PRIMITIVE GUT

1. PRIMITIVE GUT • It is a portion of the Endoderm-lined yolk sac which is incorporated in the embryo as a result of cephalcaudal and lateral folding. • It is divided into : • Fore, Mid and Hind guts.

2. PRIMORDIAL GUT • It gives rise to most of the Epithelial lining and Glands of the digestive tract. • The muscular and connective tissue is derived fro the Splanchnicmesoderm. • The epithelium at the cranial end (Stomodeum) and the caudal end (Proctodeum) is derived from Ectoderm.

3. FORE GUT • It is the part incorporated in the cephalic part of the embryo. • It forms a blind tube which lies caudal to the pharyngeal tube. • It extends as far caudally as the primordium of the liver.

4. DERIVATIVES OF THE FORE GUT • 1. Primordial pharynx (oral cavity, tongue, salivary glands, tonsils and upper respiratory system). • 2. Lower respiratory system. • 3. Esophagus and Stomach. • 4. Liver and biliary apparatus.

5. ESOPHAGUS • Origin: • It develops caudal to the primitive pharynx. • The Tracheoesophageal septum divides the fore gut into a Ventral portion (RespiratoryDiverticulum) and a Dorsal portion (Esophagus).

6. LENGTH • Initially the esophagus is short. • It elongates because of the growth and descent of the heart and lungs. • The final length is reached by the seventh week.

7. STRUCTURE • The epithelial lining and glands are from the Endoderm. • Muscles : • Upper third : Striated. From the mesenchyme • in the caudal pharyngeal arches. • Lower third : Smooth. • From splanchnic mesoderm.

8. ESOPHAGUS • Rapid proliferation of the epithelium obliterates the lumen temporarily. • Recanalization of the esophagus normally occurs at the end of the embryonic period.

9. STOMACH • It appears in the middle of the (4TH ) week as a fusiform dilatatation at the caudal end of the foregut. • It is initially in the median plane.

10. SHAPE • Its Cranial and Caudal ends are in the median plane. • It has Dorsaland a Ventral borders. • The dorsal border grows faster than the ventral. • This demarcates the Greater Curvature of the stomach.

11. ROTATION • The stomach rotates (90) degrees in a Clockwise direction around its longitudinal axis.

12. RESULTS OF ROTATION • (1) Lesser Curvature moves to the Right. • The Greater Curvature moves to the Left. • (2) The Left side becomes the Ventral surface. • The Right side becomes the Dorsal surface.

13. RESULTS OF ROTATION • This explains the Left Vagus nerve supplies the anterior wall of the adult stomach and the RightVagus supplies the posterior wall.

14. RESULTS OF ROTATION • (3) The Cranial region moves to the Left and Inferiorly. • The Caudal region (pyloric) end moves to the Right and Superiorly.

15. RESULTS OF ROTATION • (4) The Long axis of the stomach becomes almost Transverse to the Long axis of the body.

16. MESENTRIES • The stomach is attached to the dorsal body wall by the Dorsal Mesogastrium and to the ventral body wall by the VentralMesogastrium.

17. OMENTAL BURSA • Isolated intercellular clefts appear in the Dorsalmesogastrium. • These clefts fuse and form a single cavity (Lesser Peritoneal Sac). • Rotation of the stomach pulls the Dorsal mesogastrium to the left and enlarges the bursa.

18. OMENTAL BURSA • The bursa represents the extension of the right side of the peritoneal cavity behind the stomach. • The developing diaphragm cuts off the superior part of the bursa. If it persists, it lies medial to the base of the right lung.

19. OMENTAL BURSA • The persisting bursa forms a closed sac (InfraCardiac bursa). • Superior Recess of the omental bursa: • It represents the inferior portion of the superior part of the bursa.

20. OMENTAL BURSA • Inferior Recess of omental bursa: • It is developed due to the enlargement of the stomach. • It is found between the layers of the elongated part of the dorsal mesogastrium (GreaterOmentum).

21. OMENTAL BURSA • The omental bursa communicates with the peritoneal cavity through the OmentalForamen.

22. DUODENUM • It develops from the caudal par of the Foregut and the cranial part of the Midgut. • The junction of the two guts is caudal to the origin of the Bile Duct.

23. DUODENUM • The duodenum forms a C- shaped loop. • By the Rotation of the stomach, the duodenal loop moves to the Right. • This rotation and the rapid growth of the head of the pancreas swings the duodenum from the initial midline position to the left side of the abdominal cavity.

24. RETROPERITONEAL POSITION • Initially the duodenum has a Dorsal mesoduodenum. • It fuses with the peritoneum of the posterior abdominal wall. • Later the two layers disappear and the duodenum becomes fixed.

25. RECANALIZATION • In the 4th and 5th weeks, the lumen is temporarily obliterated by Proliferation of the epithelial cells. • Degeneration of the cells causes normal vacuolation and the duodenum becomes recanalized.

26. SPLEEN • It is a large vascular lymphatic organ. • It is derived from a mass of Mesenchymalcells between the layers of the Dorsal Mesogastrium. • It is developed during the (5th ) week. • It acquires its characteristic shape in the fetal period.

27. SPLEEN • The Fetal spleen is Lobulated. • The lobules disappear Beforebirth. • The notches in the upper border of the adult spleen are remnants of the grooves that separated the fetal lobules.

28. SPLEEN • Because of the Rotation of the stomach, the portion between the spleen and the dorsal midline swings to the left. • It fuses with the peritoneum over the left kidney.

29. SPLEEN • The peritoneum along this line of fusion degenerate. • The spleen is connected to the body wall in the region of the left kidney by the lienrenal ligament. • This explains the tortuous course of the adult splenic artery.

30. HEPATIC DIVERTICULUM • It arises as a ventral outgrowth from the caudal part of the fore gut during the (4th ) week. • It extends into the septum transversum. • Its larger cranial part is the primordium of the Liver. • The small caudal part becomes the GallBladder.

31. SEPTUM TRANSVERSUM • It is a mass of Splanchnic mesoderm between the developing heart and the mid gut.

32. DERIVATIVES OF SEPTUM TRANSVERSUM • 1. Central tendon of the diaphragm. • 2. Ventral mesentery with its two parts : • (a) Falciform ligament. • (b) Lesser omentum. • 3. Kupffer cells and fibrous tissue of the liver. • 4. Visceral peritoneum of the liver.

33. STRUCTURE OF THE LIVER • The proliferating Endodermal cells give rise to : • 1. Parenchyma (liver cells). • 2. Epithelial lining of the (Intrahepatic) portion of the biliary apparatus.

34. STRUCTURE OF THE LIVER • The Vitelline and Umblical veins will form • the Hepatic Sinusoids. • The development and functional segmentation of the liver is determined by the quantity of oxygenated blood passing to the liver through the umbilical vein.

35. SIZE • The liver fills a large part of the abdominal cavity between (5th -10th ) weeks. • It accounts about (10%) of the total weight of the fetus by the 9th week. • It is formed of right and left lobes of almost the same size. • The right lobe soon becomes the larger one.

36. FUNCTION • Hematopoiesis begins during the 6th week. • It is responsible for : • 1. Bright reddish appearance of the liver. • 2. Large size of the liver (7th -9th ) weeks. • Bile formation begins during (12th ) week.

37. EXTRA HEPATIC BILIARY APPARATUS • Ahepatic ductcomes out from each lobe. • The two ducts unite to form asingleCommonHepatic duct. • Bile duct: • The connection between the hepatic diverticulum and the fore gut (duodenum) narrows to form the bile duct. • Gall bladder and Cystic duct: • Formed from a small ventral outgrowth of the bile duct.

38. EXTRA HEPATIC BILIARY APPARATUS • The bile duct initially opens into the Ventral aspect of the duodenal loop. • Growth and Rotation of the duodenum brings this opening Dorsally.

39. EXTRA HEPATIC BILIARY APPARATUS • The proliferating epithelial cells cause temporary occlusion of the biliary apparatus. • Recanalization is restored by degeneration of these cells. • Bile enters the duodenum during (13th ) week. It gives the meconium its dark green color.

40. PANCREAS • It is formed from two Pancreatic Buds (Dorsal and Ventral). • They arise from Endodermal cells in the proximal part of the duodenum (caudal part of the foregut) between the layers of the dorsal and ventral mesentery respectively.

41. PANCREATIC BUDS • The Dorsalbud is the larger. • It appears first and is slightly cranial to the ventral bud. • The Ventral bud develops near the entry of the bile duct into the duodenum.

42. ROTATION • The Rotation of the duodenum to the right and its C shaped appearance shifts the entry of the bile duct and the ventral pancreatic duct Dorsally.

43. FUSION • The ventral bud comes to lie immediately below and dorsal to the dorsal bud. • The parenchyma of the buds fuse and their ducts anastomose.

44. DERIVATIVES OF THE BUDS • The Ventral bud forms : • 1.The Uncinate process. • 2. The Inferior part of the head. • The Dorsal bud forms the remaining part of the pancreas.

45. DUCTS • Main pancreatic duct • It is formed from the distal part of the dorsal pancreatic duct and the entire ventral pancreatic duct. • It opens with the bile duct into the Major duodenal papilla.

46. DUCTS • Minor pancreatic duct : • It is the Proximal part of the dorsal pancreatic duct (if persists). • It opens into the Minorduodenal papilla (2 cm) cranial to the main duct.

47. FUNCTIONS • (1) Insulin secretion • Pancreatic islets ( of Langerhans) develop from the parenchymatous pancreatic tissue. • They produce insulin by the 10th week.

48. FUNCTIONS • (2) Glucagon and Somatostatin. • Their containing cells develop before differentiation of the insulin secreting cells. • Glucagon has been detected in the fetal plasma at 15 weeks.