University:
Stanford University
Course:
MED 101 | Human Anatomy
Academic year:

2023
Views:

306

Pages:

1
Author:

larbi43100

EVOLUTION OF THE MIDDLE PRIMITIVE INTESTINE The midgut represents the middle portion of the primitive intestine, in communication via the yolk duct with the yolk sac. It extends from the outlet of the common bile duct to the junction with the hindgut upstream of the outlet of the allantois. These limits correspond in adults to the duodenal segment downstream of the common bile duct up to the junction of the middle and distal thirds of the transverse colon. Three main stages mark the evolution of this segment of the digestive tract: a significant elongation which will give rise to the handles intestinal;development partly outside the abdominal cavity (physiological hernia) a double rotation of the intestinal loops in the hernia and time of reintegration into the abdomen. 1) Formation of the primitive intestinal loop At the 6th week, the rapid lengthening of the mid intestine determines the formation of the primitive intestinal loop, in communication at its top with the vitelline duct. The opening of this canal marks the limit between the cranial part of the primitive intestinal loop (which will give rise to the distal third of the duodenum and the jejuno-ileum) and the caudal part of the loop (which will give rise to the terminal segment of the ileum, the ascending colon and the proximal third of the transverse colon). Due to this elongation and the very rapid development of the hepatic anlage, the volume of the abdominal cavity becomes insufficient to accommodate the entire digestive tract. The primitive intestinal loop therefore continues its development partially in the umbilical cord and forms a physiological hernia between the 6th and 10th week of development. 2) Rotation of the intestinal loops Two successive rotations will occur at the level of the primitive intestinal loop: A first rotation of 90° counterclockwise according to a ventral view then a second of 180° still counterclockwise. a watch, or 270° in total. The first 90° rotation is located in the physiological hernia: the cranial part of the primitive loop moves towards the right of the embryo. This first rotation is completed at the 8th week. At the same time, the vitelline loop continues to lengthen and forms the jejuno-ileal loops which begin their reintegration from the 10th week. This reintegration is done in an orderly manner:The proximal part of the jejunum first comes to be placed to the left of the abdominal cavity following the duodenum, then the loops follow one another and are placed to the right of the previous one. The cecum, which is a dilatation that appears just downstream of the vitelline duct, is placed last under the liver. At the same time as reintegration takes place, the second 180° rotation occurs, bringing the caudal part of the primitive loop (future colon) in front of the duodenum. This caudal portion lengthens little but sees its diameter increase. At the end of the reintegration and the double rotation, the cecum slides downward and ends up returning to its anatomical position in the right iliac fossa. From the 8th week, the yolk canal begins to regress and it has normally disappeared over most of its course when the loops reintegrate.