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Development of the Foregut, Midgut, and Hindgut

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Overview: Primordial Gut Formation

• The cranial end by the oropharyngeal membrane
• The caudal end by the cloacal membrane

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1. FOREGUT

Extent and Blood Supply

Derivatives

Esophagus

• Develops from the foregut immediately caudal to the pharynx
• The tracheoesophageal septum partitions the respiratory diverticulum from the esophagus (4th week)
• Initially short; elongates rapidly due to descent of the heart and lungs; reaches final length by week 7
• Lumen temporarily obliterated by epithelial proliferation, then recanalizes by week 8
• Upper 1/3 muscularis: striated muscle (from pharyngeal arch mesenchyme), innervated by vagus nerve (CN X)
• Lower 1/3 muscularis: smooth muscle (from splanchnic mesenchyme)
• Key anomaly: Esophageal atresia (1:3000-4500 births) - associated with tracheoesophageal fistula in >90% of cases

Stomach

• Begins as a fusiform dilation of the foregut in week 4, in the primitive thoracic region
• Undergoes two rotations:
  • Rotation around longitudinal axis (90° clockwise): left side becomes anterior (greater curvature); right side becomes posterior (lesser curvature) - this also explains why the left vagus nerve becomes the anterior gastric nerve
  • Rotation around dorsoventral axis: gastric antrum shifts upward to the right, while fundus shifts down and left - gives the stomach its final position
• Forms the omental bursa (lesser sac) as it rotates
• Key anomaly: Hypertrophic pyloric stenosis (1:150 males) - hypertrophy of pyloric musculature causing projectile vomiting

Duodenum

• Develops from caudal foregut + cranial midgut - junction is just distal to the bile duct origin
• Grows into a C-shaped loop projecting ventrally (week 4), then rotates to the right and becomes retroperitoneal
• Lumen temporarily obliterated (week 5-6) then recanalizes
• Supplied by both celiac trunk and superior mesenteric artery (dual origin)
• Key anomaly: Duodenal atresia - failure of recanalization; associated with Down syndrome and polyhydramnios ("double bubble" sign on X-ray)

Liver and Biliary Apparatus

• The hepatic diverticulum (liver bud) grows from the caudal foregut endoderm into the septum transversum (mesoderm) at ~day 25
• Liver cord cells proliferate into the septum transversum, which thins to form the falciform ligament, lesser omentum, and peritoneum of the liver
• The hematopoietic function begins in week 6; the liver is the main hematopoietic organ in the fetus until week 12, when the spleen takes over
• The gallbladder and cystic duct arise from the hepatic diverticulum
• Key anomaly: Biliary atresia - progressive obliteration of bile ducts; acholic (clay-colored) stools, jaundice, dark urine

Pancreas

• Arises from dorsal and ventral pancreatic buds of the caudal foregut endoderm
• The dorsal bud (larger) appears first; gives rise to most of the pancreas (body, tail, part of head)
• The ventral bud (near bile duct) rotates with the duodenum (clockwise), swings behind and fuses with the dorsal bud
• Ventral bud forms the uncinate process and part of the head
• The main pancreatic duct (of Wirsung) = duct of ventral bud + distal dorsal bud duct
• Accessory duct (of Santorini) = proximal portion of dorsal bud duct
• Key anomaly: Annular pancreas - failure of ventral bud rotation; ring of pancreatic tissue encircles duodenum causing obstruction

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2. MIDGUT

Extent and Blood Supply

Derivatives

Primary Intestinal Loop and Physiological Herniation

• Rapid elongation of the midgut forms the primary intestinal loop, connected at its apex to the yolk sac via the vitelline (omphaloenteric) duct
• The loop has a cranial (cephalic) limb - forms distal duodenum, jejunum, and part of ileum
• Caudal limb - forms distal ileum, cecum, appendix, ascending colon, proximal 2/3 transverse colon
• Due to rapid growth of the gut and liver, the abdominal cavity cannot accommodate the intestinal loops - they herniate into the umbilical cord around week 6 (physiological umbilical herniation)

Rotation of the Midgut Loop

Retraction and Fixation (Week 10)

• Intestinal loops return to the abdomen as the liver and kidneys grow relatively smaller
• Small intestine occupies the center; colon forms its final positions
• The cecum descends from just below the liver to the right iliac fossa; the appendix develops as a small outgrowth of the cecum

Key Anomalies

• Malrotation: Incomplete 270° rotation (e.g., only 90°); colon and cecum settle on the left; predisposes to volvulus and vascular compromise
• Reversed rotation (90° clockwise): Transverse colon passes behind the duodenum
• Omphalocele: Failure of the gut to return to the abdominal cavity; gut covered by peritoneum/amnion (associated with chromosomal defects)
• Gastroschisis: Protrusion through the body wall lateral to umbilicus; gut NOT covered by peritoneum (not associated with chromosomal defects; increasing in young mothers)
• Meckel's diverticulum: Persistence of the vitelline duct (~2-4% of population); ~40-60 cm from ileocecal valve on anti-mesenteric border; may contain ectopic gastric or pancreatic tissue; "rule of 2s"
• Vitelline fistula: Patent vitelline duct from umbilicus to ileum; fecal discharge at umbilicus

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3. HINDGUT

Extent and Blood Supply

Derivatives

The Cloaca and its Partitioning

• The hindgut terminates in the cloaca - a dilated chamber lined with endoderm, in contact with surface ectoderm at the cloacal membrane
• The urorectal septum (wedge of mesoderm) grows caudally and divides the cloaca into:
  • Dorsal part: anorectal canal (future rectum and upper anal canal)
  • Ventral part: urogenital sinus (bladder, urethra, lower vagina)
• The urorectal septum fuses with the cloacal membrane by week 7, dividing it into an anal membrane (dorsal) and a urogenital membrane (ventral)

Anal Canal Formation

Key Anomalies

• Anorectal agenesis / imperforate anus: Failure of the anal membrane to rupture, or abnormal development of the urorectal septum; most common anorectal malformation
• Rectovesical, rectovaginal, or rectourethral fistulas: Result from incomplete division of the cloaca by the urorectal septum
• Hirschsprung disease (congenital megacolon): Failure of neural crest cell migration into the hindgut wall (absence of Meissner's and Auerbach's plexuses); functional obstruction, most commonly involving the rectosigmoid

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Summary Table

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