University:
Kentucky State University
Course:
BIO 301 | Human Anatomy
Academic year:

2023
Views:

352

Pages:

22
Author:

Tarime9wn

Midgut • Extend from the duodenum (distal to the opening of bile duct) to the proximal 2/3rd of transverse colon. 7 • In 5th week, midgut is suspended from posterior abdominal wall by a short mesentery. Midgut is connected with yolk sac by vitelline duct or omphaloenteric duct. Stages of Midgut Development • Physiological umbilical hernia • Rotation of the midgut loop • Retraction of intestinal loops to abdomen. • Fixation of intestines Physiological Umbilical Herniation • At 6th wk, the midgut elongates to form a ventral U-shaped midgut (intestinal) loop. • Midgut loop communicates with the yolk sac by vitelline duct or yolk stalk. • As a result of rapidly growing liver, kidneys & gut, the abdominal cavity is temporarily too small to contain the developing rapidly growing intestinal loop. • Thus, Midgut loop projects into the umbilical cord …this is called physiological umbilical herniation (begins at 6th wk). Midgut development • Midgut is supplied by the superior mesenteric artery. • Elongation of the gut with its mesentery forms the primary intestinal loop. • The intestinal loop has two limbs (cephalic and caudal). • At the Apex, the loop is connected with yolk sac by vitelline duct. Midgut development • Superior mesenteric artery runs in the axis, between the 2 limbs. • The cephalic limb forms the distal part of the duodenum, jejunum and part of the ilium. • The caudal limb forms the lower portion of the ilium, cecum, appendix, ascending colon, and proximal 2/3rd of transverse colon. Rotation of the Midgut • Midgut loop rotates around the axis of the superior mesenteric artery. • Midgut loop rotates first 90 degrees to bring the cranial limb to the right and caudal limb to left during the physiological hernia. • The cranial limb of midgut loop elongates to form the intestinal coiled loops (jejunum & ileum). • When viewed from the front, this rotation is counterclockwise and it is completed to 270 degrees, so after reduction of physiological hernia it rotates to about 180 degrees. Retraction of herniated loops • Begins during 10th week. • It is called reduction of midgut hernia. • The proximal part of the jejunum is the first part to reenter & comes to lie on the left side. • The cecal bud (cecum) is the last part to reenter. 90° 180° 270° Development of Cecum and Appendix • Cecal bud appears during 6th week, lie at first in the right upper quadrant below the Rt lobe of the liver, then descends to the right iliac fossa. • During its descent with colon, appendix appears as a narrow diverticulum from the distal end of the cecal bud. • Appendix may have different positions, retrocecal, retrocolic and pelvic appendix. Development of Cecum and Appendix Following positions of the Tip of the appendix: ■ Retrocecal or retrocolic position: posterior to the cecum or the lower ascending colon. ■ Suspended over the pelvic brim in a pelvic or descending position. ■ Subcecal location: below the cecum. ■ Pre-ileal position: anterior to the terminal ileum, possibly contacting the body wall. ■ Postileal position: posterior to the terminal ileum. The first and second positions are the most common sites. Mesenteries of the intestinal loops • Mesentery proper • Mesentery of ascending & descending colon fused with Post. Abd wall. • Mesoappendix, Transverse mesocolon • Mesosigmoid and mesorectum Fixation of Intestines • The mesentery of jejunoileal loops is at first continuous with that of the ascending colon. • When the mesentery of ascending colon fuses with the posterior abdominal wall, the mesentery of small intestine becomes fan-shaped and acquires a new line of attachment that passes from duodenojejunal junction to the ileocecal junction. Fixation of Intestines Ø The enlarged colon presses the duodenum & pancreas against the posterior abdominal wall. ØMost of duodenal mesentery is absorbed, so most of duodenum (except for about the first 2.5 cm derived from foregut) & pancreas become retroperitoneal. Intestines prior to fixation Intestines after fixation Midgut Anomalies q Abnormalities of the mesenteries: The failure of ascending portion of the colon to attached to Post. Abd wall, a mobile colon results, which may allow it to twist upon itself causing volvulus of cecum and colon, and it may give rise to retrocolic pockets in which portion of small intestine entrapped their ”retrocolic hernia. Midgut Anomalies q Abnormalities of the Body Wall: o Omphalocele: • Herniation of abdominal contents into the proximal part of umbilical cord, covered by amnion. • It is due to failure of the bowel to return to the body cavity from its physiological herniation during 6th to 10th week. • It is associated with other defects such as cardiac anomalies, neural tube defects & chromosomal abnormalities. o Gastroschisis: • Protrusion of abdominal contents through the body wall directly into the amniotic cavity not covered by amnion or peritoneum. • The bowel may be damaged by exposure to the amniotic fluid. • This defect is due to the abnormal closure of the body wall around the connecting stalk. • It occurs lateral to the umbilicus usually on the right. Umbilical Hernia • The intestines return to abdominal cavity at 10th week, but herniate through an imperfectly closed umbilicus. • It is a common type of hernia. • The herniated contents are usually the greater omentum & small intestine. • The hernial sac is covered by skin & subcutaneous tissue. • It protrudes during crying, straining or coughing. • Surgery is performed at age of 3-5 years. q Vitelline Duct Abnormalities • 2-4 % of people. • Persistence of a small portion of vitelline duct forms: outpocketing of ileum, Meckel’s diverticulum or ileal diverticulum. • Enterocystoma or vitelline cyst • Umbilical fistula or vitelline fistula. q Gut Rotation Defects Malrotation of the intestinal loop may result in twisting of the intestine [volvulus] and a compromise of the blood supply. A. Rotation amount to 90° only of the primary intestinal loop. The colon is on the left side of the abdomen, and the small intestinal loops are on the right. The ileum enters the cecum from the right, and you have left-side colon. B. Reversed rotation of the intestinal loop ocurrs when the primary intestinal loop is rotated 90° clockwise [reversed rotation]. The transverse colon passes behind the duodenum. q Gut Atresia and Stenosis Atresias and stenoses may occur anywhere along the intestine. Most occur in the duodenum, fewest occur in the colon, and equal numbers occur in the jejunum and ileum. Atresias in the upper duodenum are probably due to a lack of recanalization. From the distal portion of the duodenum caudally, however, stenoses and atresias were thought to be caused by vascular “accidents” that resulted in compromised blood flow and tissue necrosis in a section of the gut resulting in the defects.

Development of Midgut

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