Development of Esophagus
The esophagus is the first section of the foregut and begins at the distal end of the pharynx. As with the rest of the digestive tract, the cells that line the lumen of the esophagus are derived from endoderm. The sup porting structures of the esophagus come from two different sources, although they are all innervated by the vagus nerve. The muscles and connective tissues of the esophagus’s proximal third are derived from the mesenchyme of the pharyngeal arches. Like the pharynx, the skeletal muscle of the superior esophagus is innervated by axons from the nucleus ambiguus traveling in the vagus nerve. The muscles and connective tissues of the distal third of the esophagus are derived from the visceral mesoderm that surrounds the gut tube. For this reason, the muscular layers of this area are composed of smooth muscle, innervated largely by the dorsal vagal motor nucleus, also traveling in the vagus nerve. The middle third of the esophagus blends the characteristics of the other two, containing both skeletal and smooth muscle.
The development of the esophagus is intimately related to that of the trachea. At the distal end of the pharynx during the fourth week, the laryngotracheal groove forms, leading to a short blind pouch, the respiratory diverticulum. The respiratory diverticulum is a pouch of endoderm that extends ventrally into the nearby visceral mesoderm. By the fifth week the respiratory diverticulum has elongated into a tracheal bud that stretches inferiorly and is completely separate from the esophagus except for its connection at the primordial laryngeal inlet, which will eventually become the glottis. The separation of the two tubes is effected by two ridges of visceral mesoderm, the tracheoesophageal ridges. These ridges grow medially between the respiratory diverticulum/tracheal bud and the esophagus, eventually forming a tracheoesophageal septum. This process occasionally goes awry, resulting in congenital anomalies such as blind pouches and tracheoesophageal fistulas.
The esophagus is initially relatively short but elongates to its normal relative length by the seventh week. If it fails to lengthen appropriately, it can produce a congenital hiatal hernia from the traction placed on the developing stomach as it passes through the diaphragm. The proliferation of the endodermal cells of the esophageal lumen is one of the factors that allow it to lengthen. The lumen typically becomes obstructed by these epithelial cells but recanalization opens the lumen during the eighth week. Failure of the lumen to recanalize may lead to an abnormal narrowing, esophageal stenosis, as well as polyhydramnio, because the fetus is unable to swallow amniotic fluid.