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Midgut & Hindgut Development
Here we will learn how the mid- and hindgut develop.
Four key steps of midgut development that occur between weeks 6-10.
1. Herniation of the primary intestinal loop and its 90-degree rotation.
The midgut comprises the primary intestinal loop, which elongates so rapidly that it temporarily outgrows the abdominal cavity.
The superior mesenteric artery supplies the primary intestinal loop and forms the axis around which it will rotate.
The vitelline duct (aka, yolk stalk) connects the primary loop to the yolk sac, and, the cecal diverticulum is an outpouching at the caudal limb of the loop.
The primary loop rotates 90 degrees counterclockwise around the superior mesenteric artery (when viewed anteriorly).
2. The cranial limb of the rotated primary loop elongates rapidly, creating the jejunoileal loops; indicate the cecum, for context.
By week 10, the loops return to the abdominal cavity.
3. The returning intestinal loop rotates an additional 180 degrees counterclockwise as it continues to elongate. Notice this means the midgut has rotated a total of 270 degrees.
To illustrate this, we show the distal end of the stomach for anatomical context with the jejunoilieal loops, and the large intestine; the cecum is now in the upper right quadrant.
4. The final positions:
The small intestine is framed by the large intestine; the cecum now lies in the lower right quadrant.
The midgut ultimately gives rise to the distal duodenum, jejunum, ileum, the ascending colon and proximal 2/3 of the transverse colon, as well as the cecum and appendix.
Be aware that, though we've omitted it here for simplicity, the intestinal loops carry their dorsal mesentery with them as they rotate around the superior mesenteric artery. The root of the mesentery proper ultimately acquires an oblique attachment to the posterior abdominal wall.
The mesocolons of the ascending and descending colons generally fuse with the posterior parietal peritoneum, making these portions of the colon secondarily retroperitoneal. The transverse mesocolon persists and becomes adherent to the posterior layer of the greater omentum.
Next, let's turn our attention to the hindgut, starting approximately week 4, in three key steps.
1. Initially, the cloaca receives the hindgut and communicates with the allantois.
We show the caudal end of the embryo with the hindgut and allantois; the hindgut opens into the cloaca.
The cloacal membrane closes off the cloaca; the membrane comprises layers of endoderm and ectoderm, with no intervening mesoderm.
The urorectal septum is a wedge of mesenchyme between the allantois and hindgut.
In the conventional developmental model, the septum extends caudally as the cloaca is partitioned during approximately weeks 4 through 7; modern studies suggest that the process of separation may be more complex, involving differential growth and tissue rearrangement.
2. The urorectal septum extends towards the cloacal membrane.
According to the conventional model, the urorectal septum separates the ventral primitive urogenital sinus from the dorsal anorectal canal.
3. Mesenchyme associated with the caudal end of the urorectal partition contributes to the perineal body, which lies between the future urogenital and anal openings. The membranes will rupture and establish separate external openings.
Ultimately, the hindgut gives rise to the distal 1/3rd of the transverse colon, the descending and sigmoid colons, rectum, and the upper portion of the anal canal.
Though not shown, here, the anal pit, aka, proctodeum, is an ectodermal depression that forms at the caudal end of the embryo. It contributes to the lower portion of the anal canal. Internally, the junction between the endoderm-derived upper portion and ectoderm-derived lower portion is marked by the pectinate line.
Midgut Development
Hindgut