Digestive System Embryology - Lecture Notes PDF

The development of Digestive System (Lecture 1) Dr. Aida Al-Obaidi 23-09-2024 1 Learning outcomes By the end of these 2 lectures, you should be able to: 1.Understand how the process of folding in the embryo during the 3rd and 4th w...

The development of Digestive System (Lecture 1) Dr. Aida Al-Obaidi 23-09-2024 1 Learning outcomes By the end of these 2 lectures, you should be able to: 1.Understand how the process of folding in the embryo during the 3rd and 4th week creates the primitive gut, the abdominal wall. 2. Describe the development of the pregut derivatives. 3.Describe how the coelomic cavity and peritoneal cavity develop. 4.Describe the fate of the embryonic dorsal and ventral mesenteries. 5.Understand why some abdominal organs possess mesenteries and some are retroperitoneal. 6.Understand the disposition of the viscera and the developmental basis for congenital defects of the Gastrointestinal tract 2  Embryonic folding lateral folding ❖ During the 3ed and 4th weeks, the top layer (ectoderm) of the trilaminar embryonic disc forms the neural plate that rolls up into a tube to form the brain and spinal cord by the process called neurulation. 3 ❖ The ventral layer (endoderm) rolls down to form the gut tube, such that the embryo consists of a tube on top of a tube: the neural tube dorsally and the gut tube ventrally. 4 ❖The middle layer (mesoderm) holds the two tubes together. ❖The lateral plate component of this mesoderm layer also splits into visceral and somatic( Parietal) layers. 5 ❖The visceral layer rolls ventrally and is intimately connected to the gut tube and the somaticl layer, together with the overlying ectoderm, forms the lateral body wall folds (one on each side of the embryo), which move ventrally and meet in the midline to close the ventral body wall. ❖ The space between visceral and somatic layers of lateral plate mesoderm is the primitive body cavity, which at this early stage is a continuous cavity. 6 ❖ Cephalocaudal folding 7  Division of the gut tube ❖ The cephalocaudal and lateral folding of the embryo will lead to form the primitive gut tube. ❖ 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 temporally connected to the yolk sac by means of the vitelline duct, or yolk stalk 8 Poll #1 The Division of the gut tube happened through the: A- Lateral folding B- Cephalocaudal folding C- Both of them 9 The primitive gut tube will developed to: 10 ❖How some portions of the gut tube and its derivatives are suspended from the dorsal and ventral body wall? ❖ Mesenteries (peritoneum) ❖Intraperitoneal organs: When double layers of peritoneum enclose an organ and connect it to the body wall. ❖retroperitoneal organs: organs that lie against the posterior body wall and are covered by peritoneum on their anterior surface only (e.g., the kidneys). ❖Peritoneal ligaments : ❖are double layers of peritoneum(mesenteries) that pass from one organ to another or from an organ to the body wall. 11  Functions of 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 posterior abdominal wall. ❖By the fifth week, 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. 12 ❖The dorsal mesentery divided into: ❖Dorsal mesogastrium or greater omentum. ❖ Dorsal mesoduodenum. ❖ Dorsal mesocolon. ❖Mesentery proper 13 ❖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. ❖The Growth of the liver into the mesenchyme of the septum transversum divides the ventral mesentery into: ❖The lesser omentum, extending from the lower portion of the esophagus, the stomach, and the upper portion of the duodenum to the liver. ❖The falciform ligament, extending from the liver to the ventral body wall. 14  Foregut (4th week) Esophagus development 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. 15 Poll #2 Both the respiratory system and the digestive system develop from the same source (the endoderm). A- T B- F 16  Foregut Esophagus The esophagus is short then it lengthens rapidly. The muscular coat is striated in its upper two-thirds and innervated by the vagus and smooth in the lower third and is innervated by the splanchnic plexus. 17  Clinical Correlates (Esophageal Abnormalities) 1.Esophageal atresia and/or tracheoesophageal fistula Results either from spontaneous posterior deviation of the tracheoesophageal septum or from some mechanical factor pushing the dorsal wall of the foregut anteriorly. 18 2-Esophageal stenosis usually in the lower third. Stenosis may be caused by incomplete recanalization, vascular abnormalities, or accidents that compromise blood flow. 3-Congenital hiatal hernia The esophagus fails to lengthen sufficiently, and the stomach is pulled up into the esophageal hiatus through the diaphragm. The result is a congenital hiatal hernia. https://ars.els-cdn.com/content/image/1-s2.0-S2213576620301949-gr1.jpg 19 Stomach ❖The stomach appears as a fusiform expansion of the foregut in the 4th week of development. ❖ The stomach rotates 90° clockwise around its longitudinal axis, causing its left side to face anteriorly and its right side to face posteriorly ❖The left vagus nerve, innervates the anterior wall and the right nerve innervates the posterior wall. ❖ The original posterior wall of the stomach grows faster than the anterior portion, forming the greater and lesser curvatures. 20  Stomach position The stomach originally lie in the midline, however during the stomach rotation around an anteroposterior axis: 1-The caudal or pyloric part moves to the right and upward 2- The cephalic or cardiac portion moves to the left and slightly downward. 21  Since the stomach is attached to the dorsal body wall by the dorsal mesogastrium and to the ventral body wall by the ventral mesogastrium its rotation and disproportionate growth alter the position of these mesenteries. ❑ 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) and also pulls the ventral mesogastrium to the right. As this process continues in the 5th week of development, the spleen primordium appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium. 22  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 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 23  Stomach Abnormalities 1.Pyloric stenosis occurs when the longitudinal musculature of the stomach in the region of the pylorus hypertrophies. It’s one of the most common abnormalities of the stomach in infants, pyloric stenosis is believed to develop during fetal life. There is an extreme narrowing of the pyloric lumen, and the passage of food is obstructed, resulting in severe vomiting. In a few cases, the pylorus is atretic. 2.Other malformations of the stomach such as duplications and a prepyloric septum, are rare. 24 ▪Duodenum is formed from the terminal part of the foregut and the cephalic part of the midgut. As the stomach rotates, the duodenum takes on the form of a C- shaped loop and rotates to the right. This rotation and the rapid growth of the head of the pancreas, causing them to be pressed against the dorsal body wall and fixed as a retroperitoneal organs. ❑ foregut is supplied by the celiac artery ❑ midgut is supplied by the superior mesenteric artery ❑the duodenum is supplied by branches of both arteries 25  Liver 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. Liver cords differentiate into: 1. The parenchyma (liver cells) 2. The lining of the biliary ducts and gall bladder. Epithelial liver cords mix with the vitelline and umbilical veins, which form hepatic sinusoids 26  The septum transversum is the mesodermal plate between the pericardial cavity and the stalk of the yolk. The mesoderm of the septum transversum forms: 1. Hematopoietic cells 2. Kupffer cells 3. connective tissue cells AND (a) peritoneum of the liver (b) falciform ligament (c) lesser omentum (d) central tendon of the diaphragm 27  Bare area of the liver ❖Mesoderm on the surface of the liver differentiates into visceral peritoneum except on its cranial surface. ❖In this region, the liver remains in contact with the rest of the original septum transversum. This portion of the septum, which consists of densely packed mesoderm, 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 28  Clinical Correlates 1-extrahepatic biliary atresia patients with extrahepatic biliary atresia ❖15% to 20% correctable defect ❖ 80%-85%usually die unless they receive a liver transplant. 2-intrahepatic biliary duct atresia and hypoplasia: This rare abnormality (1/100,000 live births) may be caused by fetal infections. 29 Reference Langman's Medical Embryology 30 Thanks for your attention 31