Sunday, March 15, 2015

DEVELOPMENT of The abdomen, pelvis and perineum

Posted by Radiology Madeeasy

Development of the gut


The gut develops from a primitive endodermal tube. Itis divided into three parts:

foregut: extends to the entry of the bile duct into the duodenum (supplied by the coeliac axis)
midgut: extends to distal transverse colon (supplied by superior mesenteric artery)

hindgut: extends to ectodermal part of anal canal (supplied by inferior mesenteric artery).


Foregut

  • Starts to divide into the oesophagus and the laryngotracheal tube during the 4th week.
  • If it fails to do so correctly, there may be pure oesophageal atresia (8% of cases), or atresia associated with tracheo-oesophageal fistula (the commonest,80% of cases), the fistula being between the lower end of the trachea and the distal oesophagus
  • Distal to the oesophagus the foregut dilates to form the stomach.
  • Rotates so that the right wall of the stomach now becomes its posterior surface, forming the lesser sac behind.
  • Vagus nerves rotate with the stomach so that the right vagus nerve becomes posterior and the left anterior.
  • As the stomach rotates to the left, so the duodenumswings to the right, its mesentery fusing with the peritoneum of the posterior abdominal wall, leaving all but the first inch retroperitoneal.


 

Midgut

  • Enlarges rapidly in early fetal life, becoming too big for the developing abdominal cavity, and herniates into the umbilical cord.
    The apex of the herniated bowel is continuous with the vitellointestinal duct into the yolk sac.
  • While the midgut is within the cord it rotates 90 counterclockwise around the axis of the superior mesenteric artery, bringing the third and fourth parts of the duodenum across to the left of the midline behind the superior mesenteric artery; this part of the duodenum is now fixed retroperitoneally.
  • The midgut returns to the abdomen at the 10th week and during this time it continues to rotate counterclockwise through a further 180, bringing the ascending colon to the right side of the abdomen with the caecum lying immediately below the liver.
  • The caecum descends into its definitive position in the right iliac fossa, pulling the colon with it.
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