Three-dimensional imaging clarifies the process of tracheoesophageal separation in the rat

The process of tracheoesophageal separation during early development of the foregut has been disputed and has led to difficulties explaining how congenital abnormalities of the trachea and esophagus might occur. This study clarifies the embryogenesis of tracheoesophageal separation by using sequential 3-dimensional imaging at crucial stages of foregut development.

Timed pregnant Sprague Dawley rats were killed at days 11, 11.5, 12, 12.5, and 13. The embryos were harvested, histologically sectioned, and stained with H&E. Digitized photographs were taken of sequential serial transverse sections and their tracings layered in a 3-dimensional rendering program before being "skinned" to produce a 3-dimensional object.

The first respiratory structures to develop are the bronchi on day 11.5. They are evident first as bulges on the ventrolateral wall of the foregut approximately two thirds of the way between the lowest pharyngeal pouch and the level of the hepatic diverticulum and pancreatic buds. Lateral grooves dorsal to the respiratory bud on the lateral walls extend cranially. On day 12 the lateral bulges have developed into the 2 main bronchi, although the trachea is yet to separate from the foregut. On days 12.5 to 13 the trachea progressively elongates, and by day 13 tracheoesophageal separation is complete.

After the main bronchi have developed, the trachea forms when the ventral component of the foregut is "cut" away from the dorsal component. There is an area of apoptosis at the point of tracheoesophageal separation, and, as the embryo grows, this causes the separation point to stay at a constant distance from the pharynx. Meanwhile, the trachea and esophagus distal to it increase dramatically in length. The area immediately caudal to the initial point of tracheoesophageal separation ultimately forms the stomach.