Development Of Digestive System PDF

Development of digestive system 1st year, Medicine Faculty of Medicine and Dentistry Department of Histology and Embryology Primitive gut Omocznia The primitive gut forms during the fourth week as t...

Development of digestive system 1st year, Medicine Faculty of Medicine and Dentistry Department of Histology and Embryology Primitive gut Omocznia The primitive gut forms during the fourth week as the head, caudal eminence (tail), and lateral folds incorporate the dorsal part of the umbilical vesicle (yolk sac) - - Week Prome Ve e 4th Week - Umbilical Vesich(yolk Sac) Primitive gut At the beginning of the 4th week, the primordial gut is closed at its cranial end by the oropharyngeal membrane and its caudal end by the cloacal membrane. The primordial gut is divided into three parts: 2th o - 0 In the cranial region – foregut In the middle region – midgut In the caudal region – hindgut The midgut remains temporarily connected to the yolk sac Primitive gut a livor Organ (lung, liver, pancreas) connected with the gut begins to form from the endoderm. - - lung area The connection between the midgut and the yolk sac becomes narrower lung+ liver + panosent ↳ endoderm The endoderm of the primordial gut gives rise to most of the epithelium and glands of the alimentary system (liver and pancreas). The epithelium of the cranial and caudal ends of the alimentary tract is derived from ectoderm of the stomatodeum and anal pit (proctodeum), respectively. The muscular and connective tissue and other layers of the wall of the digestive tract are derived from the splanchnic mesenchyme surrounding the primordial gut. Pancreas en epithelium ofthe cramal +candal c end Primitive gut midget The midgut remains connected to the yolk sac via the yolk duct I  mesoderm Splanchnic 1. The intra-embryonic body cavity is connected to the extra-embryonic body cavity 2. The intra-embryonic body cavity loses connection with the extra-embryonic body cavity 3. The layers of the splanchnic mesoderm merge in the midline and form a two-layer membrane - the dorsal mesentery (4 weeks) dorsal mesenter The derivatives of the foregut: The primordial pharynx and its derivatives The lower respiratory system The esophagus and stomach The duodenum, just distal to the opening of the bile duct The liver, biliary apparatus (hepatic ducts, gallbladder, and bile duct), and pancreas The derivatives of the midgut: Cr Lower duodenum Jejunum and ileum Cecum and vermiform appendix Ascending colon Line du Cranial half of transverse colon lowe due The derivatives of the hindgut: Caudal half of transverse colon Descending colon Rectum ↓ Midgul Superior part of anal canal luce - dr Foregut - esophagus /edays 4, 4ost Posterin phe I Most - - Just caudal to the most posterior pharyngeal pouches of a 4-week-old embryo, the pharynx becomes abruptly narrowed, and a small ventral outgrowth (respiratory diverticulum) appears. Longitudinal tracheoesophageal folds develop in the diverticulum, approach each other, and fuse to form a partition, an esophagotracheal septum, at the end of the 5th week. This septum divides the cranial portion of the foregut into: foregut a ventral part, the laryngotracheal tube (the primordium of the larynx, trachea, bronchi and lungs) into a dorsal part (the primordium of the oropharynx and esophagus For t Foregut - esophagus Although the esophagus grossly resembles a simple tube, it undergoes a series of differentiative changes at the tissue level. In its earlier stages, the endodermal lining epithelium of the esophagus is stratified columnar. By 8 weeks, the epithelium has partially occluded the lumen of the esophagus, and large vacuoles appear. In succeeding weeks, the vacuoles coalesce, and esophageal lumen recanalizes, but with a multilayered ciliated epithelium. During the 4th month, this epithelium finally is replaced with stratified squamous. The esophagus elongates rapidly and reaches its final relative length by the seventh week startified Columnar ↳citrated ↳ Stratified squamant Foregut - stomach 0 ⑧ 0 Very early in the formation of the digestive 0 0 tract, the stomach is recognizable as a dilated region with a shape remarkably similar to that of the adult stomach. - Two concomitant positional shifts bring the stomach to its adult configuration: ⑧ 0 1. The first is an approximately 90- degree rotation about its craniocaudal axis so that is originally dorsal convex border faces left, and its ventral concave border faces right 00 2. The other positional shift consists of a minor tipping of the caudal (pyloric) end of the stomach in a cranial direction so that the long axis of the stomach is positioned diagonally across the body Foregut - stomach During rotation of the stomach, the dorsal mesogastrium is carried with it, leading to the formation of a pouchlike structure – omental bursa The spleen is recognizable as a mesenchymal condensation in the dorsal mesogastrium at 4th weeks The dorsal mesogastrium enlarges and the part between the spleen and the dorsal midline fuses with the peritoneum of the posterior abdominal wall The spleen is connected to the body wall by the lienorenal ligament and to the stomach by the gastrolienal ligament abdominal Wall dorsal mesogastrium -> Posterior Foregut - stomach As the stomach rotates, the dorsal mesogastrium and the omental bursa that is encloses enlarge dramatically Soon, part of the dorsal mesogastrium, which becomes the greater omentum, overhangs the transverse colon and portions of the small intestines as a large, double flat of a fatty tissue The two sides of the greater omentum ultimately fuse, obliterating the omental bursa within the greater omentum  candal cranial port Part Foregut – liver and biliary apparatus I Part ventral candal Granial Liver I ott rece prof cranial caudalport - The liver, gallbladder and biliary duct system arise as a ventral endodermal outgrowth – hepatic diverticulum, from the distal part of the foregut early in the ventral endoc 4th week The diverticulum invades into the septum transversum (a mass of splanchnic lin mesenchyme separating the pericardial and peritoneal cavities). The septum forms the ventral mesogastrium in this region ·ventral The hepatic diverticulum enlarges rapidly and is divided 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 smaller caudal part becomes the primordium of the gallbladder The proliferating endodermal cells form interlacing cords of hepatocytes and give rise to the epithelial lining of the intrahepatic part of the biliary apparatus Foregut – liver and biliary apparatus The small caudal part of the hepatic diverticulum becomes the gallbladder, and the stalk of the diverticulum forms the cystic duct. 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 of the diverticulum connecting the hepatic and cystic ducts to the duodenum becomes the bile duct Foregut – liver and biliary apparatus When liver enlarges, the ventral mesentery gives rise to: Lesser omentum – passing from the liver to the lesser curvature of the stomach and from the liver to the duodenum Falciform ligament – extending from the liver to the ventral abdominal wall The umbilical vein passes through the free border of the falciform ligament on its way from the umbilical cord to the liver. The ventral mesentery also forms the visceral peritoneum of the liver. The liver is covered by peritoneum, exept for bare area, which is in direct contact with the diaphragm. Foregut - duodenum In the fourth week, the duodenum begins to develop from the caudal part of the foregut and cranial part of the midgut. The developing duodenum grows rapidly, forming a C-shaped loop During the 5th and 6th weeks, the lumen of the duodenum becomes progressively smaller and is temporarily obliterated because of 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 candal Crani - al - Ea rid I Minget ⑱ - ⑧ forgut - - Foregut - pancreas The pancreas develops between the layers of the mesentery from: Dorsal pancreatic bud Ventral pancreatic bud (grows between layers of the ventral mesentery) As the duodenum rotates to the right and becomes C-shaped, the ventral bud is carried dorsally with the bile duct. It soon lies posterior to the dorsal pancreatic bud and later fuses with it. The ventral pancreatic bud forms the uncinate process and part of the head of the pancreas As the pancreatic buds fuse, their duct anastomose, or open into one another. The pancreatic duct forms from 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 that opens into the minor duodenal papilla. The two ducts often communicate with each other. In approximately 9% of people, the pancreatic ducts fail to fuse, resulting in 2 ducts. Midgut As the midgut elongates, it forms a ventral U-shaped midgut loop, that projects into the remains of the umbilicard cord The loop is a physiologic umbilical herniation, which occurs at the beginning of the 6 th week The midgut loop has two limbs: a cranial (proximal) limb – grows rapidly and forms small intestine loops a caudal (distal) limb – undergoes very little change exept for the development of the cecal swelling (diverticulum), the primordium of the cecum and appendix While the midgut loop is in the umbilical cord, components of the loop rotate 90 degrees counterclockwise around the axis of the superior mesenteric artery. This bring the cranial limb (small intestine) of the loop to the right and the caudal limb (large intestine) to the left During rotation, the cranial limb elongates and forms intestinal loop (e.g., the primordia of the jejunum and ileum) During the 10th week, the intestinal loop returns to the abdomen – this is the reduction of the midgut hernia The small intestine (formed from the cranial lamb) returns first and occupies the central part of the abdomen As the large intestine returns, it undergoes 180-degree counterclockwise rotation. The descending colon and sigmoid colon move to the right side of the abdomen. The ascending colon becomes recognizable with the elongation of the posterior abdominal wall Rotation of the stomach and duodenum causes the duodenum and pancreas to fall to the right. The enlarged colon presses the duodenum and pancreas against the posterior abdominal wall. As a result, most of the duodenal mesentery is absorbed. Consequently, the duodenum, exept for the first part (derived from the foregut), has no mesentery and lies retroperitoneally. Similarly, the head of the pancreas becomes retroperitoneal. midgut except 16th were enlargement Midgut The attachment of the dorsal mesentery to the posterior abdominal wall is greatly modified after the intestines return to the abdominal cavity. At first, the dorsal mesentery is in the median plane. As the intestines enlarge, lengthen, and assume their final position, their mesenteries are pressed against the posterior abdominal wall. The mesenteries of the ascending and descending colon fuse with the parietal peritoneum on this wall and disappears. Consequently, the ascending and descending colon also become retroperitoneal. - -send ↑ des The cecal swelling (the primordium of the cecum and appendix) appears in the 6 th week as an elevation on the antimesentering border of the caudal limb of the midgut loop The apex of the cecal swelling does not grow as rapidly as the rest of the it. Therefore, the appendix is initially a small pouch or sac opening from the cecum. The appendix increases rapidly in length, so at birth, it is long tube arising from the distal end of the cecum. Ascensing desending - - Refo Hindgut In early embryos, the cloaca (expanded terminal part of the hindgut) is the chamber into which the hindgut and allantois empty Cloaca is endoderm-lined chamber that is in contact with the surface endoderm at the cloacal membrane. This membrane is composed of endoderm of the cloaca and ectoderm of the anal pit. etoden coloca -> endoder(Membrana: Hindgut the cloaca id divided into dorsal and ventral parts by a mesenchymal urorectal septum, that develops in the angle between the allantois and hindgut. As the septum grows toward the cloacal membrane, it develops fork-like extension that produce infoldings of the lateral walls of the cloaca. These folds grow toward each other and fuse, forming a partition that divides the cloaca into three parts: The rectum The cranial part of the anal canal The urogenital sinus Cloacal membrane raptures by apoptosis and anorectal lumen is temporarily closed by an epithelial plug. Mesenchymal proliferation produce elevations of the surface ectoderm around the epithelial anal plug. Recanalization of the anorectal canal occur by apoptosis of epithelial anal plug, which forms the anal pit The superior two thirds of the adult anal canal are derived from the hindgut, the inferior one third develops from the anal pit. The junction of epithelium derived from the ectoderm of the anal pit and endoderm of the hindgut is indicated by pectinate line White line – line where the composition of the anal epithelium changes from columnar to stratifies squamous cells 0 ⑩me Sale R nat 0 - O esplagui - ·c Clinical correlation Atresia of the esophagus results from deviation of the tracheoesophageal septum in a posterior direction and incomplete separation of the esophagus from the laryngotracheal tube Stagin Hypetr Hypertrophic pyloric stenosis – in infants there is a marked muscular thickening of the pylorus. The circular muscle and, to a lesser degree, the longitudinal muscle in the pyloric region are hypertrophied. This results in severe stenosis of the pyloric canal and obstruction of the passage of food. As a result, the stomach becomes markedly distended. Biliary atresia - this malformation can involve any level ranging from the bile canaliculi to the major bile-carrying ducts. Some cases can be treated surgically; for others, a liver trasplant is necessary. Annular pancreas – a ring of pancreatic tissue completely encircles the duodenum. It may cause duodenal obstruction. An annular pancreas probably results from the growth of a bifid ventral pancreatic bud around the duodenum. The parts of the bifid ventral bud then fuse with the dorsal bud, forming a panreatic ring. Surgical intervention may be required for the management of this condition. king around d Clinical correlation Omphalocele – represent the failure of return of the intestinal loops ito the body cavity during the 10th week. The primary defect in omphalocele is most likely a reduced prominence of the lateral body wall that does not provide sufficient space for the complete return of the intestines to the body cavity. After birth, the intestinal loops can be easily seen within analmost transparent sac consisting of amnion on the outside and peritoneal membrane on the inside. Gastroschisis – a birth defet ofthe abdominal wall, results from a defect lateral to the median plane of the aterior abdominal wall. The linear defect permits extrusion of the abdominal viscera without involving the umbilical cord. The viscera protrude ontothe amniotic avity and are bathed by amniotic fluid. Clinical correlation abum Ileal diverticulum and omphaloenteric remnants – the most common family of anomalies of the intestinal tract is some form of persistence of the vitteline (yolk) duct. The most common member of this family is Meckel’s diverticulum – a blind pouch a few cm long located at the antimesenteric border of the ileum. In some cases, a ligament connects a Meckel’s diverticulum to the umbilicus. Rarely, a vitelline cyst is present along the lenght of the vitelline ligament A persistent vitelline duct can take form of a vitelline fistula, which constitutes a direct connection between the intestinal lumen and the outside of the body via the umbilicus Clinical correlation Birth defects of midgut rotation – results from incomplete rotation and/or fixation of the intestine Clinical correlation Hindgut fistulas – in many cases, anal atresia is accompanied by a fistula linking the patent portion of the hindgut to the another structure in the region of the original urodenital sinus region. Common types of fistules connect the hindgut with the vagina, the urethra, or the bladder