1. The foregut, consisting of the pharynx, thoracic oesophagus and abdominal foregut, of which the abdominal foregut is supplied by the coeliac artery
2. The midgut, from the midpoint of the duodenum to the distal third of the transverse colon, supplied by the superior mesenteric artery
3. The hindgut, from the distal third of the transverse colon to the pectinate line of the anal canal, supplied by the inferior mesenteric artery.

• In the 6^th week, the endodermal lining of the gut tube proliferates, occluding the lumen before it racanalises over the next 2 weeks.
• In the 9^th week, the endoderm of the new gut lumen has formed the definitive mucosal epithelium.

The abdominal foregut

• This part of the foregut extends from the diaphragm to the midpoint of the duodenum, and forms the abdominal oesophagus, stomach, superior half of the duodenum, liver, pancreas and gallbladder.
• It has both a ventral mesentery and a dorsal mesentery attaching it to the respective body walls.
• Initially the abdominal foregut is attached to the ventral body wall by the septum transversum, but as the cranial portion of the septum transversum contributes to the diaphragm, its caudal portion thins, becoming the ventral mesentery.

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Development of the stomach, liver, pancreas, gallbladder and spleen

• The stomach develops from an expansion of the gut tube in the 4^th and 5^th weeks, with its dorsal wall growing faster than the ventral wall to produce the greater and lesser curvatures, respectively.
• In the 7^th and 8^th weeks, the stomach rotates around 2 axes:

1. A craniocaudal axis, swinging its dorsal surface (greater curvature) around to the left
2. A dorsoventral axis, dropping the greater curvature caudally

• The liver, pancreas and gallbladder develop from endodermal diverticula, which bud from the duodenum, eventually forming the glandular/duct epithelia of these organs, whilst the connective and vascular tissue forms from the surrounding mesenchyme.

• The liver develops from the ventral part of the duodenum as the hepatic bud on day 22, growing into the caudal part of the septum transversum that will later become ventral mesogastrium.

• The liver also produces blood cells in the embryo, in mesenchyme derived from the septum transversum.

• The gallbladder also develops into the ventral mesentery from the cystic diverticulum, just caudal to the hepatic bud, on day 26; with the endoderm around the openings of these 2 diverticula forming the common bile duct.

• The pancreas develops from 2 buds: the dorsal and ventral pancreatic buds, on opposite sides of the duodenum.
• The dorsal pancreatic bud sprouts into the dorsal mesentery on day 26, whilst the ventral pancreatic bud sprouts into the ventral mesentery a few days later, just caudal to the cystic diverticulum, with its duct connecting to the common bile duct
• In the 5^th week, the ventral pancreatic bud migrates dorsally, carrying with it the proximal end of the common bile duct, where the bud will fuse.

• The islets of Langerhans are from endodermal origin, developing near microcirculation via the process of mycidioblastosis from the endodermal ducts leading to the acini portions.
• The pancreas will become secondarily retroperitoneal.

• The spleen, however, develops from within the dorsal mesogastrium, during the 4^th week, dividing the dorsal mesogastrium into the gastrosplenic ligament (stomach ---> spleen) and the lienorenal ligament (spleen ---> left kidney).
• The spleen, initially a haematopoietic organ, later takes on its lymphoid function, having B and T lymphocytes by 20 weeks.

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Organisation of the mesenteries

• Growth of the liver divides the ventral mesentery into 3 structures:

1. The falciform ligament (liver ---> ventral body wall)
2. The visceral peritoneum of the liver and gall bladder, which, at the bare area of the liver, reflects onto the diaphragm as the coronary ligament
3. The lesser omentum, further divided into gastrohepatic (liver ---> stomach) and hepatoduodenal (liver ---> duodenum) ligaments.

• The lesser sac of the peritoneal cavity is created by:

1. Rotation of the stomach to the left and the liver to the right
2. Contact and fusion of the pancreas and duodenum to the dorsal body wall, making these organs secondarily retroperitoneal.

• The greater omentum is created by ventral expansion of the dorsal mesogastrium over the transverse colon and small intestine, folding back on itself to form a temporary inferior recess between the 2 folds of mesentery before they fuse, and a superior recess behind the stomach.

Folding and rotation of the midgut

• In the 5^th week, the gut tube begins a process of rotation and elongation that will produce its final configuration, starting with the sagittally oriented primary intestinal loop, with the vitelline duct at the crux, and the caecum appearing on its caudal limp.
• The primary intestinal loop herniates into the umbilicus, rotating around its long axis (90 degrees anticlockwise) until its cranial limb is on the right, and the caudal limb is on the left, completing by the 8^th week.
• In the 8^th week, the jejunum and ileum elongate, throwing the small intestine into jejunal-ileal loops, whilst the appendix develops from the caecum.
• In the 10^th week, the primary intestinal loop retracts into the abdomen, rotating another 180 degrees counterclockwise (viewed from the front), with the large intestine assuming its definitive position, and the ascending and descending colons becoming secondarily retroperitoneal.

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Formation of the anal canal

• The cloaca is the expanded distal portion of the gut tube, attached to the cloacal membrane, into which the allantois of the connecting stalk opens, at its cranioventral apex.

• In the 4^th – 6^th weeks, the urorectal septum begins growing in the cloaca from 3 folds:
  • 1 cranial Tourneux fold, growing down from the roof of the cloaca
  • 2 coronal Rathke folds, growing in from the sides

• The urorectal septum divides the cloaca and covering cloacal membrane into:

1. The primitive urogenital sinus and its urogenital membrane ventrally
2. The rectum and its anal membrane dorsally

• The primitive urogenital sinus will eventually form the bladder, pelvic urethra, and, caudally, the definitive urogenital sinus.

• The allantois will develop into the urachus ligament.

• The anorectal canal is of composed of 2 compartments, united by the breakdown of the anal membrane, which leaves the pectinate line as its remnant.