Development of the Gastrointestinal System Reading: Langman’s Medical Embryology chapter 14
Early Development
As discussed previously, the embryo undergoes lateral body folding and during this process the endoderm forms into a gut tube. This G.I. tube begins cephalically at the oral plate and continues until it reaches the cloacal or anal plate. If we look at the gut tube at the time the body folding is completed, it can be subdivided into three divisions: a foregut, a midgut and a hindgut.
Development of the Stomach and Intestines
The digestive system develops from the foregut, midgut and the hindgut. The foregut gives rise to the G.I. structures from the esophagus to the second part of the duodenum. In addition, the liver and pancreas develop as outgrowths from the foregut. The midgut gives rise to the structures from the 3rd part of the duodenum to the first 2/3 of the large intestine. The hindgut then gives rise to the structures from the remaining large intestine through the rectum.
The primitive gut begins development as an endodermally lined tube surrounded by splanchnic mesoderm which envelopes the gut and forms a dorsal mesentery.
The Stomach
The stomach develops from the foregut starting at the 4th week . It begins as a dilation of the gut tube and during development it shifts position by both moving caudally and rotating. As this occurs the left wall of the stomach will become the anterior surface and the right side of the gut tube will become the posterior surface of the stomach.
The caudal movement of the stomach results in the pyloric portion of the stomach lying at the same level as the body of the stomach.
Intestines
The first part of the duodenum and the beginning of the 2nd part are derived from the foregut. The remaining duodenum, jejunum, and ileum will be derived from the midgut. The appendix, cecum, ascending colon and 2/3 of the transverse colon are derived from the midgut. The last 1/3 of the transverse colon and the descending and sigmoid colon are derived from the hindgut.
The midgut can be divided into two portions. The cephalic limb of the midgut extends from the duodenum to the yolk stalk. The point of attachment of the yolk stalk is located, in the adult, about 2 - 3 feet from the ileocecal junction. The cephalic limb will then form the lower duodenum, the jejunum and the upper ileum. The caudal limb will form the lower ileum, the appendix, the cecum, the ascending colon and 2/3 of the transverse colon.
During development the midgut lengthens and as it does there is a twisting of the gut tube around the superior mesenteric artery. In addition, the gut tube begins to extend into the belly stalk. By ten weeks of development the abdomen has enlarged so that the entire length of the midgut can be accommodated. The rotation of the gut continues as the entire gut reenters the abdomen (to view an animation of the midgut rotation, go to the GI animation page). This results in the cephalic limb of the midgut being positioned on the left of the abdominal cavity and the caudal limbs occupying the right lower portion of the abdominal cavity.
The hindgut joins with the allantois and together form the cloaca. The development of the cloaca will be discussed during the lecture on the development of the urogenital system. However, you should know that the cloaca divides into the ventrally located urogenital sinus and a dorsally located rectum and anal canal. The lower portion of the anal canal, below the anal plate, develops from the proctodeum.
Mesenteries
The mesentery is the splanchnic mesoderm that connects the primitive gut to the body wall. During development the ventral mesentery exists only between the liver and the stomach, and the liver and the duodenum. The ventral mesentery goes on to form the lesser omentum, between the liver and the stomach and duodenum, and the falciform ligament between the liver and the anterior body wall.
The dorsal mesentery surrounds the rest of the primitive gut. In the region of the stomach the dorsal mesogastrium develops into the splenorenal and gastrosplenic ligaments. The dorsal mesentery persists and develops into the mesentery proper, between the small intestine and the posterior body wall. It also persists as the greater omentum. Finally, the mesentery of the colon develops into the transverse mesocolon. The development of the mesenteries and the location of these structures in the adult are largely dictated by the rotation of the gut and the other movements of the developing stomach and G.I. tract. During these rotations some structures come to lie close to the posterior body wall and as they do, the mesentery will be absorbed on the posterior surface of the organ. That organ is then said to be located in a retroperitoneal position. Organs that were originally in the dorsal mesentery but then came to occupy a retroperitoneal position include parts of the duodenum, the pancreas, the ascending and the descending colon.
The blood supply to the G.I. structures largely follows along with the development of these structures. If an organ developed from the foregut, it receives its blood supply from the celiac artery. If the organ developed from the midgut it receives its blood supply from the superior mesenteric artery. If the organ develops from the hindgut it gets its blood supply from the inferior mesenteric artery.
Pancreas
The pancreas develops as an outgrowth of the duodenum. It develops as two pancreatic buds: the dorsal pancreatic bud and the ventral pancreatic bud. They fuse to form the pancreas with the dosal bud giving rise to the body and tail of the pancreas and the ventral pancreas giving rise to the head and uncinate process of the pancreas (to view an animation of the pancreatic bud fusing, go to the GI animation page).
The liver develops as a diverticulum from the duodenum. It grows toward the septum transversum and eventually fuses with it. The portion of the diverticulum closest to the duodenum develops into the hepatic ducts, the bile duct and the gallbladder. The remaining portion becomes the epithelial plates of the liver. The vitelline veins become incorporated and give rise to the hepatic sinusoids.
Links to Other Sites:
For a more clinical take on gastrointestional developement, check out this site from the University of Texas - Houston Medical School.
The UHRAD case of congenital diaphramtic hernia
The Vanderbilt University Medical School's site on Hirschsprung's Disease
The Pediatric Surgery Update's articles on Hirschsprung's Disease and other gastrointestinal congenital abnormalities.
The Pediatric Pathology Index: Look specifically for conditions such as omphalocele and other GI related cases.