Development of the Gastrointestinal Tract
Formation of the Gut Tube
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 endoderm lined tube narrowly connected to the yolk sac by the yolk stalk.
This 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.
Transverse Folding of the Embryo
At the same time the embryo is folding in the cephalocadual direction, it is folding in from the lateral direction. 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.
Divisions of the Primitive Gut Tube
The primitive gut tube is lined by endoderm and divided into three regions: foregut, midgut, and hindgut. It's outer wall is derived from splanchnic mesoderm.
Each of the three subdivisions of the gut tube gives rise to specific organs and parts of the gastrointestinal tract.
- Foregut - esophagus, stomach, proximal duodenum, liver, gallbladder and pancreas
- Midgut - distal duodenum, jejunum, ileum, cecum, appendix and proximal colon
- Hindgut - distal colon, rectum, anal canal and anus
There is a dorsal mesentery that suspends the gut tube from the posterior abdominal wall and a ventral mesentery that anchors it to the anterior body wall.
The ventral mesentery disappears from all but the foregut region where is persists as the ventral mesogastrium.
The dorsal mesentery is named for the regions of the gut tube with which it is associated.
- Dorsal mesogastrium - stomach
- Dorsal mesoduodenum - duodenum
- Mesentery 'proper' - jejunoileum
- Dorsal mesocolon - colon
As the developing organs of the abdomen come to their final locations, some parts of the dorsal mesentery will be absorbed.
Each division of the embryonic gut has blood supply from a specific unpaired branch of the abdominal aorta.
- celiac artery - derivatives of the foregut
- superior mesenteric artery - derivatives of the midgut
- inferior mesenteric artery - derivatives of the handout
The enteric ganglia, derived from neural crest cells, provide the source of postsynaptic parasympathetic neurons.
Presynaptic Sympathetic innervation of the gut tube is from neurons in spinal cord levels T5 to T12 that form the greater (T5 - T9), lesser (T10 - T11) and least (T12) splanchnic nerves. These synapse upon postsynaptic sympathetic neurons of prevertebral ganglia which are also derived from neural crest cells.
Prevertebral ganglia are located adjacent to the origins of the unpaired branches of the aorta that supply the divisions of the gut tube. The ganglia are named celiac, superior mesenteric & inferior mesenteric, so easy to remember. The celiac ganglia provide sympathetic innervation to the foregut derivatives, superior mesenteric ganglia to midgut derivatives and inferior mesenteric ganglia to hindgut derivatives.
Sensory axons for physiologic reflexes reach the CNS by traveling back to the brainstem in the vagus nerves while those for visceral pain reach the spinal cord by travraversing back through the sympathetic pathways. All nerve fibers passing to and from the parts of the GI tract travel in plexuses on the surface of the blood vessels.
Rotation of the Gut Tube
A section through a 6-week old embryo reveals a loop of midgut along with the yolk sac, herniates into the umbilical cord. The foregut region expands to form the stomach and beginning of the duodenum. An hepatic bud and pancreatic buds emerge from the endoderm of the distal foregut. A mass of mesoderm in the dorsal mesentery of the stomach condenses into the spleen.
Within the umbilical cord, the midgut loop twists along its' longitudinal axis a total of 270 degrees. Beginning in the 10th week the twisted loop of bowel begins to return to the abdominal cavity in such a way that the duodenum is covered by peritoneum, the colon forms a frame around the small intestine. The process of returning to the abdomen is usually completed by week 12 of development.
GIF Animation - Click Image
Development of the Peritoneal Cavity
The abdominopelvic cavity is the space inferior to the diaphragm. Like the thoracic cavity, the abdominal cavity is lined by a moist mesothelium derived from the somatic and splanchnic layers of lateral plate mesoderm. In the case of the abdomen, this membrane is called peritoneum. Parietal peritoneum lines the walls of the cavity and visceral peritoneum covers the outer surfaces of the organs contained within the abdomen.
Double folds of peritoneum, called mesenteries, attach the visceral organs to the anterior (ventral) and posterior (dorsal) body wall.
Some abdominal organs retain their mesenteries after birth. Such organs are said to be intraperitoneal because they have relative freedom of movement within the abdominal cavity.
Some of the organs, as a consequence of gut rotation during development, are displace against the posterior abdominal wall causing absorption of their mesentery. These organs are covered by peritoneum only on their anterior surfaces and are said to be retroperitoneal to describe their fixed position on the body wall.
The greater omentum is a double fold of mesentery (quadruple fold of peritoneum) that forms as a result of gut rotation.
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.
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Start Slide Show
Initially the abdominal cavity is a single continuous space referred to as the greater peritoneal sac or just greater sac.
As the gut tube develops the stomach undergoes a counterclockwise rotation (arrows) and the liver is cast to the right side of the abdomen and the stomach to the left.
The spleen and pancreas, developing in the dorsal mesentery are also pushed to the left side of the abdomen.
Before rotation of the stomach
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At the end of gut rotation, a small portion of the greater peritoneal cavity is isolated posterior to the stomach, thus forming the lesser sac. The natural communication between greater and lesser sacs is the epiploic foramen (of Winslow),.
Also a result of gut rotation, the pancreas and portions of the GI tract are pushed against the posterior abdominal wall with absorption of one side of the mesentery, fixing them into a retroperitoneal position.
After rotation of the stomach
Other parts of the mesentery form peritoneal ligaments between visceral organs as a means through which blood vessels can reach them. Two such ligaments, the splenorenal and gastrosplenic ligaments are shown.