Embryology of GIT Lecture Notes PDF

EMBROYLOGY OF GIT 1-2 2nd stage Alkindy Medical College Baghdad University By: Dr. Mohammed Emad Ghanem MBCHB, MSC, PHD (Anat,Histo,Embryo)  The primitive gut tube is formed from the incorporation of the dorsal part of the yolk sac into the em...

EMBROYLOGY OF GIT 1-2 2nd stage Alkindy Medical College Baghdad University By: Dr. Mohammed Emad Ghanem MBCHB, MSC, PHD (Anat,Histo,Embryo)  The primitive gut tube is formed from the incorporation of the dorsal part of the yolk sac into the embryo due to the craniocaudal folding and lateral folding of the embryo. The Yolk sac and the allantois remain outside the embryo. The primitive gut tube extends from the oropharyngeal membrane to the cloacal membrane and is divided into the foregut, midgut, and hindgut. In the cephalic and caudal parts of the embryo, the primitive gut forms a blind-ending tube, the foregut and hindgut, respectively. The middle part, the midgut, remains temporarily connected to the yolk sac by means of the vitelline duct, or yolk stalk  Histologically, the general plan of the adult gastrointestinal tract consists of a mucosa (epithelial lining and glands, lamina propria, and muscularis mucosae), submucosa, muscularis externa, and adventitia or serosa. Embryologically, the epithelial lining and glands of the mucosa are derived from endoderm, whereas the other components are derived from visceral mesoderm. Early in development, the epithelial lining the gut tube proliferates rapidly and obliterates the lumen. Later, recanalization occurs. At the end of the first month MESENTERIES Portions of the gut tube and its derivatives are suspended from the dorsal and ventral body wall by mesenteries, double layers of peritoneum that enclose an organ and connect it to the body wall [intraperitoneal]. organs that lie against the posterior body wall and are covered by peritoneum on their anterior surface only are considered [retroperitoneal]. Peritoneal ligaments are double layers of peritoneum (mesenteries) that pass from one organ to another or from an organ to the body wall. Mesenteries and ligaments provide pathways for vessels, nerves, and lymphatic to and from abdominal viscera.  Initially, the foregut, midgut, and hindgut are in broad contact with the mesenchyme of the posterior abdominal wall. By the fifth week, the connection between the gut and the yolk sac is narrowed, the caudal part of the foregut, the midgut, and a major part of the hindgut are suspended from the abdominal wall by the dorsal mesentery. (which extends from the lower end of the esophagus to the cloacal region of the hindgut.) In the region of the stomach, it forms the dorsal mesogastrium or greater omentum; in the region of the duodenum, it forms the dorsal mesoduodenum; and in the region of the colon, it forms the dorsal mesocolon. Dorsal mesentery of the jejunal and ileal loops forms the mesentery proper.  Ventral mesentery, which exists only in the region of the terminal part of the esophagus, the stomach, and the upper part of the duodenum is derived from the septum transversum. Growth of the liver into the mesenchyme of the septum transversum divides the ventral mesentery into: (1) the lesser omentum, extending from the lower portion of the esophagus, the stomach, and the upper portion of the duodenum to the liver and (2) the falciform ligament, extending from the liver to the ventral body wall. The pharyngeal gut, or pharynx, extends from the oropharyngeal membrane to the respiratory diverticulum and is part of the foregut; this section is particularly important for development of the head and neck. I- DERIATIVES OF THE FOREGUT DERIATIVES OF THE FOREGUT are supplied by the celiac trunk. except the intrathoracic portion of esophagus is supplied by other branches of the aorta.  Pharyngeal gut Development of gastrointestinal tract showing the foregut, midgut, and hindgut along with the adult derivatives. lung bud (LB), pharyngeal pouches (1–4), and thyroid diverticulum (TD). E 5 esophagus; ST 5 stomach; HD 5 hepatic diverticulum; GB 5 gall bladder; VP 5 ventral pancreatic bud; DP 5 dorsal pancreatic bud; CA 5 celiac artery; YS 5 yolk sac; VD 5 vitelline duct; AL 5 allantois; SMA 5 superior mesenteric artery; CL 5 cloaca; IMA 5 inferior mesenteric artery.  2- Esophagus: At 4 weeks, the foregut is divided into the esophagus dorsally and the trachea ventrally by the tracheoesophageal folds, which fuse to form the tracheoesophageal septum. The esophagus is initially short but lengthens with descent of the heart and lungs. During development, the endodermal lining of the esophagus proliferates rapidly and obliterates the lumen; later, recanalization occurs.  The muscular coat, which is formed by surrounding splanchnic mesenchyme, is striated in its upper two-thirds and innervated by the vagus; the muscle coat is smooth in the lower third and is innervated by the splanchnic plexus. The stratified squamous epithelium, mucosal glands, and submucosal glands of the definitive esophagus are derived from endoderm. The lamina propria, muscularis mucosae, submucosa, skeletal muscle and smooth muscle of muscularis externa, and adventitia are derived from visceral mesoderm. 3- Stomach Development. A fusiform dilation forms in the foregut in week 4 that gives rise to the primitive stomach. The dorsal part of the primitive stomach grows faster than the ventral part, thereby resulting in the greater and lesser curvatures, respectively. The primitive stomach rotates 90° clockwise around its longitudinal axis. The 90° rotation affects all foregut structures and is responsible for the adult anatomical relationship of foregut viscera. As a result of this clockwise rotation, the dorsal mesentery is carried to the left and eventually forms the greater omentum; the left vagus nerve innervates the ventral surface of the stomach; and the right vagus nerve innervates the dorsal surface of the stomach.  Rotation about the longitudinal axis pulls the dorsal mesogastrium to the left, creating a space behind the stomach called the omental bursa (lesser peritoneal sac) This rotation also pulls the ventral mesogastrium to the right. As this process continues in the fifth week of development, the spleen primordium appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium * primordium : organ or tissue in its earliest recognizable stage of development.  With continued rotation of the stomach, the dorsal mesogastrium lengthens, and the portion between the spleen and dorsal midline swings to the left and fuses with the peritoneum of the posterior abdominal wall. *The posterior leaf of the dorsal mesogastrium and the peritoneum along this line of fusion degenerate. The spleen, which remains intra- peritoneal, is then connected to the body wall in the region of the left kidney by the lienorenal ligament and to the stomach by the gastrolienal ligament  Initially, the pancreas grows into the dorsal mesoduodenum, but eventually, its tail extends into the dorsal mesogastrium. Because this portion of the dorsal mesogastrium (post. leaf) fuses with the dorsal body wall, the tail of the pancreas lies against the dorsal body wall. Then, the posterior leaf of the dorsal mesogastrium and the peritoneum of the posterior body wall degenerate along the line of fusion, the tail of the pancreas is covered by peritoneum on its anterior surface only and therefore lies in a retroperitoneal position. (Organs, such as the pancreas, that are originally covered by peritoneum, but later fuse with the posterior body wall to become retroperitoneal, are said to be secondarily retroperitoneal.)  The dorsal mesogastrium bulges down and continues to grow down and forms a double-layered sac extending over the transverse colon and small intestinal loops like an apron. This double-leafed apron is the greater omentum; later, its layers fuse to form a single sheet hanging from the greater curvature of the stomach. The posterior layer of the greater omentum also fuses with the mesentery of the transverse colon.  The lesser omentum and falciform ligament form from the ventral mesogastrium, which is derived from mesoderm of the septum transversum. When liver cords grow into the septum, it thins to form:(1) the peritoneum of the liver; (2) the falciform ligament, extending from the liver to the ventral body wall; and (3) the lesser omentum, extending from the stomach and upper duodenum to the liver. The free margin of the falciform ligament contains the umbilical vein , which is obliterated after birth to form the round ligament of the liver (ligamentum teres hepatis). The free margin of the lesser omentum connecting the duodenum and liver (hepatoduodenal ligament) contains the bile duct, portal vein, and hepatic artery (portal triad). Also forms the roof of the epiploic foramen of Winslow, which is the opening connecting the omental bursa (lesser sac) with the rest of the peritoneal cavity (greater sac)  The surface mucous cells lining the stomach, mucous neck cells, parietal cells, chief cells, and enteroendocrine cells comprising the gastric glands of the definitive stomach are derived from endoderm. The lamina propria, muscularis mucosae, submucosa, smooth muscle layers of the muscularis externa, and serosa of the definitive stomach are derived from visceral mesoderm. Duodenum The terminal part of the foregut and the cephalic part of the midgut form the duodenum. The junction of the two parts is directly distal to the origin of the liver bud. As the stomach rotates, the duodenum becomes a C-shaped loop and rotates to the right. This rotation, together with rapid growth of the head of the pancreas, swings the duodenum from its initial midline position to the right side and pressed agains the post abdominal wall,. then the right surface of the dorsal mesoduodenum fuses with adjacent peritoneum, then both layers disappears so that the duodenum and head of the pancreas become fixed in a retroperitoneal position. The dorsal mesoduodenum disappears entirely except in the region of the pylorus of the stomach, where a small portion of the duodenum (duodenal cap) retains its mesentery and remains intraperitoneal  During the second month, the lumen of the duodenum is obliterated by proliferation of cells in its walls. However, the lumen is recanalized shortly thereafter Because the foregut is supplied by the celiac artery and the midgut is supplied by the superior mesenteric artery, the duodenum is supplied by branches ofboth arteries 4- Liver 1. Development. The endodermal lining of the foregut forms an outgrowth (hepatic diverticulum), “in the middle of the third week”, into the surrounding mesoderm of the septum transversum through induction by fibroblast growth factors (FGFs) FGF- 1, FGF-2, and FGF-8 released by cardiac mesoderm (which is in close vicinity). The mesoderm of the septum transversum is involved in the formation of the diaphragm. While hepatic cells continue to penetrate the septum, the connection between the hepatic diverticulum and the foregut (duodenum) narrows, forming the bile duct. A small ventral outgrowth is formed by the bile duct, and this outgrowth gives rise to the gallbladder and the cystic duct Cords of hepatoblasts (called hepatic cords) from the hepatic diverticulum grow into the mesoderm of the septum transversum, where critical hepatoblast/mesoderm interactions occur. The hepatic cords arrange themselves around the vitelline veins and umbilical veins, which course through the septum transversum, and form the hepatic sinusoids.  Due to the tremendous growth of the liver, the liver bulges into the abdominal cavity which stretches the septum transversum to form the ventral mesentery. The ventral mesentery consists of the falciform ligament and the lesser omentum. The falciform ligament contains the left umbilical vein, which regresses after birth to form the ligamentum teres. The lesser omentum can be divided into the hepatogastric ligament and hepatoduodenal ligament. The hepatoduodenal ligament contains the bile duct, portal vein, and hepatic artery (i.e., portal triad).  Mesoderm on the surface of the liver differentiates into visceral peritoneum except on its cranial surface where the liver remains in contact with the rest of the original septum transversum. This portion of the septum will form the central tendon of the diaphragm. The surface of the liver that is in contact with the future diaphragm is never covered by peritoneum; it is the bare area of the liver Functions of Liver In the 10 week of development, the weight of the liver is 10% of the th total body weight. Although this may be attributed partly to the large numbers of sinusoids, another important factor is its hematopoietic function. This activity gradually subsides during the last 2 months of intrauterine life, and only small hematopoietic islands remain at birth, the weight of the liver is then only 5% of the total body weight. Formation of bile at 12th week and can enter the GIT making its content dark green, Because of positional changes of the duodenum, the entrance of the bile duct gradually shifts from its initial anterior position to a posterior one, and consequently, the bile duct passes behind the duodenum  Sources. The hepatocytes and simple columnar or cuboidal epithelium lining the biliary tree of the definitive liver are derived from endoderm. Kupffer cells, hematopoietic cells, endothelium of the sinusoids, and fibroblasts (connective tissue) of the definitive liver are derived from mesoderm. Congenital malformations of the liver are rare 5- Gallbladder and extrahepatic bile ducts Development. The connection between the hepatic diverticulum and the foregut narrows to form the primitive bile duct. An outgrowth from the primitive bile duct gives rise to the gallbladder rudiment and cystic duct. The cystic duct divides the primitive bile duct into the common hepatic duct and the definitive bile duct. During development, the endodermal lining of the gallbladder and extrahepatic bile ducts proliferates rapidly and obliterates the lumen; later, recanalization occurs.  Sources. The simple columnar epithelium lining the definitive gallbladder and simple columnar or cuboidal epithelium lining the definitive extrahepatic bile ducts are derived from endoderm. The lamina propria, muscularis externa, and adventitia of the definitive gallbladder are derived from visceral mesoderm. 6- Pancreas 1. Development. The dorsal pancreatic bud is a direct outgrowth of foregut endoderm, whose formation is induced by the notochord. The ventral pancreatic bud is a direct outgrowth of foregut endoderm, whose formation is induced by hepatic mesoderm. Within both pancreatic buds, endodermal tubules surrounded by mesoderm branch repeatedly to form acinar cells and ducts (i.e., exocrine pancreas). Isolated clumps of endodermal cells bud from the tubules and accumulate within the mesoderm to form islet cells (i.e., endocrine pancreas). The islet cells form in the following sequence (first → last): alpha cells (glucagon) beta cells (insulin) delta cells (somatostatin) and PP cells (pancreatic polypeptide).  Because of the 90° clockwise rotation of the duodenum, the ventral bud rotates dorsally and fuses with the dorsal bud to form the definitive adult pancreas. The ventral bud forms the uncinate process and a portion of the head of the pancreas. The dorsal bud forms the remaining portion of the head, body, and tail of the pancreas. The main pancreatic duct is formed by the anatomosis of the distal two thirds of the dorsal pancreatic duct (the proximal one-third regresses) and the entire ventral pancreatic duct (48% incidence). The main pancreatic duct and common bile duct form a single opening (hepatopancreatic ampulla of Vater) into the duodenum at the tip of a major papillae (hepatopancreatic papillae)  pancreatic islets (of Langerhans) develop in the third month of fetal life, Insulin secretion begins at approximately the fifth month. Sources. The acinar cells, islet cells, and simple columnar or cuboidal epithelium lining the pancreatic ducts of the definitive pancreas are derived from endoderm. The surrounding connective tissue and vascular components of the definitive pancreas are derived from visceral mesoderm. THANK YOU