Respiratory diverticulum and tracheo-oesophageal fistula

Around the fourth week of development, a small pouch appears on the front (ventral) wall of the foregut, the upper part of the gut tube. This is the respiratory diverticulum, also called the lung bud: a diverticulum is simply an outgrowth shaped like a pouch. Its lining is endoderm, the innermost embryonic layer. The bud grows downwards to form the trachea, which divides into the two main bronchi, the large airways that enter each lung.

The airway and the oesophagus start from the same tube, so a partition called the oesophagotracheal septum has to separate them. It grows in and divides the pouch at the front, which becomes the trachea and lungs, from the foregut tube behind it, which becomes the oesophagus.

If this separation is incomplete, a connection stays between the two channels. That is a tracheo-oesophageal fistula, an abnormal passage between the trachea and the oesophagus. It often occurs together with oesophageal atresia, in which the upper oesophagus ends in a blind pouch and does not join the stomach, so food cannot reach the stomach. Surgery to close the fistula and join the oesophagus is usually done soon after birth.

The mesenchyme around the bud (loose embryonic tissue, derived from mesoderm, one of the three primary layers of the early embryo) signals to the endoderm, but the lining of the trachea and bronchi is endodermal in origin. The tracheal cartilages and bronchial smooth muscle develop from the mesenchyme around the lung bud, so the bud contributes the lining but not these supporting tissues.