Embryology of the Digestive System PDF

Embryology L10 DIGESTIVE SYSTEM  DIVISIONS OF THE GUT TUBE Development of the primitive gut tube and its derivatives is usually discussed in four sections: 1-the pharynx, extends from the oropharyngeal membrane to the respiratory diverticulum and is part of the foregut. 2- the rem...

Embryology L10 DIGESTIVE SYSTEM  DIVISIONS OF THE GUT TUBE Development of the primitive gut tube and its derivatives is usually discussed in four sections: 1-the pharynx, extends from the oropharyngeal membrane to the respiratory diverticulum and is part of the foregut. 2- the remainder of the foregut. 3- the midgut. 4- hindgut. Endoderm forms the epithelial lining of the digestive tract. Muscle, connective tissue, and peritoneal components of the wall of the gut is derived from visceral mesoderm.  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. Such organ are called intraperitoneal, whereas organs that lie against the posterior body wall and are covered by peritoneum on their anterior surface only (e.g. kidneys) 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 lymphatics 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, however, the connecting tissue bridge has narrowed, and 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 form the dorsal mesogastrium or greater omentum; in the region of the duodenum, it form 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.  MESENTERIES 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 mesentry 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 (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. In this manner, the foregut divided into a ventral portion, the respiratory primordium, and a dorsal portion, the esophagus. The muscular coat, which is formed by surrounding splanchnic mesenchyme, is striated in its upper two-thirds and innervated by the vagus; the muscle coatis smooth in the lower third and is innervated by the splanchnic plexic.  FOREGUT STOMACH: In the fourth week of development, the stomach appears as a fusiform dilation of the foregut. During the following weeks, its appearance and position change greatly as a result of the different rates of growth in various regions of its wall and the changes in position of surrounding organs. Positional changes of the stomach are most easily explained by assuming that it rotates around a longitudinal and an anteroposterior axis. The stomach rotates 90 o clockwise around its longitudinal axis, causing its left side to face anteriorly and its right side to face posteriorly. Hence, the left vagus nerve, initially innervating the left side of the stomach, now innervates the anterior wall; similarly, the right nerve innervates the posterior wall, during this rotation, the original posterior wall of the stomach grows faster than the anterior portion, forming the greater and lesser curvature. The cephalic and caudal ends of the stomach originally lie in the midline, but during further growth, the stomach rotates around an anteropoaterior axis, such that the caudal or pyloric part move to the right and upward, and the cephalic or cardiac portion move to the left and slightly downward. The spleen primordium appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium. The spleen, which remains intraperitoneal, 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.  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 takes on the form of 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 of the abdominal cavity. The duodenum and head of the pancreas press against the dorsal body wall. The duodenum and head of the pancreas become fixed in a retroperitoneal position. During the second month, the lumen of the duodenum is obliterated by proliferation of cells in its wall. However, the lumen is recanalized shortly thereafter. Since the foregut is supplied by the celiac artery and the midgut is supplied by the superior mesenteric artery, the duodenum is supplied by branches of both arteries.  LIVER AND GALLBLADDER The liver primordium appears in the middle of the third week as an outgrowth of the endodermal epithelium at the distal end of the foregut. This outgrowth, the hepatic diverticulum, or liver bud, consists of rapidly proliferating cells that penetrate the septum transversum , that is, the mesodermal plate between the pericardial cavity and the stalk of the yolk sac. While hepatic cells continue to penetrate the septum, the connection between the hepatic diverticulum and the foregut (duodenum ) narrows, forming the bile duct.  LIVER AND GALLBLADDER A small ventral outgrowth is formed by the bile duct, and this outgrowth give rise to the gallbladder and the cystic duct. During further development, form hepatic sinusoids. Hematopoietic cells, kupffer cells, and connective tissue cells are derived from mesoderm of the septum transversum. Mesoderm of the septum transversum lying between the liver and the foregut and the liver and the ventral abdominal wall become membranous, forming the lesser omentum and falciform ligament, respectively. Together, having formed the peritoneal connection the foregut and the ventral abdominal wall, they are known as the ventral mesentery. In the 10 th week of development, the weight of the liver is approximately 10% of the total body weight. Although this may be attributed partly to the large numbers of sinusoids, another important factor is its hematopoietic function, large nests of proliferating cells, which produce red and white blood cells, lie between hepatic cells and walls of the vessels. 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.  PANCREAS The pancreas is formed by two buds, dorsal and ventral, originating from the endodermal lining of the duodenum. Whereas the dorsal pancreatic bud is in the dorsal mesentery, the ventral pancreatic bud is close to the bile duct. When the duodenum rotates to the right and become C-shaped, the ventral pancreatic bud moves dorsally, the ventral bud come to lie immediately below and behind the dorsal bud. Later, the dorsal and ventral buds fuse, the ventral bud forms the uncinated process and inferior part of the head of the pancreas. The remaining part of the gland is derived from the dorsal bud. The main pancreatic duct is formed by the distal part of the dorsal pancreatic duct and the entire ventral pancreatic duct. The proximal part of the dorsal pancreatic duct, the accessory pancreatic duct. The main pancreatic duct, together with the bile duct, enters the duodenum at the site of the major papilla; the entrance of the accessory duct is the site of the minor papilla. Visceral mesoderm surrounding the pancreatic buds forms the pancreatic connective tissue. Insulin secretion begins at approximately the fifth month.  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 or yolk stalk. 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. Development of the midgut is characterized by rapid elongation of the gut and its mesentery, resulting in formation of the primary intestinal loop. At its apex, the loop remains in open connection with the yolk sac by way of the narrow vitelline duct. The cephalic limb of the loop develops into the distal part of the duodenum, the jejunum, and part of the ileum. The caudal limb becomes the lower portion of the ileum, the cecum, the appendix, the ascending colon, and the proximal two thirds of the transverse colon.  HINDGUT The hindgut give rise to the distal third of the transverse colon. The descending colon, the sigmoid, the rectum, and the upper part of the anal canal. The endoderm of the hindgut also forms the internal lining of the bladder and urethra. SUMMARY THANK YOU！