Development of the Gastrointestinal Tract
Divisions of the Primitive Gut Tube
- Foregut - esophagus, stomach, proximal duodenum, liver, gallbladder and pancreas
- Midgut - distal duodenum, jejuneum, ileum, cecum, appendix and proximal colon
- Hindgut - distal colon, rectum, anal canal and anus
Sagittal Folding of the Embryo
Development of the gut begins with the appearance of the secondary yolk sac which is lined by endoderm following gastrulation. Cephalocaudal and lateral folding of the embryo during the 4th week creates an endodermally lined tube that is narrowly connected to the yolk sac by the yolk stalk. This endodermally lined primitive gut is divided into three regions: foregut, midgut, and hindgut. The stomodeum, located at the cranial end of the gut tube will perforate to form the opening to the oral cavity and the proctodeum, in the area of the cloaca, will perorate to from the anus. Note the relationship of the gut tube to the neural tube and the developing heart.
Final frame of animation.
Final frame of animation.
At the same time the embryo is folding in the cephalocadual direction, it is folding in from the lateral direction. By the end of the fourth week, the gut tube is formed and lined by endoderm. It is covered by a layer of splanchnic mesoderm, which will form the connective tissue and smooth muscle of the gut wall. A double fold of peritoneum suspends the gut tube from the posterior body wall, forming the dorsal mesentery. In the animation, note that there is also a ventral mesentery, but it disappears from the midgut and hindgut regions.
This sagittal view of an embryo about day 37 of development shows the developing gut tube and its associated mesenteries. Note that the foregut is attached to the posterior body wall by a dorsal mesentery, colored pink. It is regionally named for the part of the GI tract with which it is associated e.g.(dorsal mesogastrium (stomach); dorsal mesoduodenum; duodenum, etc.).
It is also attached to the anterior body wall by a ventral mesentery, colored blue (ventral mesogastrium,; stomach). The ventral mesentery disappears from all but the foregut region.
Growth of the liver as a derivative of the foregut, splits the ventral mesentery into a portion stretching between the developing liver and stomach, the lesser omentum. The portion between the liver and anterior body wall is the falciform ligament.
The midgut is suspended by a dorsal mesentery only, the "mesentery" proper and the hindgut is suspended by the dorsal mesocolon.
Three unpaired branches of the abdominal aorta supply blood to the GI tract and all its derivatives. All of the derivatives of the foregut receive blood supply from the celiac artery. The midgut derivatives receive blood supply from the superior mesenteric artery and the derivatives of the hindgut are supplied by the inferior mesenteric artery.
The autonomic innervation of the gut follows this pattern. Presynaptic parasympathetic innervation to the foregut and midgut is provided by branches of the vagus nerve while the hindgut is provided with parasympathetic presynaptic innervation via the pelvic splanchnic nerves.
The enteric ganglia provide the source of postsynaptic parasympathetic neurons.
Postsynaptic sympathetic innervation is provided by neurons of prevertebral ganglia located adjacent to the origins of the unpaired aortic branches. Their axons reach the smooth muscle and glands of the digestive organs by traveling on the surface of the blood vessels that supply the organs. The celiac ganglia provide sympathetic innervation to the foregut derivatives, the superior mesenteric ganglia to the midgut derivatives and the inferior mesenteric ganglia to the hindgut derivatives.
Development of the Peritoneal Cavity
The abdominopelvic cavity is the space inferior to the diaphragm. The upper part of the cavity is lined by mesothelium called the peritoneum. Parietal peritoneum contacts the body wall and visceral peritoneum invests most of the abdominal organs.
The liver is surrounded by a capsule formed by the visceral peritoneum. Visceral peritoneum is also attached to the wall of most of the GI tract. Double folds of peritoneum, called mesenteries anchor the abdominal visceral organs to the abdominal body wall.
The parietal peritoneum lines the entire inside of the abdominal cavity.
Roll over the image
Sagittal section through the abdominopelvic cavity of an adult showing the locations of the parietal and visceral peritoneal layers
Initially, the gut tube is suspended in the midline from the posterior body wall. Following rotation of the stomach, the liver is pushed to the right side of the abdomen and the stomach is pushed to the left. As a result, a small portion of the peritoneal cavity is located behind the stomach. It is called the lesser sac or omental bursa. The larger space within the peritoneal cavity, anterior to the stomach and liver, is called the greater sac.
Knowledge of these subdivisions is important in pathologies of organs that are located behind the stomach in the omental bursa. in particular, the pancreas, duodenum and aorta.
Before rotation of the stomach
in the sagittal view of the adult abdomen to the right, the lesser sac is colored blue and the greater sac is colored pink. This subdivision of the peritoneal cavity is the result of the rotational events that occur during development of the GI tract. The arrow is passing through the epiploic foramen (of Winslow), the only natural connection between the greater and lesser sacs.
Roll over the image
After rotation of the stomach
Sagittal section through the abdominopelvic cavity of an adult showing the subdivisions of the peritoneal sac