Midgut

Anatomy of midgut

1. Midgut Lower duodenum Jejunum Ileum Caecum Appendix Ascending colon Proximal transverse 2/3 colon

2. DISTAL DUODENUM Distal or lower duodenum arises from the cranial most portion of the midgut and is served by anterior and posterior branches of the inferior pancreaticoduodenal artery , which is a branch of the superior mesentery artery. As with the rest of the duodenum, becomes secondarily retroperitoneal as with the rest of the entire gi tract, the lumen is obliterated transiently during development and then recanalizes . Failure to recanalize the duodenum can result in stenosis (narrowing) or atresia (complete blockage )

3. Development of the midgut and colon Herniation and rotation: Growth of the GI tract exceeds volume of abdominal cavity so the tube herniates through umbilicus While herniated, gut undergoes a primary rotation (fig B) of 90° “counterclockwise” (when looking at the embryo); this corresponds with the rotation of the stomach, and positions the appendix on the left. The primary rotation also brings the right vagus n. to the FRONT (hence the change in its name to ANTERIOR vagus n. With the growth of the embryo, the abdominal cavity expands thus drawing the gut tube back within the abdominal cavity and causing an additional, secondary rotation (fig C) of 180° CCW (positioning the appendix on the RIGHT) Once in the abdominal cavity, the colon continues to grow in length, pushing the appendix to its final position in the lower right quadrant. Note the attachment of the vitelline duct to the gut at the region of the ileum. The duct normally regresses during development, but not always….

4. 1. Development . The midgut forms a U-shaped loop ( midgut loop ) that herniates through the primitive umbilical ring into the extraembryonic coelom (i.e., physiological umbilical herniation ) beginning at week 6. The midgut loop consists of a cranial limb and a caudal limb. The cranial limb forms the jejunum and upper part of the ileum . The caudal limb forms the cecal diverticulum , from which the cecum and appendix develop; the rest of the caudal limb forms the lower part of the ileum, ascending colon, and proximal two thirds of the transverse colon. The midgut loop rotates a total of 270_ counterclockwise around the superior mesenteric artery as it returns to the abdominal cavity, thus reducing the physiological herniation , around week 12.

5. 2. Sources . Simple columnar absorptive cells lining midgut derivatives, goblet cells, Paneth cells, and enteroendocrine cells comprising the intestinal glands are derived from endoderm. The lamina propria , muscularis mucosae , submucosa , and inner circular and outer longitudinal smooth muscle of the muscularis externa and serosa are derived from visceral mesoderm.

6. 3. Clinical considerations a. Omphalocele : occurs when abdominal contents herniates through the umbilical ring and persists outside the body, covered variably by a translucent peritoneal membrane sac (a light gray, shiny sac) protruding from the base of the umbilical cord. Large omphaloceles may contain stomach, liver, and intestines. Small omphaloceles contain only intestines. Omphaloceles are usually associated with other congenital anomalies (e.g., trisomy 13, trisomy 18, or Beckwith- Wiedemann syndrome) and with increased levels of alfa - fetoprotein.

7. b. Gastroschisis : occurs when there is a defect in the ventral abdominal wall, usually to the right of the umbilical ring, through which there is a massive evisceration of intestines (other organs may also be involved). The intestines are not covered by a peritoneal membrane, are directly exposed to amniotic fluid, are thickened, and are covered with adhesions.

8. c. Ileal diverticulum ( Meckel’s diverticulum ): occurs when a remnant of the vitelline duct persists, thereby forming an outpouching located on the antimesenteric border of the ileum. The outpouching may connect to the umbilicus via a fibrous cord or fistula. A Meckel’s diverticulum is usually located about 30 cm proximal to the ileocecal valve in infants and varies in length from 2 to 15 cm. Heterotopic gastric mucosa may be present, which leads to ulceration, perforation, or gastrointestinal bleeding, especially if a large number of parietal cells are present. It is associated clinically with symptoms resembling appendicitis and bright-red or dark-red stools (bloody).

9. d. Nonrotation of the midgut loop: occurs when the midgut loop rotates only 90_ counterclockwise, thereby positioning the small intestine entirely on the right side and the large intestine entirely on the left side, with the cecum located either in the left upper quadrant or the left iliac fossa . Note the small intestines (SI) on the right side and the large intestines (LI) on the left side. e. Malrotation of the midgut loop: occurs when the midgut loop undergoes only partial counterclockwise rotation. This results in the cecum and appendix lying in a subpyloric or subhepatic location and the small intestine being suspended by only a vascular pedicle (i.e., not a broad mesentery). A major clinical complication of malrotation is volvulus (twisting of the small intestines around the vascular pedicle), which may cause necrosis due to compromised blood supply. (Note: The abnormal position of the appendix due to malrotation of the midgut should be considered when diagnosing appendicitis).

10. g. Intestinal atresia and stenosis : Atresia occurs when the lumen of the intestines is completely occluded, whereas stenosis occurs when the lumen of the intestines is narrowed. The causes of these conditions seem to be both failed recanalization and/or an ischemic intrauterine event (“vascular accident”).

11. i . Intussusception : occurs when a segment of bowel invaginates or telescopes into an adjacent bowel segment, leading to obstruction or ischemia. This is one of the most common causes of obstruction in children younger than 2 years of age, is most often idiopathic, and is most commonly involves the ileum and colon (i.e., ileocolic ). It is associated clinically with acute onset of intermittent abdominal pain, vomiting, bloody stools, diarrhea, and somnolence.

12. j. Retrocecal and retrocolic appendix occurs when the appendix is located on the posterior side of the cecum or colon, respectively. These anomalies are very common and important to remember during appendectomies. Note: The appendix is normally found on the medial side of the cecum . Derivatives of the hindgut are supplied by the inferior mesenteric artery . A. Distal one third of the transverse colon, descending colon, sigmoid colon. 1. Development. The cranial end of the hindgut develops into the distal one third of the transverse colon, descending colon, and sigmoid colon. The terminal end of the hindgut is an endoderm-lined pouch called the cloaca , which contacts the surface ectoderm of the proctodeum to form the cloacal membrane.

13. Defects associated with gut herniation and rotation: vitelline duct abnormalities Vitelline duct abnormalities of some sort occur in ~2% of all live births. Note that these aberrant structures are almost always found along the ileal portion of the GI tract. Langman’s fig 14-32

14. Defects associated with gut herniation and rotation: oomphalocoele Langman’s fig 14-31

15. Midgut Anomalies Clinical correlation – Duodenal atresia is due to failed canalization. Omphalocele results from failure of the midgut loop to return to the abdomen. Meckel’s diverticulum occurs when a remnant of the yolk sac (Vitelline duct) persists. Malrotation occurs if the midgut does not complete the rotation prior to returning to the abdomen.

16. Malrotation Slide from S Baker, MD

17. Defects associated with gut herniation and rotation: abnormal rotation Absent or incomplete secondary rotation Reversed secondary rotation (90 CCW primary rotation occurs as usual but followed by abnormal 180 CW rotation. Net rotation is 90° CW; viscera are in their normal location, but note that the duodenum is anterior to the transverse colon) Langman’s fig 14-33

18. Defects associated with gut herniation and rotation: volvulus Fixation of a portion of the gut tube to the body wall; subsequent rotation causes twisting of the tube, possibly resulting in stenosis and/or ischemia. Carlson fig 15-13