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.
