Harvard-MIT Division of Health Sciences and Technology HST.121: Gastroenterology, Fall 2005 Instructors: Dr. Jonathan Glickman Overview of Gastrointestinal EmbryologyThe Primitive Gut • The primitive gut forms during the 4th week of gestation when the flat embryonic disc folds in median and horizontal planes to form a tubular structure that incorporates part of the yolk sac into the embryo • Ventral folding of lateral sides forms the midgut • Ventral folding of cranial and caudal ends (head and tail folds) form the foregut and the hindgutFolding of the Embryonic Disc Figure removed due to copyright reasons. Please see: Moore, Keith L., and T. V. N. Persaud. The Developing Human: Clinically Oriented Embryology. Philadelphia, PA: W.B. Saunders Company, 1998. ISBN: 0721669743.Formation of human gastrointestinal tract (A) Stomach Heart Vitelline duct Allantois Cloacal membrane Liver bud Midgut Hindgut Image by MIT OCW.The Foregut • The foregut gives rise to the: – Pharynx – Lower respiratory system – Esophagus – Stomach – Proximal duodenum – Liver and the biliary tree – PancreasPartitioning of the foregut by the tracheoesophageal septum Figure removed due to copyright reasons. Please see: Sadler, Thomas W. Langman's Medical Embryology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins, 1990. ISBN: 0683074938.Errors of the Foregut Development Errors in partitioning of the laryngo-tracheal tube from the esophagus by the tracheo-esophageal septum result in various forms of esophageal atresia and tracheo-esophageal fistulas or EA/TEF (1 in 3000-4500 8 5 % 8 -1 0 % 3 -4 % Figure by MIT OCW. live births, M>F)Tracheoesophageal fistula Images removed due to copyright reasons.Development of the Lumina Epithelial Plug Vacuolization Lumen RecanalizationAbnormal Development of the Lumina Epithelial Plug Vacuolization Lumen Atresia Stenosis Duplication, Cyst or Septum RecanalizationJejunoileal atresia Image removed due to copyright reasons.Congenital Hypertrophic Pyloric StenosisErrors of the Foregut Development • Congenital Hypertrophic Pyloric Stenosis is the most common congenital anomaly of thestomach and occurs in 1-8:1000 live births with a 4-6:1 M:F ratio • Pyloric stenosis is a multifactorial andprogressive disease that classically presentswith non-bilious projectile vomiting in the firstfew weeks of lifeThe Liver and the Pancreas Figure removed due to copyright reasons. Please see: Moore, Keith L., and T. V. N. Persaud. The Developing Human: Clinically Oriented Embryology. Philadelphia, PA: W.B. Saunders Company, 1998. ISBN: 0721669743.Errors in Pancreatic Development • Annular pancreas • Pancreas divisum • Ectopic pancreatic tissueThe Midgut • The midgut gives rise to: – Distal duodenum – Jejunum and ileum – Appendix – Ascending colon – Proximal transverse colonEpithelial cytodifferentiation Images removed due to copyright reasons.Intestinal epithelial differentiation Images removed due to copyright reasons.The Midgut Rotation • The midgut forms a U-shaped loop that herniates into the umbilical cord during the 6th weeks of gestation • While in the umbilical cord, the midgut loop rotates 90 degrees • During the 10th week of gestation, the midgut loop returns to the abdomen, rotating an additional 180 degreesErrors in Midgut Rotation Anything can happen, but it usually doesn’t!Errors in Midgut Development • Omphaloceles result from failure of the intestines to return to the abdominal cavity • Umbilical hernias occur when intestines do return to the abdomen, but later herniate through the umbilicus • Gastroschisis is a linear defect of the abdominal wall that permits extrusion of the viscera without involving the umbilicusImage removed due to copyright reasons. Infant with gastroschisisInfant with omphalocele Image removed due to copyright reasons.DIFFERENCES BETWEEN GASTROSCHISIS AND OMPHALOCELE Gastroschisis Omphalocele Maternal age Younger Older Associated anomalies 10% (usually intestinal atresia) 50% (structural and chromosomal) Sac covering abdominal contents No Yes Liver out through abdominal wall defect No Often Intestinal dysfunction (hypomotility and malabsorption) Yes NoRemnants of the omphalomesenteric duct (yolk stalk) are found in 1-4% of infants, making them the most common congenital anomaly of the GI tract Figure removed due to copyright reasons. Please see: Moore, Keith L., and T. V. N. Persaud. The Developing Human: Clinically Oriented Embryology. Philadelphia, PA: W.B. Saunders Company, 1998. ISBN: 0721669743.Meckel’s Diverticulum • Meckel’s or ileal diverticulum accounts for up to 80%of omphalomesenteric remnants • Typical Meckel’s diverticulum measures 3-5 cm, and is located in the anti-mesenteric wall of the ileum 40-50 cm from the ileocecal valve • Most symptomatic cases present in childhood • The M:F incidence ratio is ~1, but there is a 3:1 M:F ratio in clinically symptomatic casesThe Hindgut • The hindgut gives rise to: – Distal transverse colon – Descending colon, sigmoid, and rectum – Proximal anal canal (superior to the pectinateline) • The caudal part of the hindgut known as thecloaca, is divided by the urorectal septuminto the urogenital sinus and the rectumPartitioning of the Cloaca by the Urorectal Septum Figure removed due to copyright reasons. Please see: Moore, Keith L., and T. V. N. Persaud. The Developing Human: Clinically Oriented Embryology. Philadelphia, PA: W.B. Saunders Company, 1998. ISBN: 0721669743.Errors in Hindgut Development Figure removed due to copyright reasons. Please see: Moore, Keith L., and T. V. N. Persaud. The Developing Human: Clinically Oriented Embryology. Philadelphia, PA: W.B. Saunders Company, 1998. ISBN: 0721669743.Hirschsprung’s Disease • HD is the partial or total absence of autonomic ganglia resulting from failure of migration of the neural crest cells into the colonic wall during 5th-7th week of gestation • With an incidence of 1 in 5000 liver births, HD is the most common cause of neonatal colonic obstruction, and can result in congenital megacolon • HD has been associated with several genetic abnormalities including Trisomy 21, mutations of the RET gene and the endothelin receptor type B geneIntraoperative photograph of Hirschsprung's disease Image removed due to copyright reasons.Hirschsprung’s Disease Figure removed due to copyright reasons. Please see: Fenoglio-Preiser,