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Midgut & Hindgut Development
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Midgut & Hindgut Development

Midgut
  • The midgut comprises the primary intestinal loop, which elongates so rapidly that it temporarily outgrows the abdominal cavity
    • It herniates into the umbilical cord, growing and rotating 90 degrees
    • As the loops are retracted a few weeks later, they continue to grow and rotate an additional 180 degrees.
  • The midgut ultimately gives rise to the distal duodenum, jejunum, ileum, the ascending colon and proximal 2/3 of the transverse colon.
Midgut development in 4 Steps
1. Herniation & Counterclockwise Rotation
    • The first step is herniation of the primary loop, which begins approximately week 7
    • Counterclockwise rotation occurs about the axis of the superior mesenteric artery
- Notable landmarks include: The vitelline duct (aka, yolk stalk), which connects the primary loop to the yolk sac. The cecal diverticulum; this outpocket is a helpful anatomical landmark to track as the small intestine elongates, rotates, and shifts position. 2. Elongation
    • The rotated primary loop elongates rapidly, creating the jejunoileal loops; note the location of the cecum.
3. Retraction & Counterclockwise Rotation
    • By week 10, the loops retract into the abdominal cavity.
    • The intestines rotater an additional 180 degrees counter clockwise as they continue to elongate.
    • The cecum is in the upper right quadrant.
4. Final Positions
    • The large intestine "frames" the small intestine.
    • The cecum now lies in the lower right quadrant.
Be aware that, though we've omitted it here for simplicity, the mesentery proper twists around the superior mesenteric artery during rotation of the midgut, while the mesenteries of the ascending and descending colons become fused to the posterior body wall (the mesentery of the transverse segment fuses to the greater omentum, which is discussed elsewhere).
Hindgut
  • The cloacal region differentiates to form separate urogenital and gastrointestinal channels.
  • Ultimately, the hindgut gives rise to the distal 1/3rd of the transverse colon, the descending and sigmoid colons, and the proximal 2/3rd of the anorectal canal.
Hindgut development in 3 Steps.
1. Cloaca
    • Initially, the cloaca is the common end of the hindgut and urogenital tract.
    • The cloacal membrane closes off the cloaca; it comprises layers of endoderm and ectoderm.
    • The urorectal septum, which is a wedge-like mass between the allantois and hindgut that forms during weeks 4-6; many authors now agree it comprises two fused mesodermal structures.
2. Urorectal Septum
    • The urorectal septum grows towards the cloacal membrane.
    • Urorectal septum now separates the primitive urogenital sinus, which appears as an anterior swelling, from the anorectal canal.
    • The cloacal membrane ruptures before the urorectal septum reaches it.
3. Perineal Body
    • The perineal body, which represents the tip of the urorectal septum, separates the urogenital and anal membranes.
    • The urorectal septum now completely divides the urinary and digestive tracts, so that the urinary bladder lies anteriorly, and the anorectal canal remains posteriorly.
Though not shown, here, the anal pit, aka, proctodeum, forms as mesenchymal cells proliferate and form a raised border around the anal canal. Thus, the cranial portion of the anal canal derives from the hindgut, while the caudal-most portion arises from the anal pit.

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