| Entodermal derivatives: formation of the gut, liver, and pancreas |
| Mike Gershon |
| Folding forms the gut |
| Primitive gut extends from buccopharyngeal to cloacal membrane. | ||
| Move toward each other | ||
| Cardiogenic mesenchyme is originally rostral, but folding brings it caudal to buccal membrane. | ||
| Foregut and hindgut become recognizable | ||
| Portion of yolk sac is incoporated into the embro as bowel. | ||
| Midgut remains open. | ||
| Cephalocaudal and lateral folding occur simultaneously |
| Meeting and fusion of cranial, lateral, and caudal edges of the embryo create the primordial foregut and hindgut | ||
| Slow fusion of midgut-due to presence of yolk sac. Midgut remains open until week 6-donnects to yolk sac via vitelline duct. | ||
| Buccopharyngeal membrane opens at 4 and cloacal membrane at 7 weeks | ||
| Flexion delimits the bowel |
| After the gut forms, it is attached to the body wall by dorsal and ventral mesenteries; ventral is lost except in region of liver. Vetelline duct remains in umbilical cord. |
| Anterior-posterior and lateral folding form the primitive gut |
| Embryonic disc grows faster in length than the yolk sac causing the embryo to bend. | ||
| Dorsal surface grows more rapidly than the ventral | ||
| Lateral folding | ||
| Fusion with apposing side except in the region of the yolk sac, and allantois | ||
| Folding brings the heart and septum transversum caudal to bucco-pharyngeal membrane. | ||
| The dorsal mesentery thins to allow the gut to be flexibly suspended |
| The foregut has many derivatives |
| Pharynx and its derivatives | |
| Lower Respiratory tract | |
| Esophagus | |
| Stomach | |
| Duodenum proximal to ampulla of Vater | |
| Liver | |
| Biliary Apparatus | |
| Pancreas |
| Esophagus elongates rapidly |
| Appears to grow faster at its cranial than caudal end. | ||
| Stomach does not descend but arises from a region just caudal to septum transversum that has been fated to be stomach. | ||
| Epithelium obliterates lumen of esophagus and is recanalized by apoptosis (week 8). | ||
| Failure causes polyhydramnios | ||
| Esophageal atresia or tracheo-esophageal fistula. | ||
| Stomach enlarges and rotates | ||
| Obliteration of the lumen and recanalization occurs |
| The stomach rotates 90° in a clockwise direction |
| Dorsal surface grows faster than the ventral to create the greater and lesser curvature. Acquires a transverse position |
| Rotation of the stomach creates the lesser sac |
| Dorsal mesogastrium moves to left. | ||
| Ventral mesogastrium attaches to liver and body wall. | ||
| Inferior recess form the greater omentum | ||
| Layers fuse to obliterate the lesser sac | ||
| Rotation of the stomach forms the omental bursa |
| Movements of the mesentery and stomach are made possible by vacuolization due to selective apoptosis |
| Liver, biliary system and pancreas arise from the duodenum |
| Hepatic diverticulum grows from the duodenum into the ventral mesentery |
| Begins ~ week 4 | ||
| Divides into cranial and caudal buds. | ||
| Cranial bud grows faster and becomes the hepatic parenchyma; | ||
| Hematopoietic colonists arrive ~ week 6 | ||
| Caudal bud gives rise to the biliary system. | ||
| Ventral mesentery forms falciform ligament, hepatic peritoneum, and lesser omentum |
| Ventral mesogastrium supports liver and stomach |
| Rotation of the stomach shapes the pancreas |
| Pancreas arises from dorsal and ventral buds. | ||
| Rotation brings ventral to dorsal bud. | ||
| Buds fuse. | ||
| Ventral duct becomes the main pancreatic duct but the dorsal bud forms most of the pancreas | ||
| Ventral bud forms only the uncinate process and inferior part of the head of the pancreas. | ||
| Aberrant rotation causes an annular pancreas |
| Review of the Gut Tube |
| Derivatives of the midgut |
| Small intestine (except for the proximal duodenum. | |
| Cecum | |
| Appendix | |
| Ascending colon | |
| Right 1/2 to 2/3 of the proximal transverse colon | |
| All are supplied by the superior mesenteric artery (“the artery of the midgut”) |
| The midgut grows rapidly and herniates into the umbilical cord |
| Slide 23 |
| The midgut rotates around
an axis of the superior mesenteric artery: 1. 90° 2. 180° |
| Rotation of the midgut |
| 1. Cranial and caudal loop form. | ||
| 2. Cranial growth >>> caudal growth. | ||
| 3. Apex of loop is vitelline duct. | ||
| 4. Cranial loop moves to right and caudal loop to left (90° counterclockwise). | ||
| 4. Reduction of midgut hernia with rotation a further 180°. | ||
| Brings cecum to right | ||
| Moves down | ||
| Becomes secondarily retroperitoneal. | ||
| Loops of bowel fuse with the body wall and become secondarily retroperitoneal |
| Slide 27 |
| Volvulus is a serious complication of excessive flexibility |
| Slide 29 |
| Derivatives of the hindgut |
| Left 1/3 to 1/2 of the distal transverse colon | |
| Descending colon | |
| Sigmoid colon | |
| Rectum | |
| Superior part of anal canal | |
| Epithelium of unrinary bladder and most of the urethra | |
| All are supplied by the inferior mesenteric artery, “the artery of the”. hindgut |
| The hindgut is originally a cloaca-partioned to form rectum and urogenital sinus |
| Urorectal septum divides the cloaca |
| Hindgut forms superior 2/3 of rectal canal; proctodeum forms lower 1/3; divided at pectinate line |
| Never forget the pectinate line |
| If anything can go wrong it will; anorectal malformations |
| The END |
| Have a nice day! |