Development of the GI Tract Block III 2023 PDF

Development of the GI tract Block III – 2023 Author: Dr. Rocha Class objectives • Know the embyologic division of the gut tube • Know the embryologic layer origin of the structures of the GI tract • Know the classification, location and parts of the mesenteries • Know the development of the differ...

Development of the GI tract Block III – 2023 Author: Dr. Rocha Class objectives • Know the embyologic division of the gut tube • Know the embryologic layer origin of the structures of the GI tract • Know the classification, location and parts of the mesenteries • Know the development of the different structures of the foregut: • Esophagus, stomach, duodenum, liver, gallbladder and pancreas • Know the stages in which the midgut develops: • Herniation, rotation and retraction • Know how the hindgut develops Divisions of the Gut Tube 1. Foregut a. b. Pharynx. From the oropharyngeal membrane to the respiratory diverticulum Reminder of the foregut. From the respiratory diverticulum to the liver bud 2. Midgut a. From the liver bud to the junction of the right two thirds and left third of the transverse colon in the adult 3. Hindgut a. From the left third of the transverse colon to the cloacal membrane Germ layers derivatives in the GI tract • Ectoderm • Enteric nervous system • Mesoderm • Mesenteries (Peritoneum) • Stroma (connective tissue) of accesory organs • Muscles and connective tissue • Endoderm • Epithelial lining of the digestive tract • Parenchima (specific cells) of the accesory organs (i.e. salivary glands, liver, pancreas, gallbladder) Dorsal mesentery • Extends from the lower end of the esophagus to the cloaca. • Stomach – d. mesogastrium (greater omentum) • Duodenum – d. mesoduodenum • Jejunum and ileum – mesentery • Colon – d. mesocolon Ventral mesentery • Derived from the septum transversum • Growth of the liver into the mesenchyme of the septum transversum divides the ventral mesentery into: (a) the lesser omentum, extending from the lower portion of the esophagus, the stomach, and the upper portion of the duodenum to the liver and (b) the falciform ligament, extending from the liver to the ventral body wall Foregut Esophagus • When the embryo is approximately 4 weeks old, the respiratory diverticulum (lung bud) appears at the ventral wall of the foregut at the border with the pharyngeal gut • The tracheoesophageal septum gradually partitions this diverticulum from the dorsal part of the foregut Stomach ● ● Appears as a fusiform dilation of the foregut in the 4th week It is formed by rotating around a longitudinal and an anterioposterior axis Duodenum ● ● ● It is formed by the terminal part of the foregut and the cephalic part of the midgut On the opposite side to where the liver bud orginates The rotation of the stomach and the rapid growth of the head of the pancreas, swings the duodenum to the right side and gives it its C-shape form Liver and Gallbladder ● ● ● The liver bud appears in the middle of the 3rd week as an outgrowth of the endodermal epithelium at the distal end of the foregut Hepatic cells proliferate penetrating the septum transversum forming the liver The connection between the hepatic bud and the foregut forms the bile duct that in turn, forms another outgrowth that gives rise to the cyst duct and gallbladder Liver Epithelial liver cords intermingle with the vitelline and umbilical veins, which form hepatic sinusoids. ● Liver cords differentiate into the parenchyma (liver cells) and form the lining of the biliary ducts. ● Hematopoietic cells, Kupffer cells, and connective tissue cells are derived from mesoderm of the septum transversum. PANCREAS • It is formed by 2 buds: Dorsal and Ventral, both have their origin in the endoderm of the duodenum. • The Ventral Bud forms the “minor pancreas” that will become the inferior portion of the head of the pancreas • When the duodenum rotates to the right, the ventral bud moves dorsally fusing with the dorsal bud to form the pancreas. • The Pancreatic Islets (Langerhans) are developed from the parenchymal pancreatic tissue in the 3rd month of gestation. • INSULINE secretion begins at the 5th month. Midgut Midgut • In the 5-week embryo, the midgut is suspended from the dorsal abdominal wall by a short mesentery and communicates with the yolk sac by way of the vitelline duct • In the adult, the midgut begins immediately distal to the entrance of the bile duct into the duodenum and terminates at the junction of the proximal two thirds of the transverse colon with the distal third. • Over its entire length, the midgut is supplied by the superior mesenteric artery. • Three processes: herniation, rotation and retraction Physiological herniation • Development of the primary intestinal loop is characterized by rapid elongation, particularly of the cephalic limb. • As a result of the rapid growth and expansion of the liver, the abdominal cavity temporarily becomes too small to contain all the intestinal loops, and they enter the extraembryonic cavity in the umbilical cord during the sixth week of development. Rotation of the Midgut • Coincident with growth in length, the primary intestinal loop rotates around an axis formed by the superior mesenteric artery • When viewed from the front, this rotation is counterclockwise, and it amounts to approximately 270° when it is complete • Even during rotation, elongation of the small intestinal loop continues, and the jejunum and ileum form a number of coiled loops • The large intestine likewise lengthens considerably but does not participate in the coiling phenomenon. • Rotation occurs during herniation (about 90°), as well as during return of the intestinal loops into the abdominal cavity (remaining 180°) Retraction • During the 10th week, herniated intestinal loops begin to return to the abdominal cavity. • The proximal portion of the jejunum, the first part to reenter the abdominal cavity, comes to lie on the left side • The cecal bud, is the last part of the gut to reenter the abdominal cavity. Temporarily, it lies in the right upper quadrant directly below the right lobe of the liver. • From here, it descends into the right iliac fossa, placing the ascending colon and hepatic flexure on the right side of the abdominal cavity • During this process, the distal end of the cecal bud forms a narrow diverticulum, the appendix Hindgut Hindgut • Gives rise to the distal third of the transverse colon, the descending colon, the sigmoid, the rectum, and the upper part of the anal canal. • At the end of the seventh week, the cloacal membrane ruptures, creating the anal opening for the hindgut • The upper part (two-thirds) of the anal canal is derived from endoderm of the hindgut; the lower part (one-third) is derived from ectoderm around the proctodeum • Since the caudal part of the anal canal originates from ectoderm, it is supplied by the inferior rectal arteries, branches of the internal pudendal arteries. • However, the cranial part of the anal canal originates from endoderm and is therefore supplied by the superior rectal artery, a continuation of the inferior mesenteric artery, the artery of the hindgut. Hindgut Video BIBLIOGRAPHY • Chapter 15 • LANGMAN'S MEDICAL EMBRYOLOGY. Sadler, T W (Thomas W); Langman, Jan. Medical embryology. 14th ed. / T.W. Sadler. Philadelphia : Wolters Kluwer Health/Lippincott Williams & Wilkins, c2018.