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The midgut is the middle segment of the early gut tube that ultimately produces most of the small intestine and a significant portion of the large intestine. The proper development of the midgut is crucial to the overall function of the human digestive tract. Early in development, the incipient embryonic gut is an endodermal-derived tube divided based on its arterial supply into three segments: the foregut, midgut, and hindgut. Each segment of the primordial gut gives rise to different parts of the adult gastrointestinal tract and outgrowth organs. Midgut-derived organs are supplied by the superior mesenteric artery and include the duodenum distal to the entrance of the bile duct, the jejunum, ileum, cecum, appendix, ascending colon, and proximal two-thirds of the transverse colon. These organs are integral components of the digestive tract and collectively contribute to the digestion and absorption of nutrients, water, and salts that humans consume through their diet. The midgut development is a captivating process involving the rapid growth of the gut segment that outpaces the space available within the developing abdominal cavity. This forces the midgut to herniate outside of the abdominal cavity at the future site of the umbilicus and into the umbilical cord. Later in development, the segments along the midgut loop will derive structures of the mature gastrointestinal tract.[1]

The midgut is subject to a variety of potential pathologies during embryologic development. Intestinal atresia can present with bilious vomiting early in the life of the newborn. Duodenal atresia is often caused by a failure of the intestine to fully recanalize, leaving a non-patent lumen, and correlates with various conditions, including Down syndrome. A double bubble sign on a radiograph will be present due to the dilated stomach and proximal duodenum. Jejunal and ileal atresia is often due to a disruption in the blood supply of the mesenteric vessels, leading to bowel discontinuity and ischemic necrosis.[19] Midgut volvulus is a condition that infants can be predisposed to by intestinal malrotation during GI tract development. Malrotation can happen anywhere along the bowel and increases the likelihood that the child will advance to acquiring a midgut volvulus, which is the intestines winding upon one another; this condition can include disruption of blood flow to the involved segments of the bowel, leading to necrosis.[20] The vitelline duct, or omphaloenteric duct, omphalomesenteric duct, or yolk stalk, is a tubular structure that allows communication between the midgut and the fetal yolk sac. This channel typically separates and disappears spontaneously during weeks 5-9 in fetal development. Pathologies associated with abnormal separation and obliteration of the vitelline duct include vitelline cyst, vitelline fistula, and Meckel diverticulum. Meckel diverticulum occurs when the vitelline duct is not entirely obliterated, and the remnant forms an outpouching of the ileum. Meckel diverticulum is the most common congenital anomaly of the gastrointestinal tract. It is usually located within two feet of the ileocecal valve, is roughly two inches long, is twice as common in males, and occurs in approximately 2% of newborn babies.[21] It is a true diverticulum because it contains all the layers of the small intestine. Still, it can also possess ectopic tissue from the pancreas or gastric mucosa, leading to ulceration and bleeding.[18]

The vitelline duct, or omphaloenteric duct, omphalomesenteric duct, or yolk stalk, is a tubular structure that allows communication between the midgut and the fetal yolk sac. This channel typically separates and disappears spontaneously during weeks 5-9 in fetal development. Pathologies associated with abnormal separation and obliteration of the vitelline duct include vitelline cyst, vitelline fistula, and Meckel diverticulum. Meckel diverticulum occurs when the vitelline duct is not entirely obliterated, and the remnant forms an outpouching of the ileum. Meckel diverticulum is the most common congenital anomaly of the gastrointestinal tract. It is usually located within two feet of the ileocecal valve, is roughly two inches long, is twice as common in males, and occurs in approximately 2% of newborn babies.[21] It is a true diverticulum because it contains all the layers of the small intestine. Still, it can also possess ectopic tissue from the pancreas or gastric mucosa, leading to ulceration and bleeding.[18] Intussusception is the telescoping or invagination of one segment of the bowel into another. It most commonly appears at the junction of the ileum and cecum. Intussusception is most often due to a lead point such as a malignancy or Meckel diverticulum. Associations with recent viral infections leading to hypertrophy of ileal lymphoid follicles and the rotavirus vaccine have also been implicated.[22] Patients will present with intermittent severe abdominal pain, red currant jelly stools, and an ultrasonographic finding known as a “target sign.” Gastroschisis and omphalocele are conditions that can occur when the midgut fails to return to the peritoneal cavity after its herniation and rotation around week six of gestation. The critical difference between gastroschisis and omphalocele is that an omphalocele is herniated bowel covered by the amnion. In contrast, in gastroschisis, the viscera are not covered and have direct exposure to amniotic fluid. Omphalocele is due to an abdominal wall defect. In comparison, gastroschisis is thought to be due to herniated bowel not being able to return to the abdominal cavity correctly.[23] A congenital umbilical hernia can arise when the umbilical ring fails to close following the herniation of the midgut.