The tracheo-esophageal septum--fact or fantasy? Origin and development of the respiratory primordium and esophagus.

The so-called tracheo-esophageal septum is in fact the curved primitive floor that results from the ventrocaudal out-growth of the respiratory primordium from the caudal end of the laryngeal sulcus of the foregut. This floor, which separates the openings of the respiratory diverticulum and esophagus, is apparent in transverse sections as a bridge of tissue separating the lumina. The respiratory and hepatic primordia are contiguous initially, but they are separated very early by the rapid growth of the heart and liver, and the resultant stretching of the slower-growing foregut. The portion of foregut between the primordia is drawn out into a narrowing tube that develops into the esophagus and stomach. The respiratory primordium is drawn cranialward; it proliferates, dilates, bifurcates, and grows caudally, dragging out a stalk from the ventral aspect of the foregut. Most of the stalk above the bifurcation will develop into the trachea.