3: Small intestine

Small intestine: anatomy and structural anomalies

Deborah C. Rubin1 and Jacob C. Langer2

1Washington University School of Medicine, St Louis, MO, USA

2University of Toronto and Hospital for Sick Children, Toronto, ON, Canada

Embryology of the small intestine

The primitive human gut forms when the dorsal part of the yolk sac is incorporated into the embryo at 4 weeks’ development, giving rise to the foregut, midgut, and hindgut. The foregut is the progenitor of the esophagus, stomach, duodenum up to the biliary duct ampulla, pharynx, respiratory tract, liver, pancreas, and biliary tract. The midgut gives rise to the duodenum distal to the common bile duct, jejunum, ileum, cecum, appendix, ascending colon, and one‐half to two‐thirds of the transverse colon. The rest of the colon and superior anal canal are derived from the hindgut.

The gut endoderm is the precursor of the gastrointestinal tract epithelium. Its endothelium arises from the ectoderm of the stomodeum and proctodeum as well as the endoderm. The splanchnic mesenchyme supplies the muscular and connective tissue components of the gastrointestinal tract. The midgut first freely communicates with the yolk sac and then narrows to be connected by the omphalomesenteric or vitelline duct. The primitive gut forms a U‐shaped loop that grows so rapidly compared with the embryo that it herniates into the umbilical cord at the sixth week of gestation (Figure 3.1). The proximal limb of the loop elongates into multiple intestinal loops, whereas the distal limb simply develops into the cecal diverticulum. The first stage of rotation is 90° counterclockwise around the superior mesenteric artery axis. At 10 weeks, the intestines return into the abdominal cavity and rotate a further 180°counterclockwise in the second stage. Finally, the cecum and appendix descend from the right upper quadrant to the right lower quadrant, and the proximal part of the colon elongates to form the hepatic flexure and ascending colon (third stage of rotation). Fixation occurs as the ascending colonic mesentery fuses with the parietal peritoneum and becomes fixed retroperitoneally. The mesentery of the small intestine attains a broad‐based attachment to the posterior abdominal wall and extends from the duodenojejunal junction to the ileocecal region. The end‐result of this process is the normal location of the small and large intestines.

Congenital anomalies

A brief review of the main features of the common congenital anomalies is presented and illustrated with pictures of surgical specimens.

Meckel diverticulum

Meckel diverticulum is the most common congenital anomaly of the gastrointestinal tract. It results from failure of the vitelline duct to be completely resorbed (Figure 3.2). Large autopsy series indicate a 2–3% prevalence of Meckel diverticulum in the general population. Meckel diverticula are true diverticula, containing all layers of the bowel from serosa to mucosa. Heterotopic tissue is present approximately 50% of the time and includes gastric mucosa, pancreatic tissue, and, less commonly, colonic mucosa, Brunner glands, and jejunal or hepatobiliary tissue. The presence of heterotopic mucosa correlates with increased risk for symptomatic, complicated Meckel diverticulum.

The complications of Meckel diverticulum include bleeding, intestinal obstruction, diverticulitis, perforation, and carcinoma. The frequency of specific complications varies between adult and pediatric patients. Among children, the most common complications are gastrointestinal bleeding and obstruction. For adults, intestinal obstruction is by far the most frequent complication and gastrointestinal bleeding is rare.

The diagnosis of Meckel diverticulum remains a challenge. Sodium pertechnetate technetium‐99m radionuclide scanning is particularly useful in the care of children. This isotope is taken up into gastric mucosal cells and can help detect Meckel diverticula that contain ectopic gastric mucosa. Pretreatment with a histamine‐2 receptor blocker optimizes accuracy. Other examinations include enteroclysis and angiography, which may show the vitelline artery. Magnetic resonance enterography and capsule endoscopy have also been used for occult gastrointestinal bleeding when the Meckel scan is negative.

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Nov 27, 2022 | Posted by in GASTROENTEROLOGY | Comments Off on 3: Small intestine

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