Question

DEVELOPMENT OF ESOPHAGUS

Answer 1

The foregut immediately caudal to the primitive pharynx forms the esophagus. Initially, the esophagus appears as a very short tube, which ends in the primordium of the stomach that appears as a fusiform dilatation of the foregut in the cervical regin of the embryo. During the 4th week of devel­opment, the tracheobronchial diverticu-lum (also called respiratory diverticu-lum or lung bud) appears as an out­growth from the ventral wall of the foregut. This diverticulum opens into the floor of the developing esophagus by a long median slit-like opening called, laryngotracheal groove. This opening is bounded by two tracheo-esophageal ridges, one of which lies on each lateral border of the laryngotra­cheal groove. The tracheoesophageal ridges grow medially toward each other and finally fuse to form a septum called tracheoesophageal septum. This sep­tum divides the initial part of the foregut proper into a two parts: (1) a dorsl part that becomes the primitive esophagus, and (2) a ventral part called laryngotra­cheal tube (which is the primordium of the larynx, trachea, bronchi and lungs).
Initially, the esophagus is very short because the primitive stomach lies in the cervical region. However, the esophagus elongates rapidly because of the descent of the stomach and
achieves its final relative length by the 8th week of development.
The lining epithelium and glands of the esophagus are derived from the endoderm of the foregut. The muscu-laris externa of the esophagus contains striated (skeletal) as well as smooth muscle. The striated muscle is derived from the mesenchyme of the caudal pharyngeal arches. The smooth muscle (present mainly in the inferior third of the esophagus) develops from mesen­chyme which is derived from the splanchnic mesoderm surrounding the foregut.
During the 5th and 6th weeks the endodermal epithelium of the esophagus proliferates and almost completely obliterates the lumen. How­ever, recanalization by programmed cell death subsequently occurs, so that the lumen is restored by the end of the embryonic period (8th week).