mesoderm. The older idea that islet cells come from neural crest has been disproved.
Pancreatic islets
- Islets bud from ductal endoderm
- Appear in month 3
- Insulin from month 5
- Stroma from splanchnic mesoderm
Remember Exocrine and endocrine pancreas: both endoderm.
Exam trap Neuroendocrine features do not mean neural crest origin.
Why the other options are wrong
- Mesoderm Forms the stroma and vessels, not the islets.
- Neural crest Islet cells are not crest-derived, though they share some neuroendocrine features.
- Surface ectoderm Forms epidermis and the oral lining, not the pancreas.
- Neuroectoderm Forms the nervous system, not islets.
A 35-year-old non-drinker has had four attacks of acute pancreatitis without gallstones. MRCP shows the main pancreatic duct from the tail and body draining through the minor papilla, with a short separate duct in the head draining through the major papilla. What is the embryological cause?
Answer: Failure of dorsal and ventral ducts to fuse
The dorsal and ventral pancreatic buds each have a duct. After the ventral bud rotates to meet the dorsal bud, the ventral duct normally joins the distal dorsal duct to form the main duct of Wirsung, which opens with the bile duct at the major papilla. In pancreas divisum the ducts fail to fuse, so most of the gland drains through the small dorsal duct (Santorini) and minor papilla. Relative outflow obstruction may cause recurrent pancreatitis.
Pancreas divisum
- Dorsal and ventral ducts fail to fuse
- Most drainage via minor papilla
- Commonest pancreatic anomaly
- May cause recurrent pancreatitis
Remember Big duct to the minor papilla: pancreas divisum.
Exam trap Most people with pancreas divisum never have symptoms.
Why the other options are wrong
- Abnormal migration of a bilobed ventral bud Gives annular pancreas round the duodenum.
- Persistence of the vitelline duct Gives Meckel diverticulum.
- Failure of duodenal recanalisation Gives duodenal atresia or stenosis.
- Abnormal rotation of the midgut Gives malrotation with Ladd bands.
The glandular epithelium of the prostate develops from which embryonic structure?
Answer: Urethral endodermal buds
In the third month, multiple outgrowths of endoderm bud from the prostatic urethra, which is part of the pelvic urogenital sinus. They grow into the surrounding mesenchyme, which forms the stroma and smooth muscle. The bulbourethral glands form in the same way from the spongy urethra. The seminal vesicles and ejaculatory ducts, by contrast, come from the mesonephric duct.
Prostate
- Epithelium: urethral endoderm
- Stroma: surrounding mesenchyme
- Same origin: bulbourethral glands
- Seminal vesicle: mesonephric duct
Remember Prostate buds from the urethra it surrounds.
Exam trap The seminal vesicle and the prostate lie together but have different origins.
Why the other options are wrong
- Mesonephric duct Forms the epididymis, vas, seminal vesicle and ejaculatory duct.
- Paramesonephric duct Regresses in the male, leaving the prostatic utricle and appendix testis.
- Genital tubercle Forms the penis.
- Mesonephric tubules Form the efferent ductules of the testis.
Where this sits in the CPSP syllabus
- Paper 1: General and systemic embryology, Gut development: rotation, atresia, Meckel, Hirschsprung; liver and pancreas
- Surgery & Allied, Paper 2: Embryology for surgery, Gut development: rotation, atresia, Meckel, Hirschsprung; liver and pancreas
- Diagnostic Radiology, Paper 2: Embryology, Development of the gut, liver, pancreas and urogenital system