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GI EMBRYOLOGY OVERVIEW Primordial gut is closed at 4th week by oropharyngeal membrane at cranial end and at caudal end by cloacal membrane Endoderm gives rise to gut epithelium and glands Epithelium at cranial and caudal end is from ectoderm of stomodeum and proctodeum Fibroblast growth factors are involved early in anterior-posterior axial patterning o Ex) FGF 4 signals endoderm induction by ectoderm and mesoderm Muscular, connective tissue, and other layers of the wall of digestive tract are derived from splanchnic mesenchyme surrounding primordial gut Gut is divided into foregut, midgut, and hindgut FOREGUT Derivatives are o Primordial pharynx and its derivatives o Lower respiratory system o Esophagus and stomach o Duodenum, distal to opening of bile duct o Liver, biliary apparatus (hepatic ducts, gall bladder, and bile duct), and pancreas o All of these except for pharynx and lower respiratory tract and most of esophagus are supplied by the CELIAC TRUNK—the artery of the foregut DEVELOPMENT OF ESOPHAGUS o Develops from foregut immediately caudal to pharynx o Is partitioned from trachea by tracheoesophageal septum o Reaches final relative length by week 7 o Epithelium and glands from endoderm and they grow to completely obliterate the lumen, then are recanalized o Striated muscle forming superior third of esophageal muscular externa is derived from mesenchyme in the caudal pharyngeal arches o Smooth muscle develops from splanchnic mesenchyme o Both types are innervated by X which serve the caudal pharyngeal arches PG. 213 for esophageal atresia, stenosis, and short esophagus (hiatal hernia) DEVELOPMENT OF STOMACH o At week 4, slight dilation in the tube indicates the site of primordium of the stomach o Is initially oriented in the median plane o Soon enlarges and broadens ventrodorsally Dorsal border grows faster than ventral border, forming greater curvature o ROTATION OF STOMACH Rotates 90 degrees in a clockwise direction (viewed from cranial end) around its longitudinal axis This has several effects Lesser curvature (ventral border) moves towards the right and the greater curvature (dorsal border) moves to the left Left side becomes ventral surface and right side becomes dorsal surface Cranial region of the stomach moves left and slightly inferiorly and its caudal region moves right and superiorly Explains why the left vagus nerve supplies anterior wall and the right vagus innervates the posterior wall o MESENTERIES OF STOMACH Stomach is suspended from dorsal wall of abdominal cavity by the primordial dorsal mesogastrium—a dorsal mesentery Is carried left during rotation of the stomach and formation of the omental bursa/lesser sac Primordial ventral mesogastrium attaches to stomach as well as connecting the duodenum to the liver and the ventral abdominal wall OMENTAL BURSA o Isolated clefts develop in the mesenchyme forming the thick dorsal mesogastrium—they coalesce to form a single cavity—omental bursa/lesser sac o Rotation of stomach pulls the dorsal mesogastrium to the left, enlarging the bursa o Bursa expands transversely and cranially to lie between stomach and the posterior abdominal wall o Superior part is cut off as diaphragm develops forming a closed space—infracardiac bursa If it persists, it lies medial to the base of the right lung o Inferior part of omental bursa persists as superior recess of the omental bursa o Inferior recess of omental bursa is formed between the layers of elongated dorsal mesogastrium—which forms the greater omentum PG. 215 for figure The greater omentum overhangs the developing intestines Inferior recess disappears as the layers of the greater omentum fuse o Omental bursa communicates with main part of the peritoneal cavity through the omental foramen—located posterior to the free edge of lesser omentum in adult DEVELOPMENT OF DUODENUM o Duodenum develops from distal foregut, the proximal part of midgut, and splanchnic mesenchyme Its blood supply is therefore from both CELIAC TRUNK and SUPERIOR MESENTERIC ARTERY o Junction of two parts of duodenum is just distal to origin of the bile duct o As the stomach rotates, the duodenal loop rotates right and comes to lie retroperitoneally (external to peritoneum) o Lumen is obliterated and recanalized By this time, most of the ventral mesentery of duodenum has disappeared o PG. 216 for congenital hypertrophic pyloric stenosis, duodenal stenosis, and duodenal atresia DEVELOPMENT OF LIVER AND BILIARY APPARATUS o Liver, gallbladder, and biliary duct system arise as ventral outgrowth—hepatic diverticulum—from caudal or distal part of the foregut early in week 4 o Diverticulum extends to septum transversum—mass of splanchnic mesoderm between developing heart and midgut Septum transversum forms ventral mesentery in this region o Hepatic diverticulum enlarges and divides into two parts as it grows between layers of ventral mesogastrium o Larger cranial part is liver primordium Proliferating endoderm gives rise to hepatic cords and to epithelial lining of the intrahepatic part of biliary apparatus Hepatic cords anastomose around endothelium-lined spaces—hepatic sinusoids Fibrous and hematopoietic tissue, as well as Kupffer cells are derived from mesenchyme in the septum transversum o Small caudal part of the hepatic diverticulum becomes the gallbladder, and the stalk of the diverticulum forms the cystic duct Stalk connecting the hepatic and cystic ducts to the duodenum becomes the bile duct Initially, the stalk connects to the ventral aspect of the duodenal loop, but as it rotates, the entrance to bile duct is carried posteriorly VENTRAL MESENTERY o Thin double layered membrane gives rise to Lesser omentum, passing from liver to the lesser curvature of the stomach Hepatogastric ligament Lesser omentum also passes from liver to duodenum Hepatoduodenal ligament Falciform ligaement, extending from liver to the ventral abdominal wall. o Umbilical vein passes in the free border of the falciform ligament on its way from the umbilical cord to the liver o Ventra mesentery is derived from mesogastrum Also forms the visceral peritoneum of the liver o Liver is covered with peritoneum except for bare area in contact with diaphragm PG. 220 anomalies of liver DEVELOPMENT OF PANCREAS o Develops between the layers of mesentery from dorsal and ventral pancreatic buds of endodermal cells, arising from foregut o Is derived from dorsal pancreatic bud and ventral pancreatic bud o As duodenum rotates right, the ventral bud is carried dorsally with the bile duct Eventually it lies posterior to the dorsal bud and fuses with it o Ventral pancreatic bud forms the uncinate process of the head of pancreas o As the two buds fuse, their ducts anastomose Pancreatic duct forms from ventral bud and the distal part of the duct of the dorsal bud Proximal part of dorsal bud persists as accessory pancreatic duct in some people that opens into the minor duodenal papilla Sometimes the ducts fail to fuse, and you get two ducts o HISTOGENESIS OF PANCREAS Parenchyma is derived from endoderm Pancreatic acini develop from cell clusters around the ends of these tubules Pancreatic islets develop from cells that separate from the tubules and lie between the acini o PG. 222 pancreas anomalies DEVELOPMENT OF SPLEEN o Derived from mesenchymal cells located between the layers of the dorsal mesogastrium o Is a vascular lymphatic organ o As the stomach rotates, the left surface of mesogastrium fuses with the peritoneum over the left kidney—the fusion explains the dorsal attachment of the splenorenal ligament and why the adult splenic artery follows a tortuous course posterior to the omental bursa and anterior to the left kidney o HISTOGENESIS OF SPLEEN Mesenchymal cells differentiate to form the capsule, connective tissue framework, and parenchyma of the spleen o PG. 224 accessory spleens/polysplenia MIDGUT Derivatives are o Small intestine, including duodenum distal to the opening of bile duct o Cecum, appendix, ascending colon, and the right one half to two thirds of the transverse colon These are supplied by the SUPERIOR MESENTERIC ARTERY, the midgut artery As the midgut elongates, it forms a ventral U shaped loop of gut—the midgut of the intestine, projecting into the remains of extraembryonic coelom in the proximal part of the umbilical cord o This is a physiologic umbilical herniation o It communicates with yolk sac through omphaloenteric duct/yolk stalk o Occurs because there is not enough room in the abdominal cavity for the midgut Due to huge liver and kidneys at this point in development Omphaloenteric duct is attached to the apex of the midgut loop where the cranial and caudal limbs join Cranial end grows rapidly and forms loops, but caudal end only develops cecal swelling/diverticulum (primordium of cecum and appendix) o Midgut loop is suspended from dorsal abdominal wall by mesentery ROTATION OF MIDGUT LOOP o Midgut loop rotates 90 degrees counterclockwise (looking from the ventral side) around the axis of the SMA o This brings the cranial limb (small intestine) of the midgut loop to the right and the caudal limb (large intestine) to the left o During rotation, the cranial end elongates and forms intestinal loops (primordial of jejunum and ileum) o RETURN OF MIDGUT LOOP TO THE ABDOMEN Not known what causes this to happen Small intestine returns first, passing posterior to the SMA and occupies central part of the abdomen As the large intestine returns, it undergoes a 180 degree counterclockwise rotation, and later occupies the right side of the abdomen—ascending colon FIXATION OF INTESTINES o Rotation of stomach and duodenum causes the duodenum and pancreas to fall to the right o Enlarged colon presses the duodenum and pancreas against the posterior abdominal wall, so most duodenal mesentery is absorbed Therefore the duodenum, except for approximately the first part derived from foregut, has no mesentery and lies retroperitoneally (posterior to peritoneum) o 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 median plane As intestines enlarge and lengthen, their mesenteries are pressed against the posterior abdominal wall Mesentery of ascending colon fuses with parietal peritoneum on this wall and disappears—so ascending colon is also retroperitoneal o Other derivatives of midgut loop retain mesenteries (jejunum and ileum) After the mesentery of the ascending colon disappears, the fan-shaped mesentery of the small intestine acquires a new line of attachment that passes from the duodenojejunal junction inferolaterally to the ileocecal junction CECUM AND APPENDIX o Primordium of these is cecal swelling o Appendix eventually comes to enter the cecum’s medial side because of unequal growth of cecum after birth o Position of appendix is variable PG. 228-235—more anomalies of the gut HINDGUT Derivatives are o Left one third to one half of the transverse colon, the descending and sigmoid colon, the rectum, and the superior part of the anal canal o Epithelium of urinary bladder and most of the urethra They are all supplied by INFERIOR MESENTERIC ARTERY, the artery of the hindgut The junction between the segment of transverse colon derived from the midgut and that originating from the hindgut is indicated by the change in blood supply from a branch of the SMA to a branch of the IMA The descending colon becomes retroperitoneal as its mesentery fuses with the peritoneum on the left posterior abdominal wall and then disappears Mesentery of sigmoid colon is retained, but it is shorter than in the embryo CLOACA o Is the expanded terminal portion of the hindgut o Lined with endoderm o Contacts surface ectoderm at the cloacal membrane, which is composed of endoderm of cloaca and ectoderm of the proctodeum/anal pit o Cloaca receives the allantois ventrally—a fingerlike diverticulum o PARTITIONING Divided into ventral and dorsal parts by urorectal septum—a wedge of mesenchyme Two parts Rectum and cranial part of anal canal dorsally Urogenital sinus ventrally Urorectal septum fuses with the cloacal membrane, dividing it into a dorsal anal membrane and larger ventral urogenital membrane You see this fusion in the adult as the perineal body—tendinous center of the perineum Several muscles converge and attach here urorectal septum also divides the cloacal sphincter into anterior and posterior parts posterior becomes external anal sphincter anterior part becomes superficial transverse perineal, bulbospongiosus, and ischiocavernosus muscles this explains why the pudendal nerve supplies all these muscles anal membrane is supposed to rupture by the end of the 8 th week, bringing the anal canal in communication with amnionic cavity ANAL CANAL o Superior two thirds derived from hindgut o Inferior third is from proctodeum o Pectinate line indicates where epithelium derived from ectoderm joins with epithelium derived from endoderm It is located at the inferior limit of the anal valves Approximates where the anal membrane was o 2 cm superior to anus is anocutaneous line where composition of anal epithelium changes from columnar to stratified squamous o Other layers of the wall of the anus are derived from splanchnic mesenchyme o Superior two thirds of anal canal are supplied by SUPERIOR RECTAL ARTERY, continuation of the IMA The hindgut artery Venous drainage of superior part is via SUPERIOR RECTAL VEIN, a tributary of IMV Nerves are from ANS o Inferior third of anal canal, because it’s from proctodeum, is supplied by INFERIOR RECTAL ARTERIES, branches of INTERNAL PUDENDAL ARTERY Venous drainage from inferior rectal vein, tributary of internal pudendal vein that drains into internal iliac vein Nerves from inferior rectal nerve, sensitive to pain, temperature, touch, and pressure o IMPORTANT when considering metastasis Characteristics of tumors of the two parts differ Tumors of superior part are painless and arise from columnar epithelium, while those in inferior part are painful and arise from stratified squamous MORE ANOMALIES PG. 238-241 o