Study Notes on Embryonic Gut, Mesenteries, and Associated Anatomical Structures

Embryonic Gut and Mesenteries

  • Primitive Gut Tube

    • Extends from the mouth to the rectum.

    • Suspended in the abdomen by mesenteries:

    • Dorsal Mesentery

      • Major component, mostly retained.

      • Supports most of the intestine and serves as the main route for vessels, nerves, and lymphatic drainage.

    • Ventral Mesentery

      • Exists early in development but is mostly lost.

      • Remains primarily around the foregut region.

  • Key Associations

    • Ventral mesentery predominantly connected to foregut structures, especially in the liver and stomach region.

    • Dorsal mesentery associated with the majority of the intestine.

Arterial Supply Pattern (The “3 Unpaired Branches”)

  • The abdominal gastrointestinal (GI) tract is primarily supplied by three unpaired branches of the aorta:

    • Coeliac Artery (supplies foregut derivatives):

    • Supplies:

      • Stomach

      • Liver

      • Spleen

      • Proximal duodenum (varying details based on species)

    • Cranial Mesenteric Artery (supplies midgut derivatives):

    • Supplies:

      • Most small intestine

      • Caecum

      • Ascending colon

      • Proximal/transverse colon (up to mid-transverse colon)

    • Caudal Mesenteric Artery (supplies hindgut derivatives):

    • Supplies:

      • Distal colon

      • Rectum (often includes branches/continuations)

  • Clinical Insight: Knowing what structures belong to the foregut, midgut, and hindgut aids in deducing the respective artery.

Stomach Development: The Two Rotations

  • The stomach development initiates as a simple enlargement of the foregut, followed by two key rotations:

    • Rotation 1: Approximately 90° around the longitudinal axis.

    • Brings the left side of the stomach anteriorly and the right side posteriorly.

    • Aids in establishing the final left-sided orientation of the stomach.

    • Rotation 2: Around the anteroposterior axis.

    • Positions the greater curvature more ventrally and caudally (towards the tail).

    • Positions the lesser curvature more dorsally and cranially (towards the head).

Mesenteries Transformation into Omenta and Ligaments

  • Dorsal Mesogastrium: Becomes the greater omentum, which appears as a large apron-like fold and aids in attaching to the spleen.

  • Ventral Mesogastrium: Develops into the lesser omentum, positioned between the liver and the lesser curvature of the stomach/proximal duodenum.

  • Includes significant components:

    • Hepatogastric Ligament

    • Hepatoduodenal Ligament (houses the portal triad):

    • Portal Vein

    • Hepatic Artery

    • Bile Duct

    • This is a common clinical query regarding the contents of this ligament.

Final Stomach Position in Dogs (High-Yield)

  • In dogs, the stomach is predominantly located on the left side.

    • The pylorus (the outlet of the stomach) is situated on the right side.

    • The fundus tends to lie left/dorsal and often contains gas, as visible in radiographic imaging.

  • Comparison of Cardia and Pylorus (Functional Note):

    • Dog cardia is relatively weak; hence dogs can vomit more easily.

    • The pyloric sphincter is tightly regulated, thereby controlling gastric outflow.

Intestinal Layout in Dogs: Simplicity vs Complexity

  • Small Intestine:

    • Duodenum: More fixed position (short mesentery), with a right-sided course.

    • Jejunum: Longest section and the most mobile (long mesentery).

    • Ileum: Short segment preceding the large intestine.

  • Large Intestine in Carnivores (Dog):

    • Caecum: Relatively small in dogs.

    • Colon: Comparatively simple structure consisting of:

    • Ascending colon (right side)

    • Transverse colon (crosses from right to left)

    • Descending colon (left side)

  • Comparison with Other Species:

    • In Horse and Pig: Larger and more complex caecum and colon due to fermentation processes.

    • In Ruminants: Extensive forestomachs occupy ample abdominal space, with intestines moved to periphery.

Midgut Rotation: Organ Positions

  • The midgut loop completes a significant rotation, commonly taught as a total of approximately 270° around the axis of the cranial mesenteric artery.

  • Final Arrangement in Dogs:

    • Caecum ends up on the right side.

    • Ascending colon positioned on the right side.

    • Descending colon located on the left side.

    • The transverse colon crosses between ascending and descending colon.

  • Embryological Logic: This rotation explains the normal positioning of these organs.

Liver Essentials: Development, Blood Flow and Gall Bladder Note

  • Blood Supplies to the Liver:

    • Hepatic Artery: Supplies oxygenated blood for the liver tissue.

    • Portal Vein: Drains nutrient-rich blood from the:

    • Intestines

    • Spleen (and often the pancreas)

    • This dual blood supply emphasizes the liver's roles in metabolism, detoxification, and bile production.

  • Gall Bladder: Generally present in most species, but notably absent in horses.

    • In horses, bile flows continuously into the duodenum due to lack of gall bladder storage.

Spleen Development and Positioning

  • The spleen forms within the dorsal mesogastrium.

  • As the stomach rotates, the spleen concludes its positioning on the left side, anchored by:

    • Gastrosplenic Ligament

    • Related splenic attachments within the area of the greater omentum.

Radiographic Orientation Cues (Dog Abdomen)

  • Clinical hints typically observed include:

    • Presence of gas in the fundus (frequently visible radiographically).

    • Stomach predominantly located on the left.

    • Pylorus positioned on the right side.

    • Liver mostly under the rib cage, where only the margins are palpable or visible based on the size and body condition of the animal.