The Foregut

The Foregut At first the esophagus is short but due to enlargement and descent of the heart and lungs it lengthens rapidly downward. The muscular coat, which is formed by surrounding splanchnic mesenchyme, is striated in its upper two-thirds so innervated by the vagus nerve while the lower third is smooth muscle so innervated by the splanchnic plexus. Esophagus high up descend downward pierce the Diaphragm at level of (T10) on the left which is accompanied by the two vagi. Esophageal Abnormalities: 1)ESOPHAGEAL ATRESIA (blind end of esophagus) and/or tracheoesophageal fistula (connection between esophagus and trachea) results either from spontaneous posterior deviation of the tracheoesophageal septum or from some mechanical factor pushing the dorsal wall of the foregut anteriorly toward a trachea . Notes about this figure: A) very common 90% of abnormalities ( proximal part blind end and fistula in the distal end with trachea we call it tracheoesophageal fistula connection between respiratory system and GIT. B) rare (atresia distal and proximal without fistula C)common fistula between the trachea (proximal and Distal part) H shaped fistula. D)proximal fistula and distal atresia. E) separated proximal and distal fistula. • In its most common form, the proximal part of the esophagus ends as a blind sac, and the distal part is connected to the trachea by a narrow canal just above the bifurcation. (From slides) • Other types of defects in this region occur much less frequently Atresia of the esophagus usually associated with polyhydramnios (amount of amniotic fluid around the fetus accumulated and increased) the cause is fluid enter the esophagus ang found blind end so return to amniotic cavity. Symptoms: vomiting while breastfeeding, the amount of amniotic fluid is large bcz the amniotic fluid has secretion, absorption and excretion which is enter through the oral cavity and exit with fluids (polyhydramnios), when the baby cries the abdomen distended bcz there is an amount of air coming from the trachea through the fistula and infection if we don’t treat it early (pneumonia) . Fistula accompanied by other abnormalities like renal agenesis or cardiac abnormalities 2) ESOPHAGEAL STENOSIS : usually in lower third of the esophagus . Notes about this figure: A) normal (esophagus enters the diaphragm and end in the cardia of the stomach and the site of the fundus below the diaphragm) B) sliding hiatus hernia (Occasionally the esophagus fails to lengthen sufficiently and the stomach is pulled up into the esophageal hiatus through the diaphragm) C)paraoesophageal hiatus hernia (fundus of stomach moves upward through the diaphragm cause hernia parallel to the lower end of the esophagus). • Stenosis may be caused by incomplete recanalization, vascular abnormalities, or accidents that compromise blood flow. • The result is congenital hiatal hernia Causes acidity in the chest. Stomach * The stomach appears as a fusiform (spindle) dilation of the foregut in the fourth week of development 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◦ clockwise around its longitudinal axis(the first rotation), causing its left side to face anteriorly and its right side to face posteriorly(from lesser and greater curvature). The development in the greater curvature is more rapid than in lesser curvature , more rapid so become larger. From sheet 019 The stomach attached with the anterior wall of the abdomen by ventral mesentery (ventral gastromesentery) and dorsal mesentery (posterior gastromesentery )attached with posterior abdominal wall. Note : gastromesentery =mesogastrium With rotation: dorsal become left side / ventral become right side The dorsal mesentery forms the greater omentum , greater curvature and the legaments of the stomach with spleen and with other parts(posteriorly) The ventral mesentery forms the lesser omentum and ligaments of the liver The left and right vagus nerve with rotation the left become anterior and the right posterior to the stomach. • Hence the left vagus nerve, initially innervating the left side of the stomach, now innervates the anterior wall • similarly, the right vagus 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 curvatures The cephalic and caudal ends of the stomach originally lie in the midline Ceohalic end The cephalic and caudal ends of the stomach originally lie in the midline, but during further growth the stomach rotates around an anteroposterior axis, such that the caudal or pyloric part moves to the right and upward and the cephalic or cardiac portion moves to the left and slightly downward. The stomach thus assumes its final position, its axis running from above left to below right. Caudal end 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) This rotation also pulls the ventral mesogastrium to the right forms the ligament of the liver - The pancreas and duodenum go backward and become retroperitoneal(posterior part of remain attached to the posterior peritoneum or mesogastrium become disappearance ) - The small intestine and the transverse colon Ventral mesogastrium abdominal wall. - 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. - Posterior mesogastrium 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. Note about this figure: Between the stomach and spleen there is ligament called gastrosplenic ligament With rotation: posterior to the stomach the lesser sac will form. 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. Lengthening and fusion of the dorsal mesogastrium to the posterior body wall also determine the final position of the pancreas and duodenum. Stomach abnormalities: pyloric stenosis: thickening in the inner circular(mainly) and outer longitudinal muscles cause pylorus hypertrophy Baby after delivery has projectile vomiting (strong vomiting) One of the most common abnormalities of the stomach in infants, pyloric stenosis is believed to develop during fetal life. (3-6) weeks Treatment: pyloroplasty which is surgery to dilate the sphincter. At the end of the third week, intraembryonic mesoderm on each side of the midline differentiates into a paraxial portion, an intermediate portion, and a lateral plate . When intercellular clefts appear in the lateral mesoderm, the plates are divided into two layers: the somatic mesoderm layer(outer /gives parietal peritoneum) and the splanchnic mesoderm layer (gives visceral peritoneum around the yalk sac). endoderm The latter is continuous with mesoderm of the wall of the yolk sac. The space bordered by these layers forms the intraembryonic cavity (body cavity). When the embryo folds the intraembryonic cavity has a connection with the extra embryonic cavity and with folding. The peritoneal cavity is derived from the intraembryonic coleam caudal to the septum transversum(that is located between the pericardium and the stalk of the umbilicus) At first the right and left sides of the intraembryonic cavity are in open connection with the extraembryonic cavity, but when the body of the embryo folds cephalocaudally and laterally, this connection is lost -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. -The dorsal mesentery 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(make the duodenum backward and become retroperitoneal )and in the region of the colon it forms the dorsal mesocolon(connected with posterior abdominal wall and go to the anterior border of the pancreas ). -Dorsal mesentery of the jejunal and ileal loops forms the mesentery proper attached to the the posterior abdominal wall. Dorsal mesogastrium gives (the greater omentum, splenic ligament-lienorenal and gastrosplenic-) Spleen from the mesenchyme of the dorsal (mesoderm). 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: (a) the lesser omentum, extending from the lower portion of the esophagus, the stomach, and the upper portion of the duodenum to the liver and (b) the falciformligament, extending from the liver to the ventral body wall,and the coronary and the triangular ligaments Ventral mesogastium gives the lesser omentum and ligament of the liver- falciform, coronary and triangular ligament. -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. -As a result of rotation of the stomach about its anteroposterior axis, the dorsal mesogastrium bulges down -It continues to grow down and forms a doublelayered 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 Note : metaphorically we say the ascending 2 layers of greater omentum surround the transverse colon and form mesocolon but in the embryo it ends in the transverse colon and meet the mesentery of transverse colon . When liver cords grow into the septum, it thins to form: a (b) the falciform ligament, extending from the liver to the ( ) the peritoneum of the liver ventral body wall c ( ) 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). -This free margin 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). Summary: dorsal mesogastrium moves backward to the left while the ventral anterior to the right Ventral form the lesser omentum and the ligaments of the liver Dorsal moves backward make the pancreas and duodenum retroperitoneal and extend to the left and surround the spleen and make it intraperitoneal and form the ligaments of the stomach(greater omentum / gastrosplenic /lienorenal ligaments) And the result of the rotation the lesser sac will form behind the stomach . 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 -A small ventral outgrowth is formed by the bile duct, and this outgrowth gives rise to the gallbladder and the cystic duct. -During further development, epithelial liver cords intermingle with the vitelline and umbilical veins, which form hepatic sinusoids. -Liver cords differentiate into the parenchyma (liver cells) and form the lining of the biliary ducts. -Hematopoietic cells, Kupffer cells, and connective tissue cells are derived from mesoderm of the septum transversum. Notes about this figure: We see liver bud (come from anterior wall of duodenum) after grow up and meet with septum transversum (lies between the pericardium of the heart and the umbilical cord stalk of the umbilicus). Liver and gallbladder abnormalities: Variations in liver lobulation are common but not clinically significant, Accessory hepatic ducts and duplication of the gallbladder are also common and usually asymptomatic However, they become clinically important under pathological conditions. In some cases the ducts, which pass through a solid phase in their development, fail to recanalize This defect, extrahepatic biliary atresia, occurs in 1/15,000 live births. patients with extrahepatic biliary atresia, 15 to 20% have patent proximal ducts and a correctable defect, but the remainder usually die unless they receive a liver transplant Another problem with duct formation lies within the liver itself; it is intrahepatic biliary duct atresia and hypoplasia This rare abnormality (1/100,000 live births) may be caused by fetal infections. It may be lethal but usually runs an extended benign course. DUODENUM The terminal part of the foregut and the cephalic part of the midgut form the Duodenum(upper cephalic and lower caudal parts) . 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 so the rotation of duodenum with the rotation of the stomach) the rotation in the right side and backwards and form the C-shaped of duodenum. This rotation, together with rapid growth of the head of the pancreas, swings the duodenum from its initial midline position to the left side of the abdominal cavity - With rotation the common bile duct that was in the right side become in the left side of the duodenum- -The duodenum and head of the pancreas press against the dorsal body wall, and the right surface of the dorsal mesoduodenum fuses with the adjacent peritoneum. -Both layers subsequently disappear, and the duodenum and head of the pancreas become fixed in a retroperitoneal position. -The entire pancreas thus obtains 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 Remember: the duodenum is retroperitoneal except the 1st and last inches. 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 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 (upper half from celiac through superior pancreaticoduodenal artery and the lower part from superior mesenteric through inferior pancreaticoduodenal artery) . Any organ containing a duct or lumen the first thing that happen ( proliferation of cells) then canalization disappearance of cells ) . Pancreas -The pancreas is formed by two buds originating from the endodermal lining of the duodenum(epithelial growth) Whereas the dorsal pancreatic bud is in the dorsal mesentery(in the left side), the ventral pancreatic bud is close to the bile duct -When the duodenum rotates to the right and becomes Cshaped, the ventral pancreatic budmoves dorsally in a manner similar to the shifting of the entrance of the bile duct. -Finally the ventral bud comes to lie immediately below and behind the dorsal bud -Later the parenchyma and the duct systems of the dorsal and ventral pancreatic buds fuse together. -The ventral bud forms the uncinate process and inferior part of the head of the pancreas -The remaining part of the gland(pancreas) is derived from the dorsal bud. -The main pancreatic duct (of Wirsung) 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 either is obliterated or persists as a small channel, the accessory pancreatic 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. -Splanchnic mesoderm surrounding the pancreatic buds forms the pancreatic connective tissue. Pancreatic Abnormalities -The ventral pancreatic bud consists of two components that normally fuse and rotate around the duodenum so that they come to lie below the dorsal pancreatic bud. -Occasionally, however, the right portion of the ventral bud migrates along its normal route, but the left migrates in the opposite direction. -In this manner, the duodenum is surrounded by pancreatic tissue, and an annular pancreas is formed -The malformation sometimes constricts the duodenum and causes complete obstruction Accessory pancreatic tissue may be anywhere from the distal end of the esophagus to the tip of the primary intestinal loop Most frequently it lies in the mucosa of the stomach and in Meckel’s diverticulum, where it may show all of the histological characteristics of the pancreas itself.