GIT Embryo

• Lateral folding of the embryo

  The development of the GIT starts with the lateral folding of the embryo

  1. Embryo folds from a flat structure into a tubular structure

  2. Endoderm-lined part of the yolk sac gets incorporated into the embryo
• Vitelline duct formation

  Part of the yolk sac which gets incorporated into the embryo becomes narrow and forms a vitelline duct
• Primitive gut

  Part of the yolk sac which gets incorporated into the embryo
• Derivatives of the primitive gut

  • Cephalic part = Foregut
  • Continuous with the yolk sac at this point = Midgut
  • Caudal end = Hindgut
• Foregut derivatives

  • Pharynx (and its derivatives)
  • Lower respiratory system
  • Oesophagus
  • Stomach
  • Proximal duodenum
  • Liver
  • Pancreas
• Midgut derivatives

  • Proximal limb = distal part of duodenum, jejenum & proximal part of ileum
  • Distal limb = distal part of ileum, cecum, appendix, ascending colon and proximal 2/3 of tranverse colon
• Hindgut derivatives

  • Distal 1/3 of the transverse colon
  • Descending colon
  • Sigmoid
  • Rectum
  • Proximal part of the anal canal
• Oropharyngeal membrane

  Termination of the tube at the cranial end

  • It remains bilaminar after gastrulation, i.e. it doesn’t have mesoderm between ectoderm and endoderm
  • The part that is more cranial to the oropharyngeal membrane is the stomodeum
• Cloacal membrane

  Separates the hindgut from the proctodeum

  • Remains bilaminar after gastrulation. When it ruptures, the proctodeum becomes continuous with the gut tube.
• Membrane ruptures

  • The oropharyngeal membrane ruptures around the 4th week of development
  • The cloacal membrane ruptures around the 7th week of development
• Divisions of the primitive gut

  • Foregut lies caudal to the pharynx and extends as far caudally as the liver outgrowth
  • Midgut begins caudal to the liver bud and extends to the junction of the right two-thirds and left third of the transverse colon in the adult
  • Hindgut extends from the left third of the transverse colon to the cloacal membrane
• Intestinal junctions

  • Anterior intestinal portal: Junction between the foregut and the midgut
  • Posterior intestinal portal: Junction between the midgut and the hindgut
• Blood supply of guts

  • Celiac artery: Foregut (all derivatives except pharynx, respiratory tract and most of the oesophagus)
  • Superior mesenteric artery = midgut
  • Inferior mesenteric artery = hindgut
• Oesophagus development

  The oesophagus develops caudal to the pharyngeal gut

  1. The epithelial lining of the oesophagus will proliferate profusely such that the cells obliterate the lumen of the oesophagus.

  2. The tube will re-canalise so that the lumen will reappear

  3. Oesophagus lengthens rapidly as heart and lungs descend

  4. This will cause the primordium of the stomach (which lies distal to the oesophagus) to descend from its original cervical levels into the abdominal cavit
• Hiatus/Hiatal hernia

  Condition where part of the stomach remains in the thoracic cavity due to failure of oesophagus elongation
• Stomach rotation

  1. Stomach rotates 90 degrees clockwise, such that its left side faces anteriorly & right side posteriorly.

  2. Left vagus nerve will supply the anterior side. Right vagus nerve will supply the posterior side of the stomach.

  3. Dorsal wall grows faster than ventral wall, forming greater and lesser curvatures respectively.

  4. Cardiac portion moves left in a slightly downwards direction, pyloric portion moves right in a slightly upwards direction.

  5. Therefore, the stomach will assume its final position with its axis running from above left to below right.
• Dorsal mesogastrium

  Mesentery connecting the stomach to the posterior body wall
• Ventral mesogastrium

  Mesentery connecting the stomach to the anterior body wall, forms falciform ligament and lesser omentum
• Omental bursa/Lesser peritoneal sac


  Space behind the stomach created by stomach rotation
• Greater omentum

  Double-layered sac formed by the bulging down of the dorsal mesogastrium
• Duodenum development

  1. Distal foregut and proximal midgut form the duodenum

  2. Duodenum rotates to the right and forms a C-shape

  3. Lumen gets obliterated by epithelial proliferation then re-canalises

  4. Supplied by branches of both celiac and superior mesenteric arteries
• Liver development

  1. Hepatic diverticulum/liver bud forms as an outgrowth from the endoderm at the anterior intestinal portal

  2. Hepatic cells penetrate the septum transversum

  3. Connection to foregut narrows to form bile duct

  4. Ventral diverticulum forms gall bladder and cystic duct

  5. Hepatic cells invade vitelline and umbilical veins, breaking them into sinusoids
• Septum transversum

  Mesodermal plate between the pericardial cavity and the yolk sac stalk

  >The haematopoietic cells, Kupffer cells and connective tissue cells of the liver are derived from the mesoderm of the septum transversum.
• Falciform ligament and lesser omentum

  Formed from the membranous septum transversum after the liver has invaded it

  >The mesenchyme on the surface of the liver will differentiate into visceral peritoneum, except on its cranial surface. In this region, the liver remains in contact with rest of the original septum transversum.

  >This portion of the septum will form the central tendon of the diaphragm.
• Bare area of the liver


  Surface of the liver that remains in contact with the future diaphragm and is not covered by peritoneum
• Pancreas development

  1. Ventral and dorsal pancreatic bud form from the endoderm lining of the duodenum

  2. The dorsal pancreatic bud is found in the dorsal mesentery, whereas the ventral pancreatic bud is close to the bile duct

  3. Ventral bud migrates dorsally towards the dorsal bud as the duodenum rotates

  4. Later, the parenchyma of the two buds will fuse in order to form the pancreas

  5. Most of the pancreas is formed by the dorsal pancreatic bud, whereas the ventral pancreatic bud forms the uncinate process
• Duct system of the pancreas

  1. Main pancreatic duct formed by distal part of dorsal duct and entire ventral duct

  2. Proximal part of dorsal duct obliterated or forms accessory duct
• Annular pancreas

  Two components of ventral bud fail to fuse, right migrates normally but left migrates in opposite direction, surrounding duodenum
• Islets of Langerhans

  • Develop from pancreatic parenchyma in 3rd month of gestation
  • Insulin production evident in 5th month
  • Connective tissue surrounding pancreas derived from splanchnic mesoderm
• Development of midgut - Primary intestinal loop

  1. Rapid elongation of midgut and mesentery forms primary intestinal loop

  2. Proximal limb develops into distal duodenum, jejunum, proximal ileum

  3. Distal limb develops into distal ileum, cecum, appendix, ascending and proximal transverse colon
• Physiological umbilical herniation

  Midgut loop herniates into connecting stalk (umbilical cord) due to growing liver and kidneys (6th week of development)

  • Rotates at 90 in an anti-clockwise along the longitudinal axis formed by the superior mesenteric artery. The proximal limb = right & distal limb = left of the embryo
  • A diverticulum, the cecum, forms at the transition between the two limbs and creates the vermiform appendix
  • In the 10th week, midgut loop will return to the abdominal cavity due to unknown factors.
• Rotation of midgut loop

  1. 90 degree anticlockwise rotation when herniating

  2. Further 180 degree anticlockwise rotation when returning to abdominal cavity

  3. Total 270 degree anticlockwise rotation
• Exomphalos occurs if midgut fails to return to abdominal cavity
• Caecum anomaly occurs if caecum fails to descend to right iliac fossa

  >The caecum and the vermiform appendix will remain in a subhepatic position.

  >This may lead to abnormal site of pain in acute appendicitis.
• Abnormalities of rotation of midgut

  • Abnormal rotation: 90 degree rotation only, colon and caecum enter first and settle on left side of the abdominal cavity resulting in a left-sided colon
  • Reversed rotation: clockwise rotation, transverse colon passes behind duodenum and superior mesenteric artery
• Development of hindgut part 1

  
  

  1. Continuation of midgut, separated from proctodeum by cloacal membrane

  2. Gives rise to distal transverse colon, descending colon, sigmoid, rectum, proximal anal canal

  3. Terminal hindgut enters posterior cloaca, allantois enters anterior cloaca

  4. Urorectal septum (mesodermal) divides the cloaca into the anterior primitive urogenital sinus (later forms urinary bladder)and the dorsal rectum
• Urorectal fistula

  Failure of urorectal septum to fully separate urogenital sinus and rectum, rectum opens into urethra and faecal material is deposited into the urethra
• Rectovaginal fistula

  Failure of urorectal septum to fully separate urogenital sinus and rectum, rectum opens into vagina and faecal material is deposited into the vagina.
• Imperforate anus

  Failure of proctodeum to recanalize or cloacal membrane to rupture, no anal opening
• The GIT consists of a muscular tube which extends from the oral cavity down to the anal canal.

  • Most epithelium of GIT is derived from endoderm

  • Stomodeum (which forms oral cavity) and proctodeum (which forms inferior part of anal canal) are derived from ectoderm

  • Connective tissue and muscle is derived from splanchnic mesoderm