Development of the digestive system

Gut tube

The primitive gut is formed when a portion of the yolk sac becomes incorporated into the embryo, which occurs due to the cephalocaudal and lateral folding of the embryo. The portions that remain outside the embryo are the yolk sac and the allantois. The primitive gut forms a blind-ended tube on both the cephalic and caudal ends of the embryo, forming the foregut and the hindgut, respectively. The middle part forms the midgut, but remains temporarily connected to the yolk sac via the vitelline duct (yolk stalk).

The development of the primitive gut and its derivatives are generally divided into four sections:

• the proximal foregut
• the distal foregut
• the midgut
• the hindgut

First, the proximal foregut consists of the pharyngeal gut (pharynx), which extends from the oropharyngeal membrane to the respiratory diverticulum. This region is crucial to the development of the pharyngeal apparatus and ultimately the head and neck. Second, the distal foregut extends from the pharyngeal tube to the liver bud. Third, the midgut extends from the liver bud to the junction between the right two-thirds and the left third of the transverse colon. Fourth, the hindgut extends from the left third of the transverse colon to the cloacal membrane.

Mesenteries

In the abdominal cavity, the gut tube and its derivatives are suspended from the dorsal and ventral body wall by mesenteries; these are double layers of peritoneum that enclose organs and connect them to the body wall. Such organs are known as intraperitoneal organs (i.e. stomach), whereas organs that lie directly on the posterior body wall and are covered by peritoneum on their anterior surface are known as retroperitoneal organs (i.e. kidneys). Mesenteries between two organs or one organ and the body wall are known as peritoneal ligaments. Together, mesenteries and peritoneal ligaments serve as a conduit for vessels, nerves, and lymphatics to and from the abdominal organs.

The gut tube, being initially in contact with the posterior abdominal wall, becomes suspended from it by the dorsal mesentery. At the stomach, the dorsal mesentery forms the dorsal mesogastrium (or the greater omentum); at the duodenum, it forms the mesoduodenum; at the jejunum and ileum, it forms the mesentery proper; and at the colon, it forms the dorsal mesocolon.

On the other hand, the ventral mesentery suspends only the lower esophagus, the stomach, and the upper duodenum to the ventral body wall. The ventral mesentery is derived from the septum transversum (a mass of splanchnic mesoderm separating the pericardial and peritoneal cavities) and eventually becomes divided by the growing liver into the lesser omentum and the falciform ligament; the former extends from the lower esophagus, the stomach, and the upper duodenum to the liver, whereas the latter extends from the liver to the ventral body wall.

The free margin of the falciform ligament serves as a conduit for the umbilical vein, which becomes obliterated after birth to form the round ligament of the liver. The free margin of the lesser omentum that connects the duodenum and the liver (hepatoduodenal ligament) forms the portal triad, thus serving as a gateway for the bile duct, the portal vein, and the hepatic artery. This free margin also forms the roof of the epiploic foramen of Winslow, a gateway which connects the omental bursa with the rest of the peritoneal cavity.

Foregut

The foregut gives rise to the: 

• pharynx
• the lower respiratory system
• the esophagus
• the stomach
• the proximal half of the duodenum
• the liver
• the biliary apparatus
• the pancreas

The derivatives of the foregut, except for the pharynx and the lower respiratory system, are mostly supplied by the celiac artery (trunk).

Midgut

The midgut gives rise to the:

• distal half of the duodenum
• the jejunum
• the ileum
• the cecum
• the appendix
• the ascending colon
• the proximal two-thirds of the transverse colon

The derivatives of the midgut are supplied by the superior mesenteric artery.

During the fifth week of gestation, the midgut undergoes a rapid elongation that occurs much faster than that of the abdominal cavity, resulting in the formation of the primary intestinal loop. At its apex, the loop remains in open communication with the yolk sac via the vitelline duct, while the superior mesenteric artery runs along the axis of the loop. The cranial limb of the loop will develop into the inferior half of the duodenum, the jejunum, and proximal half of the ileum. The caudal limb of the loop will develop into the distal half of the ileum, the cecum, the ascending colon, and the proximal two-thirds of the transverse colon.

By the sixth week, the continuing elongation of the midgut, combined with the pressure exerted by the dramatic growth of the abdominal organs, force the primary intestinal loop to protrude into the umbilicus (physiological herniation). Concurrently, the loop rotates 90 degrees counterclockwise around the axis of the superior mesenteric artery, resulting in the cranial limb to move caudally and to the embryo’s right, and the caudal limb to move cranially and to the embryo’s left. While this rotation takes place until the eighth week of gestation, the lengthening jejunum and ileum develop into a series of folds known as the jejunal-ileal loops, whereas the expanding cecum gives rise to a wormlike diverticulum, the vermiform appendix.

During the tenth week, the herniated midgut retracts into the abdomen. The mechanism responsible for this retraction is not fully understood, but may involve the increase in size of the abdominal cavity. As the intestinal loops reenter the abdomen, it rotates an additional 180 degrees counterclockwise around the axis of the superior mesenteric artery, thus having travelled for a total of 270 degrees. As a result, the cecum, being initially positioned under the liver, becomes displaced inferior, pulling down the proximal hindgut to form the ascending colon.

By the eleventh week, the intestines have completely retracted into the abdomen. The dorsal mesenteries of the ascending and descending colon shorten and fold, anchoring these organs to the dorsal body wall, where they become secondarily retroperitoneal. The jejunum, ileum, cecum, and the transverse and sigmoid colon remain suspended by a short mesentery from the dorsal body wall, thus becoming intraperitoneal.

Hindgut

The hindgut gives rise to the distal third of the: 

• transverse colon
• descending colon
• sigmoid colon
• rectum
• upper two-thirds of the anal canal

The derivatives of the hindgut are supplied by the inferior mesenteric artery.

During the early weeks of development, the terminal portion of the hindgut enters the posterior portion of the cloaca, whereas the allantois enters the anterior portion; the cloaca and the allantois form the future anorectal canal and the urogenital sinus, respectively. The cavity of the cloaca is lined by endoderm, whereas its ventral boundary is lined by surface ectoderm; the latter is known as the cloacal membrane.

By the seventh week of gestation, the urorectal septum (a layer of mesoderm) grows between the allantois and the hindgut. It divides the cloaca into the urogenital sinus and the anorectal canal, while its tip forms the future perineal body. The urogenital sinus forms the future bladder, parts of the urethra, and the phallus, whereas the anorectal canal develops into the rectum and most of the anal canal. Specifically, while the upper two-thirds of the anorectal canal is derived from the endoderm of the hindgut, the lower one-third is derived from the surface ectoderm of the cloaca.

Degeneration of the cloacal membrane connects the upper and lower parts of the anorectal canal, while the location of this juncture is demarcated by the pectinate line, an irregular folding of the mucosa. This is also where the epithelial lining transitions from columnar to stratified squamous. The vasculature of the anorectal canal is consistent with its dual origin: the upper two-thirds (superior to the pectinate line) is supplied by the superior rectal arteries, branches of the inferior mesenteric artery, whereas its lower one-third (inferior to the pectinate line) is supplied by the inferior rectal arteries, branches of the internal pudendal arteries.