Embryology of GI system

Embryology of the Gastrointestinal (GI) System

Introduction

  • Instructor: Dr. Najla Fiaturi

  • Courtesy of: Dr. Robert Willson

Learning Objectives

  • Describe the formation of the foregut, midgut, and hindgut and distinctions based on blood supply.

  • Explain the rotation of the stomach and its relation to the lesser and greater peritoneal sacs.

  • Discuss the development of the pancreas and biliary system from duodenal buds.

  • Connect physiological herniation of the midgut and abnormalities of rotation to clinical conditions.

  • Understand how certain GI portions become secondary peritoneal.

Embryonic GI Tube Divisions

  • The embryonic GI tube is divided into three parts:
      - Foregut
        - Begins at the level of the respiratory bud from the esophagus.
        - Structures include:
          - Esophagus
          - Stomach
          - Duodenum (up to the entrance of the common bile duct)
          - Liver
          - Pancreas
      - Midgut
        - Extends from the entry of the common bile duct to the junction of the proximal two-thirds and distal one-third of the transverse colon.
        - Structures include:
          - Distal duodenum
          - Jejunum
          - Ileum
          - Cecum
          - Vermiform appendix
          - Ascending colon
          - Proximal transverse colon
      - Hindgut
        - Extends from the transverse colon junction through the upper one-third of the rectum.
        - Structures include:
          - Distal transverse colon
          - Descending colon
          - Sigmoid colon
          - Upper one-third of the rectum/anal canal

Arterial Supply of GI Derivatives

  • Each division receives arterial supply from branches of the abdominal aorta's unpaired visceral branches derived from the yolk sac:
      - Celiac trunk: artery for the foregut
      - Superior Mesenteric Artery (SMA): artery for the midgut
      - Inferior Mesenteric Artery (IMA): artery for the hindgut

Formation of the Tri-laminar Embryo

  • Implantation: Blastocyst implantation in the endometrium completes during the second week of development.

  • Inner Cell Mass (Embryoblast): Differentiates into two layers:
      - Hypoblast layer: Composed of small cuboidal cells.
      - Epiblast layer: Composed of columnar cells.
      - The two-layered structure is termed the embryonic disc.

  • Development of the Amniotic Cavity:
      - Epiblast forms the floor of the amniotic cavity and is continuous with the amnion.

  • Exocoelomic Membrane: Formed by flattened hypoblast-derived cells, lining the primitive yolk sac.

  • Extraembryonic Mesoderm: Derived from yolk sac cells, fills the space between the trophoblast and the amniotic/exocoelomic membranes.

  • Extraembryonic Coelom: Formed by merging cavities within the extraembryonic mesoderm; it divides into:
      - Extraembryonic Somatic Mesoderm: Lining the trophoblast and amnion.
      - Extraembryonic Splanchnic Mesoderm: Covering the yolk sac.

  • Chorion Formation: Composed of the extraembryonic somatic mesoderm and trophoblast layers.

  • Amnion Formation: Composed of the extraembryonic somatic mesoderm and ectoderm.

  • Secondary or Definitive Yolk Sac Formation: From the endodermal germ layer.

  • Chorionic Cavity: The expanding coelom forms a cavity that suspends the embryo and attached structures by the connecting stalk.

  • Embryo Folding:
      - Medial Plane Folding: Results in head and tail folds; incorporates part of the yolk sac, forming the foregut and hindgut.
      - Horizontal Plane Folding: Forms lateral and ventral body walls; incorporates yolk sac into the midgut.

Formation of Body Cavities and Embryo Folding

  • Intraembryonic Coelom Development: Emerges by the end of week 3, becoming a horseshoe-shaped cavity.

  • Horseshoe Shape Representation:
      - Future pericardial cavity is represented by the curve.
      - Future pleural and peritoneal cavities represented by lateral limbs.

  • Week 4 Folding:
      - Lateral parts of the intraembryonic coelom merge ventrally, forming the peritoneal cavity.
      - Disappearance of the caudal part of the ventral mesentery allows the right and left coelomic parts to merge.

  • Dorsal Mesentery Formation: Encloses the primitive gut and suspends it from the dorsal body wall.

  • Embryonic Pericardial to Peritoneal Cavity Communication: Present until week 7 via pericardioperitoneal canals; partitions form during weeks 5 and 6 for separation.

Development of the Foregut

Stomach Development and Rotation

  • The developing stomach rotates 90 degrees clockwise around its longitudinal axis:
      - Original left side becomes a ventral surface.
      - Original right side becomes a dorsal surface.
      - Greater Curvature: Moves caudally and to the left.
      - Lesser Curvature: Moves cranially and to the right.
      - Vagal Trunks Movement: Left vagus trunk moves anteriorly, while right trunk moves posteriorly (Mnemonic: “LARP” - Left Anterior, Right Posterior).

Creation of the Lesser Sac (Omental Bursa)

  • Stomach rotation leads the duodenum to bend into a “C” shape and lie against the posterior body wall.

  • The mesentery of the duodenum fuses with the parietal peritoneum, causing:
      - Duodenum and pancreas to become secondarily retroperitoneal.
      - This classification differentiates structures that lost their mesenteries during development from those that never possessed mesenteries (primarily retroperitoneal structures include kidneys, suprarenal glands, abdominal aorta, and inferior vena cava).

  • Omental Bursa Formation:
      - Formed as the dorsal mesogastrium is pulled to the left.
      - The spleen develops within dorsal mesogastrium layers, remaining intraperitoneal.
      - Ligament Connections:
        - Splenorenal (Lienorenal) Ligament: Anchors spleen to posterior body wall near the left kidney.
        - Gastrosplenic (Gastrolienal) Ligament: Connects spleen to the stomach.

Development of the Pancreas

  • Pancreatic Bud Development:
      - Two endodermal buds from the duodenum contribute:
        - Ventral Bud: Arises in the ventral mesentery.
        - Dorsal Bud: Arises in the dorsal mesentery.

  • When the duodenum rotates:
      - The ventral bud moves dorsally, positioning itself below and behind the dorsal bud.

  • Pancreatic Structure Formation:
      - Ventral bud becomes the uncinate process and the inferior part of the pancreas's head.
      - Dorsal bud forms the remainder of the pancreas.

  • Pancreatic Ducts:
      - Main Pancreatic Duct (Duct of Wirsung): Derived from the distal dorsal duct and whole ventral duct.
      - Accessory Pancreatic Duct (Duct of Santorini): Maintains from the proximal part of the dorsal pancreatic duct.

Development of the Liver

  • Hepatic Development:
      - Begins as a diverticulum from the duodenum, growing toward the septum transversum.
      - Fuses with the septum to form:
        - Hepatic ducts,
        - Bile ducts,
        - Gallbladder.
      - Vitelline veins get incorporated, developing into hepatic sinusoids.

Development of the Midgut

  • Midgut Development Characteristics:
      - Rapid elongation of both the gut and mesentery occurs while the body cavity is still small, initiating “physiological herniation” into the umbilical cord at approximately 6 weeks of development.
      - Apex of the midgut loops maintains continuity with the yolk sac through the vitelline duct.
      - The intestinal loop rotates counterclockwise around the superior mesenteric artery (central axis) by a total of 270 degrees.

  • Return to Abdominal Cavity: Begins by the 10th week:
      - Proximal jejunum is first to re-enter (left side placement).
      - Later parts lie progressively towards the right side.
      - Prospective Cecum: Last to return, initially lies below the liver, ultimately descends to the right iliac fossa.

  • Colonic Structures Becoming Secondarily Retroperitoneal: Ascending and descending colon fuse their mesenteries to the posterior parietal peritoneum as they press against the body wall.

  • Potential Clinical Conditions due to Abnormalities in Midgut Rotation:
      - Left-sided colon.
      - Reversed rotation of intestinal loop.
      - Volvulus and compromised blood supply.

  • Meckel’s Diverticulum: Persistent vitelline duct may lead to this condition.

Development of the Hindgut

  • Derived Structures:
      - Epithelial lining of the urinary bladder and parts of the urethra are derived from hindgut endoderm.

  • Terminations: The hindgut terminates in part of the cloaca, which becomes the anorectal canal.

  • Cloacal Structure: An endoderm-lined cavity temporarily closed off by the cloacal membrane containing ectoderm, endoderm, but no mesoderm.

  • Cloaca Divisions: Divided into dorsal and ventral portions by the urorectal septum:
      - Urorectal septum develops in the angle between the allantois and hindgut, separating the urogenital sinus from the anorectal canal.
      - Urorectal septum fuses with the cloacal membrane to form the perineal body.
      - Anorectal Anomalies: Most result from abnormal partitioning of the cloaca by the urorectal septum.