Development Of The Digestive System PDF
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This document provides an outline of the development of the digestive system. It details the formation of the primitive gut, the subsequent development of the foregut, midgut, and hindgut, as well as the clinical anatomy related to the system. This document is not an exam paper but rather an overview of the topic.
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DEVELOPMENT OF THE DIGESTIVE SYSTEM OUTLINE INTRODUCTION FORMATION AND DEVELOPMENT OF PRIMITIVE GUT FOREGUT DEVELOPMENT OF THE ESOPHAGUS, STOMACH AND DUODENUM DEVELOPMENT OF LIVER AND BILIARY APPARATUS DEVELOPMENT OF PANCREAS AND SPLEEN MIDGUT AND HERNIATION OF MIDGUT...
DEVELOPMENT OF THE DIGESTIVE SYSTEM OUTLINE INTRODUCTION FORMATION AND DEVELOPMENT OF PRIMITIVE GUT FOREGUT DEVELOPMENT OF THE ESOPHAGUS, STOMACH AND DUODENUM DEVELOPMENT OF LIVER AND BILIARY APPARATUS DEVELOPMENT OF PANCREAS AND SPLEEN MIDGUT AND HERNIATION OF MIDGUT LOOP DEVELOPMENT OF CECUM AND APPENDIX HINDGUT DEVELOPMENT OF TRANSVERSE COLONS DEVELOPMENT OF CLOACA CLINICAL ANATOMY 2 INTRODUCTION The Digestive System (Alimentary System) is the digestive tract from the mouth to the anus with all its associated glands and organs. It develops from the primitive gut that is derived from the dorsal part of endodermal yolk sac. The primordial gut is initially closed at its cranial end by the oropharyngeal membrane and at its caudal end by the cloacal membrane. 3 FORMATION OF THE PRIMITIVE GUT The Primitive Gut forms during the fourth week of intrauterine life by the incorporation of a larger portion of the yolk sac (umbilical vesicle) into the embryonic disc during craniocaudal and lateral folding of embryo The endoderm of the primordial gut gives rise to most of the gut, epithelium, and glands 4 PRIMITIVE GUT 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 Vitello – intestinal duct or yolk stalk. For descriptive purpose, the primitive gut is divided into three parts: Foregut, Midgut, and Hindgut. Molecular regulation of regional differentiation of primitive gut to form its different parts is done by Hox and ParaHox genes, and sonic hedgehog (SHH) signals. 5 DEVELOPMENT OF THE PRIMITIVE GUT Development of the primitive gut and its derivatives is usually discussed in four sections: The pharyngeal gut, or pharynx, extends from the oropharyngeal membrane to the respiratory diverticulum and is part of the foregut The remainder of the foregut lies caudal to the pharyngeal tube and extends as far caudally as the liver outgrowth. The midgut begins caudal to the liver bud and extends to the junction of the right two-thirds and left third of the transverse colon in the adult. The hindgut extends from the left third of the transverse colon to the cloacal membrane 6 FOREGUT Derivatives of the foregut include: The primordial pharynx and its derivatives The lower respiratory system The esophagus and stomach The duodenum, distal to the opening of the bile duct The liver, biliary apparatus (hepatic ducts, gallbladder, and bile duct), and pancreas 7 DEVELOMENT OF ESOPHAGUS The esophagus develops from the foregut immediately caudal to the pharynx (between the pharynx and stomach) The foregut presents a median laryngotracheal groove at the ventral part of the pharyngoesophageal junction. By the end of the fourth week, the laryngotracheal groove has evaginated (protruded) to form a pouch-like laryngotracheal diverticulum (lung bud), which is located ventral to the caudal part of the foregut The tracheoesophageal septum gradually partitions this diverticulum from the dorsal part of the foregut. As a result of this, the foregut divides into; A Ventral Portion – The Respiratory Primordium A Dorsal Portion – The Esophagus. 8 DEVELOPMENT OF THE ESOPHAGUS Initially the esophagus is short but later it elongates due to: Formation of neck Descent of diaphragm Descent of heart and lungs However, it reaches its final relative length by the seventh week 9 DEVELOPMENT OF THE STOMACH During the fourth week, a slight dilation indicates the site of the primordium of the stomach. The dilation first appears as a fusiform enlargement of the caudal (distal part) of the foregut and is initially oriented in the median plane. The primordial stomach soon enlarges and broadens ventrodorsally. During the 6th week, the dorsal border of the stomach grows faster than its ventral border; this demarcates the developing greater curvature of the stomach. Gastric glands appear in the third month of the IUL. Oxyntic and zymogenic cells appear in the fourth month of IUL 10 ROTATION OF THE STOMACH Enlargement of the mesentery and adjacent organs, as well as growth of the stomach walls, contribute to the rotation of the stomach. The rotation of the stomach is easily explained by assuming it rotates its longitudinal and anteroposterior axis. First the stomach rotates 90° clockwise around its longitudinal axis. As a result, its left surface now faces anteriorly and forms anterior surface. 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. 11 ROTATION OF THE STOMACH During this rotation, the original posterior wall of the stomach grows faster than the anterior portion, forming the greater and lesser curvatures respectively. The cephalic and caudal ends of stomach originally lie in the midline. Now the stomach rotates around its anteroposterior axis causing the cardiac (cephalic) end of the stomach originally lying in the midline to move the left and slightly inferior, while the pyloric (caudal) end originally lying in the midline moves to the right and slightly superior. The stomach thus assumes its final position. Its axis running from above left to below right 12 MESENTRIES OF STOMACH The stomach is attached to the dorsal body wall by the dorsal mesogastrium and to the ventral body wall by the ventral mesogastrium (which is derived from mesoderm of the septum transversum). When liver develops in the septum transversum, the mesoderm forming the ventral mesogastrium becomes thinner and forms two parts of the ventral mesentry: The Lesser Omentum – The part extending from the stomach to the liver The Falciform Ligament – the part extending between the liver and anterior abdominal wall The epithelial lining and gastric glands of the stomach are derived from the endoderm of the primitive foregut, while the rest of the layers of the stomach (muscular and serous coats) are derived from surrounding splanchnic intraembryonic mesoderm. 13 MESENTRIES OF STOMACH Due to the stomach’s rotation and disproportionate growth, the position of the dorsal and ventral mesenteric connections is altered. 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) As the rotation of the stomach continues in the 5th week (pulling the lesser omentum to the right), the spleen primordium appears as a mesodermal proliferation between the two leaves of the dorsal mesogastrium, dividing the dorsal mesogastrium into: Gastrosplenic Ligament – The part extending from greater curvature (fundus) of the stomach to spleen Lienorenal Ligament – while the part extending from spleen to posterior abdominal wall. The dorsal mesogastrium attached to rest of greater curvature elongates and forms a large apron-like fold of peritoneum called greater omentum. 14 DEVELOPMENT OF DUODENUM Early in the fourth week, the duodenum begins to develop from the caudal part of the foregut, the cranial part of the midgut, and the splanchnic mesenchyme associated with these parts of the primordial gut. The first and second part of duodenum up to the opening of common bile duct develop from foregut The second part of the duodenum below the opening of common bile duct along with third and fourth part develop from midgut The developing duodenum grows rapidly, forming a C-shaped loop that projects ventrally and attached to posterior abdominal wall by a mesentery called mesoduodenum. The clockwise rotation of the stomach to the left makes the duodenal loop to fall on the right side. Its mesentery (mesoduodenum) is absorbed by zygosis and becomes retroperitoneal 15 DEVELOPMENT OF DUODENUM 16 DEVELOPMENT OF DUODENUM During the fifth and sixth weeks, the lumen of the duodenum becomes progressively smaller and is temporarily obliterated because of the proliferation of its epithelial cells. Normally vacuolation occurs as the epithelial cells degenerate; as a result, the duodenum normally becomes recanalized by the end of the embryonic period. 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 of both arteries. 17 DEVELOPMENT OF LIVER AND BILIARY APPARATUS The liver, gallbladder, and biliary duct system arise as a ventral outgrowth (hepatic diverticulum or liver bud) from the distal part of the foregut early in the fourth week. The diverticulum consists of rapidly proliferating cells that extends into the septum transversum - a mass of splanchnic mesoderm between the developing heart and midgut. The hepatic diverticulum enlarges rapidly and divides into two parts as it grows between the layers of the ventral mesogastrium The larger cranial part of the hepatic diverticulum is the primordium of the liver; The small caudal part of the hepatic diverticulum becomes the gallbladder, and the stalk of the diverticulum forms the cystic duct 18 DEVELOPMENT OF LIVER AND BILIARY APPARATUS 19 DEVELOPMENT OF LIVER AND BILIARY APPARATUS The proliferating endodermal cells give rise to interlacing cords of hepatocytes and to the epithelial lining of the intrahepatic part of the biliary apparatus. The hepatic cords anastomose around endothelium-lined spaces, the primordia of the hepatic sinusoids. Vascular endothelial growth factor Flk-1 (VEGF-Flk-1) signaling appears to be important for the early morphogenesis of the hepatic sinusoids (primitive vascular system). The fibrous and hematopoietic tissue and Kupffer cells of the liver are derived from mesenchyme in the septum transversum The liver grows rapidly and, from the 5th to 10th week, fills a large part of the upper abdominal cavity. The quantity of oxygenated blood flowing from the umbilical vein into the liver determines the development and functional segmentation of the liver. Initially, the right and left lobes are approximately the same size, but the right lobe soon becomes larger. 20 DEVELOPMENT OF LIVER AND BILIARY APPARATUS Hematopoiesis (formation and development of various types of blood cells) begins during the sixth week, giving the liver a bright reddish appearance. By the ninth week, the liver accounts for approximately 10% of the total weight of the fetus. Bile formation by hepatic cells begins during the 12th week. Initially, the extrahepatic biliary apparatus is occluded with epithelial cells, but it is later canalized because of vacuolation resulting from degeneration of these cells. The stalk connecting the hepatic and cystic ducts to the duodenum becomes the bile duct. Initially, this duct attaches to the ventral aspect of the duodenal loop; however, as the duodenum grows and rotates, the entrance of the bile duct is carried to the dorsal aspect of the duodenum. The bile entering the duodenum through the bile duct after the 13th week gives the meconium (intestinal contents) a dark green color 21 DEVELOPMENT OF LIVER AND BILIARY APPARATUS 22 DEVELOPMENT OF LIVER AND BILIARY APPARATUS When liver cells have invaded the entire septum transversum, so that the organ bulges caudally into the abdominal cavity, mesoderm of the septum transversum lying between the liver and the foregut and the liver and the ventral abdominal wall becomes membranous, forming the lesser omentum and falciform ligament, respectively. This thin, double-layered membrane gives rise to: The Lesser Omentum, passing from the liver to the lesser curvature of the stomach (hepatogastric ligament) and from the liver to the duodenum (hepatoduodenal ligament) The Falciform Ligament, extending from the liver to the ventral abdominal wall They are both regarded as the Ventral Mesentry 23 DEVELOPMENT OF PANCREAS The pancreas develops between the layers of the mesentery from dorsal and ventral pancreatic buds of endodermal cells, which arise from the caudal of the foregut When the duodenum rotates to right and becomes C shaped, the ventral pancreatic bud is on the right and the dorsal pancreatic bud is on the left of the duodenum. With rapid growth of right duodenal wall, the ventral pancreatic bud shifts from right to left and lies just below the dorsal pancreatic bud. The dorsal and ventral pancreatic buds grow in size and fuse with each other to form the pancreas. Eventually, The dorsal pancreatic bud forms the upper part of head, neck, body, and tail of the pancreas while ventral pancreatic bud forms the lower part of the head and uncinate process of pancreas. 24 DEVELOPMENT OF PANCREAS 25 DEVELOPMENT OF PANCREAS As the pancreatic buds fuse, their ducts anastomose. The main pancreatic duct (Duct of Wirsung) forms the duct of the ventral bud and the distal part of the duct of the dorsal bud. The proximal part of the duct of the dorsal bud often persists as an accessory pancreatic duct (Duct of Santorini) In the third month of fetal life, pancreatic islets (of Langerhans) develop from the parenchymatous pancreatic tissue and scatter throughout the pancreas. Insulin secretion begins at approximately the fifth month. Glucagon- and somatostatin-secreting cells also develop from parenchymal cells. Visceral mesoderm surrounding the pancreatic buds forms the pancreatic connective tissue. 26 DEVELOPMENT OF SPLEEN Although the spleen is not a gland of the The development of the spleen in the digestive tract but is described here dorsal mesogastrium later divides into because of its close association with the two parts: digestive tract. part that extends between hilum of the spleen and greater curvature of Development of the spleen is described with the alimentary system because this the stomach is called Gastrosplenic Ligament, organ is derived from a mass of mesenchymal cells located between the the part of dorsal mesogastrium that layers of the dorsal mesogastrium. extends between the spleen and left kidney on the posterior abdominal The spleen, a vascular lymphatic organ, wall is called Splenorenal begins to develop during the fifth week, Ligament/Lienorenal ligament but does not acquire its characteristic shape until early in the fetal period. 27 DEVELOPMENT OF SPLEEN 28 MIDGUT Derivatives of the midgut include: The small intestine, including the duodenum distal to the opening of the bile duct The cecum, appendix, ascending colon, and the right one half to two thirds of the transverse colon 29 HERNIATION OF MIDGUT LOOP As the midgut elongates, it forms a ventral, U-shaped loop of intestine (midgut loop) that projects into the remains of the extraembryonic coelom in the proximal part of the umbilical cord. The midgut loop of the intestine is a physiologic umbilical herniation which occurs at the beginning of the sixth week. This loop communicates with the umbilical vesicle through the narrow omphaloenteric/vitelline duct (yolk stalk) until the 10th week. The superior mesenteric artery divides the midgut loop into two segments: Prearterial (Cranial) Limb – gives rise to Distal half of duodenum, Jejunum, Ileum, except its terminal part Postarterial (Caudal Limb) – gives rise to Terminal part of ileum, Cecum, Appendix, Ascending colon, Proximal (right) two-third of the transverse colon 30 HERNIATION OF MIDGUT LOOP 31 ROTATION AND RETRACTION OF MIDGUT LOOP The rotation of gut occurs when herniated intestinal loops return back to the abdominal cavity, which begins at the end of the 3rd month of IUL. 32 DEVELOPMENT OF CECUM AND APPENDIX The primordium of the cecum and appendix—the cecal swelling/bud (diverticulum), appears in the sixth week as an elevation on the antimesenteric border of the caudal limb of the midgut loop. The proximal part of the bud grows rapidly and forms cecum, while its distal part remains narrow to form the appendix. After birth, the wall of the cecum grows unequally, with the result that the appendix comes to enter its medial side The appendix is subject to considerable variation in position. As the ascending colon elongates, the appendix may pass posterior to the cecum (retrocecal appendix) or colon (retrocolic appendix). It may also descend over the brim of the pelvis (pelvic appendix). In approximately 64% of people, the appendix is located retrocecally. 33 DEVELOPMENT OF CECUM AND APPENDIX 34 HINDGUT The derivatives of the hindgut include: The left one third to one half of the transverse colon, the descending colon, sigmoid colon, rectum, and superior part of the anal canal 35 DEVELOPMENT OF TRANSVERSE COLON The right two-third of transverse colon develops from the postarterial segment of the midgut loop while the left one-third of transverse colon develops from the hindgut. For this reason, the right two-third of transverse colon is supplied by superior mesenteric artery (the artery of midgut) and left one-third of transverse colon is supplied by the inferior mesenteric artery (the artery of hindgut). The descending colon becomes retroperitoneal as its mesentery fuses with the parietal peritoneum on the left posterior abdominal wall and then disappears. The mesentery of the fetal sigmoid colon is retained, but it is smaller than in the embryo 36 DEVELOPMENT OF CLOACA The expanded terminal part of the hindgut - the cloaca, is an endoderm-lined chamber that is in contact with the surface ectoderm at the cloacal membrane. The cloaca receives the allantois ventrally, which is a fingerlike diverticulum PARTITIONING OF CLOACA - The cloaca is divided into dorsal and ventral parts by a wedge of mesenchyme—the urorectal septum that develops in the angle between the allantois and hindgut. A broad ventral part called primitive urogenital sinus A narrow dorsal part is called primitive rectum. The urogenital sinus gives rise to the urinary bladder and urethra, while the primitive rectum gives rise to the rectum and upper part of the anal canal. 37 DEVELOPMENT OF CLOACA 38 DEVELOPMENT OF ANAL CANAL The anal canal develops from two sources: Hindgut proctodeum The Upper Half of the anal canal is endodermal in origin and develops from primitive rectum. The lower half of the anal canal is endodermal in origin and develops from anal pit (proctodeum). Initially, the two parts are separated from each other by anal membrane. Later when this membrane ruptures the two parts communicate with each other. The site of anal membrane is represented by pectinate line in adults 39 CLINICAL ANATOMY ESOPHAGEAL ATRESIA TRACHEOESOPAHGEAL FISTULA It occurs due to failure of recanalization of A fistula (abnormal passage) between the the developing esophagus. Esophageal trachea and esophagus occuring once in 3000 atresia is associated with to 4500 live births. tracheoesophageal fistula in more than 90% of cases. A TEF results from incomplete division of the cranial part of the foregut into respiratory and It is produced by extreme posterior deviation esophageal parts during the fourth week of tracheoesophageal septum and incomplete separation of the esophagus. Incomplete fusion of the tracheoesophageal from the laryngotracheal tube. folds results in a defective tracheoesophageal septum and a TEF between the trachea and Isolated esophageal atresia (5%–7% of esophagus cases) results from failure of recanalization of the esophagus during the eighth week of development. ESOPHAGEAL STENOSIS 40 CLINICAL ANATOMY 41 CLINICAL ANATOMY HYPERTROPHIC PYLORIC STENOSIS ANOMALIES OF THE LIVER This anomaly affects one in every 150 males and one in every 750 females. It occurs due to Intrahepatic biliary atresia – This is the hypertrophy of circular muscle layer at the most serious anomaly of the extrahepatic pylorus. biliary system. The most common form of The circular and, to a lesser degree, the longitudinal extrahepatic biliary atresia (present in 85% muscles in the pyloric region are hypertrophied. of cases) is obliteration of the bile ducts at This results in severe stenosis of the pyloric canal or superior to the porta hepatis—a deep and obstruction of the passage of food. As a result, transverse fissure on the visceral surface the stomach becomes markedly distended and the of the liver. Biliary atresia could result from infant expels the stomach’s contents with a failure of the remodeling process at the considerable force (projectile vomiting). hepatic hilum or from infections or DUODENAL ATRESIA AND STENOSIS immunologic reactions during late fetal Complete occlusion of the duodenal lumen – development. DUOENAL ATRESIA. It occurs due to the failure of Minor variations of liver lobulation recanalization of the duodenum Partial occlusion of the duodenal lumen – Polycystic disease of the liver DUODENAL STENOSIS. It occurs because of 42 incomplete recanalization of the duodenum. CLINICAL ANATOMY ANNULAR PANCREAS In this condition, the pancreatic tissue completely surrounds second part of the duodenum causing its obstruction. This anomaly is produced as follows: The bifid ventral pancreatic bud fails to fuse to form a single mass. The two lobes (right and left) of the ventral pancreatic bud grow and migrate in opposite directions around the second part of the duodenum and form a collar of pancreatic tissue before it fuses with dorsal pancreatic bud, thereby causing duodenal obstruction. Blockage of the duodenum develops if inflammation (pancreatitis) develops in the annular pancreas. An annular pancreas may be associated with Down syndrome, intestinal malrotation, and cardiac defects. 43 CLINICAL ANATOMY CONGENITAL OMPHALOCELE This anomaly is a persistence of the herniation of abdominal contents into the proximal part of the umbilical cord. It results from failure of coils of the small intestine to return into abdominal cavity from their physiological herniation into extraembryonic celom during sixth to tenth week of IUL 44 CLINICAL ANATOMY IMPERFORATE ANUS Various types of imperforated anus. A. Persistence of anal membrane. B. Failure of anal pit to develop. C. Upper and lower parts of rectum separated by a gap. D. Stenosis of the anal canal. 45 46