Foregut

Development

The foregut develops from a cranial region of endoderm created after the initial cephalocaudal folding of the embryo. Starting at the stomodeum, a rapid expansion of the primitive gut forms the esophagus, from which the respiratory bud branches off. During early foregut development, the esophagus lengthens considerably, reaching its proportional postnatal size. Simultaneously, the stomach begins to expand in width dorsally and ventrally in an asymmetric manner. This asymmetric expansion creates two curvatures, with the ventral side creating the lesser curvature and the dorsal side creating the greater curvature. The expanding dorsal stomach wall then rotates the on its transverse plane, pulling its caudal portion upward and forcing the upper duodenum into a C shape. This rotation positions the left vagus nerve anteriorly and right vagus nerve posteriorly.

While the hindgut and midgut are only attached dorsally to the body wall by a fold of peritoneum, the foregut also has a ventral attachment. Its two attachments are commonly referred to as the dorsal mesogastrium and the ventral mesogastrium.

As the stomach rotates during early development, the dorsal and ventral mesentery rotate with it; this rotation produces a space anterior to the expanding stomach called the greater sac, and a space posterior to the stomach called the lesser sac. After this rotation the dorsal mesentery thins and forms the greater omentum, which is attached to the greater curvature of the stomach. The ventral mesentery forms the lesser omentum, and is attached to the developing liver. In the adult, these connective structures of omentum and mesentery form the peritoneum, and act as an insulating and protective layer while also supplying organs with blood and lymph vessels as well as nerves. Arterial supply to all these structures is from the celiac trunk, and venous drainage is by the portal venous system. Lymph from these organs is drained to the prevertebral celiac nodes at the origin of the celiac artery from the aorta.

Signalling pathways

In vertebrates, functional differentiation continues even after birth, with the transformation into the adult phenotype occurring through epithelial–mesenchymal transition. Patterning events that determine tissue differentiation in vertebrates rely on several hox genes, the morphogen sonic hedgehog, and transcription factors such as sox2 and sox9. Recent research has suggested that most foregut malformations are due to defects in these signalling pathways, with sonic hedgehog gene knockout mice showing phenotypes similar to those seen in patients with esophageal atresia/stenosis, tracheo-esophageal fistula, and respiratory tract anomalies.

Clinical significance

• Esophageal atresia is a congenital defect of the digestive system in which the continuity of the esophageal wall is interrupted. In most cases, the upper esophagus fails to connect with the lower esophagus and stomach.
• Esophageal stricture is the narrowing of the esophagus resulting in swallowing difficulties.
• Pyloric stenosis is the thickening (hypertrophy) of the muscle that forms the pyloric sphincter, obstructing the passage of food.
• Biliary atresia is a congenital defect where the common bile duct, which connects the small intestine to the liver, is obstructed or absent.
• Pancreatic disease exist as both congenital and acquired diseases. Two of the well known types of congenital defect are: Pancreatic divisum, where the pancreatic duct fails to form, and Annular pancreas, where extra pancreatic tissue grows and wraps around the duodenum leading to obstruction by constriction.