At the end of the third week, the trilaminar disc, which is a three-layered structure of early embryonic tissue, starts to fold in two planes. [13] Among other things, this forms a single simple gut tube by week 4, beginning at the pharynx and ending at the cloaca (see diagram 2). From this structure, specifically the midgut, the intestines are formed. [12] As is evident in diagram 2, the gut tube is still very different in terms of proportional length and appearance when compared to the intestines in adults. Because of this, a few important developmental processes need to take place in order to achieve the desired outcome.
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Elongation of the Intestines
The first change that occurs in the gut tube is the rapid growth in length to the size it needs to be in a newborn. During the fifth week, endodermal cells of the midgut, which are cells that line the gut tube, begin to proliferate. [10] As the abdominal cavity is a restricted space and is not growing anywhere near as fast as the midgut, the tube folds and creates a u-shaped loop composed of a cephalic and caudal limb (see diagram 3). [10]
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Herniation of the Intestines
Associated with this, the kidneys and liver are quite large relative to the rest of the embryo and so further push the intestinal loop out of the abdominal cavity. [12] By the sixth week, the intestinal loop has herniated into the umbilicus due to a combination of the continually elongating midgut and pressure from the rapid growth of other abdominal organs. [10] Whilst here, the cephalic limb significantly elongates and becomes coiled to form the jejunum and ileum of the small intestine in the newborn. [9] The caudal limb slightly increases in length but does not coil eventually developing into the cecum, ascending and transverse colons of the large intestine. [9]
Rotation of the Intestines
The next important mechanism that takes place is the rotation of the intestinal loop in the umbilical cord to begin to achieve the correct anatomical orientation. [10] Whilst it continues to grow in size, the midgut rotates around the superior mesenteric artery, which is completed during week eight (see diagram 3). [10] This occurs 90 degrees in a counter clockwise direction from the anterior aspect as indicated by the arrow in diagram 3. [14]
Return of the Intestines
During week 10, the abdominal cavity has grown enough to fit the intestinal loop in with the liver and kidneys and so they quickly return from the umbilicus. [12] As the intestines withdraw, they undergo a further rotation of 180 degrees in the same direction as before so that the intestinal loop has rotated a total of 270 degrees. [10] Diagram 4 illustrates the final rotation of the midgut as well as the elongation and coiled nature of the cephalic limb, as opposed to the relatively linear nature of the caudal limb. Following the return of the intestines, the rectus abdominis muscles come together, closing the circular defect in the umbilical cord and restoring the opening back to its original size. [13]
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The cephalic limb is first to re-enter the abdominal cavity and occupies the left side of the abdomen with the jejunum superior or above the ileum. [14] The caudal limb returns by the 11th week with the cecum being the last to retract, filling the right side and moving inferiorly/downwards. [14] This achieves the correct, final anatomical position of the intestines with the ascending colon being on the right, the transverse colon across the top, the descending colon on the left and the small intestine located in the middle of the abdominal cavity.