Development of the digestive system

[1] At the end of the third week, the neural tube, which is a fold of one of the layers of the trilaminar germ disc, called the ectoderm, appears.

Ventral mesentery, derived from the septum transversum, exists only in the region of the terminal part of the esophagus, the stomach, and the upper portion of the duodenum.

Herniation (Latin, meaning 'rupture') takes place at around 7½ weeks in the human embryo and refers to the retraction of the intestine from the extraembryonal navel coelom into the abdomen (panel B3).

The intestinal duct rotates by 90° (counterclockwise when viewing from tail-to-head) around the body axis (see panel B1) in the same direction as the heart.

In the cephalic and caudal parts of the embryo, the primitive gut forms a tube, the foregut and hindgut, respectively.

The middle part, the midgut, remains temporally connected to the yolk sac by means of the vitelline duct.

[6] The foregut gives rise to the esophagus, the trachea, lung buds, the stomach, and the duodenum proximal to the entrance of the bile duct.

In addition, the liver, pancreas, and biliary apparatus develop as outgrowths of the endodermal epithelium of the upper part of the duodenum.

[7] The midgut forms the primary intestinal loop, from which originates the distal duodenum to the entrance of the bile duct.

At its apex, the primary loop remains temporarily in open connection with the yolk sac through the vitelline duct.

Common abnormalities at this stage of development include remnants of the vitelline duct, failure of the midgut to return to the abdominal cavity, malrotation, stenosis, and duplication of parts.

The urorectal septum divides the two regions and breakdown of the cloacal membrane covering this area provides communication to the exterior for the anus and urogenital sinus.

Ectoderm, in the region of the proctodeum on the surface of part of the cloaca, proliferates and invaginates to create the anal pit.

Subsequently, degeneration of the cloacal membrane establishes continuity between the upper and lower parts of the anal canal.

Abnormalities in the size of the posterior region of the cloaca shift the entrance of the anus anteriorly, causing rectovaginal and rectourethral fistulas and atresias.

Once the mesoderm is specified by this code, it instructs the endoderm to form components of the mid- and hindgut regions, such as the small intestine, caecum, colon, and cloaca.

Diagram showing the process by which the intestine rotates and herniates during normal development. From panel A to B (left-sided views), the midgut loop rotates 90° in a counterclockwise direction (caudal-to-rostral view), so that its position changes from midsagittal (A) to transverse (B1). The small intestine forms loops (B2) and slides back into the abdomen (B3) during resolution of the hernia. Meanwhile, the cecum moves from the left to the right side, which represents the additional 180° counterclockwise rotation of the intestine (C, central view). [ 3 ]