Objectives 1) Describe the basic formation of foregut, midgut, and hindgut 2) Understand formation of greater and less omenta 3) Understand clinical correlations 1) Describe the basic formation of foregut, midgut, and hindgut - Body folding review - Longitudinal and transverse foldng of embryo (3/4 wk) converts it from a flat trilaminar disc into cylinder, helps internalize endoderm!- Gut tube: cranial and caudal blind-ending tubes (foregut and hindgut) and central midgut, still open ventrally to yolk sac!- Neck of yolk sac narrows until it becomes vitelline duct!- both vit duct + yolk sac incorporated into umbilical cord - Primitive Gut - Basic gut tube regions are formed, then restricted gene exp. and tissue/tissue interaction --> dev. of various organs!- Endoderm: epithelial lining of GI tract and gives rise to specific cells (parenchyma) of glands!- Muscle, CT, peritoneum components, and strome= from visceral mesoderm - Mesenteries= double layers of peritoneum that enclose the organ and connect it to body wall, suspends portions of the gut tube!- provide pathways for vessels, nerves, lymphatics to and from viscera!- 1st broad mesodermal connection of primitive gut tube to post. body wall gradually disappearing= form dorsal mesentary!- Ventral mesentary= from the septum transversum!- Organs suspended IN peritoneal cavity= intraperitoneal!- Organs embedded in the BODY WALL and covered by peritoneum= retroperitoneal (incl. kidney, bladder)!- Mesentary suspends intraperitoneal organs disappears as both mesentary and organs fuse w/body wall= secondarily retroperitoneal (think about them if you have a hard time moving them around) - Foregut - Esophagus: in 4th week, lung bud appears at ventral wall and tracheoesophageal septumforms!- Stomach:!- Dilation of foregut, appears in week 4!- Rotates 90 degrees around longitudinal axis !- L side faces anteriorly (vagus n. moves --> L vagus n. becomes anterior)!- What was on L --> anterior, R --> post.!- Original post. wall of stomach grows faster than anterior --> forms the greater and lesser curvatures!- Rotates around anteroposterior axis (week 7-8)- caudal pyloric part of stomach moves R and up, and cephalic cardiac portion moves to the L and down! --> thus the dorsal mesograstrium bulges down and keeps growing (to allow turn of stomach) --> forms a double-layered sac over transverse colon and small intestine = greater omentum - Clinical correlation: Pyloric Stenosis!- when circular musculature in region of pylorus hypertrophies --> obstructs passage of food --> projectile vomiting (white)!- develops in fetus or few days after birth!- Liver and gallbladder (Day 22)!- Liver primordium appears during week 3 as outgrowth= hepatic diverticulum of endodermal epithelium at distal end of foregut !- Grows forward into ventral mesentary, anchored to wall by ventral mesentary!- Hepatic cells penetary septum transversum, and some cells derive from it (like CT, hematopoietic cells)!- Connection between hepatic diverticulum and foregut narrows --> forms bile duct!- Bile duct forms outgrowth --> gallbladder and cystic duct!- Pancreas (Day 26)!- Formed by 2 buds (dorsal and ventral), from endodermal lining of duodenum and from origin of liver bud!- Duodenum rotates to form a C --> ventral bud moves dorsally and goes below/behind dorsal bud!- Pancreatic buds fuse, ventral forms uncinate process, dorsal forms rest of pancreas!- Clinical correlation: Abnormal formation/rotation of pancreas!- Ventral pancreas may have 2 lobes!- If lobes migrate in diff. directions to fuse w/dorsal pancreatic bud= annular pancreas --> constricts duodenum --> feeding intolerance, bilous (green) vomit, abdominal distention in infants!- Peritoneal layers!- Mesentary: double layer of peritoneum between body wall (usually post.) and organ!- Ligament: double layer of peritoneum between organs or between organ and abdominal wall!- Omentum: double layer of peritoneum between stomach and another organ!- Formation of Lesser Omentum!- Lesser omentum and falciform ligament form from ventral mesogastrium!- Liver cords grow into septum transversum, which divides to form: peritoneum of liver, falciform ligament (from liver to ventral body wall), lesser omentum (from stomach and duodenum to liver)!- Free margin of falciform lig. contains umbilical v., obliterated after birth to form round ligamnet of liver !- Free margin of lesser omentum (hepatoduodenal ligament) contains bile duct, portal v., hepatic a.! - Midgut - Characterized by rapid elongation of gut + mesentery --> forms primary intestinal loop - Primary loops stays connected to yolk sac by vitelline duct!- In 6th week: physiological herniation (runs out of room! midgut comes out of body)!- Rotation happens around axis formed by SMA, 270 degrees counterclockwise overall!- Herniated intestinal loops go back into abdominal cavity during week 10)!- Primary intestinal loop herniates into umbilicus, rotates 90 deg. counterclockwise!- Primary int. loop retracts into abdomenal cavity, rotates another 180 degrees counterclockwise!- Clinical Correlation: Abnormal Rotation of Gut!- If it doesn't rotate, or if it rotates abnormally, can be many diff. anomalies!- Mostly asymptomatic, but can lead to volvulus or some type of strangulation of gut, manifests w/sudden onset of acute abdomenal pain, vomiting, GI bleeding !- CC: Meckel's (Ilial) Diverticulum!- Blind pouch in ileum, vestigal remnant of vitelline duct (yolk stalk)!- Can be free or connected to umbilicus by ligament!- Can stay open and form unbilicointestinal fistula!- Rule of 2's: in 2% of pop'n, 2x more common in males, 2% of people w/it have symptoms, usually by 2 years old, usually found 2 ft proximal to terminal ileum and is 2 in. long!- Hindgut - Caudal end of embryo--cloacal membrane!- Gut forms expansion= cloaca (region where everything empties)!- Diverticulum of cloace= allantois extends into the connecting stalk!- During week 4-6, cloaca divided into dorsal anorectal canal, and ventral urogential sinus by formation of urorectal septum - Urogenital sinus--> gives rise to bladder, pelvic urethra, phallic segment!- Before urorectal septum fuses w/cloacal membrane, cloacal membrane ruptures, opens the urogenital sinus and dorsal anorectal canal to exterior!- Urorectal septum completely sep. urogenital sinus and anorectal canal from each other, tip of it forms the future perineum!- Defective Partitioning of Cloaca!- Fistulas: failure of the urorecetal
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