Embryology of the early foregut.

In embryology, no agreement exists how the early foregut differentiates into the respiratory tract and the intestinal tract. In particular, the formation of the early lung anlage as well as the process of separation of trachea and esophagus remains unclear. This process is explained in a rather schematic way and aims more to explain pathologic findings, whereas true embryologic investigations are extremely rare in this field. Here, scanning electron microscopy of the normal foregut development illustrates the steps, which finally leads to the development of larynx and trachea on the one hand, and pharynx and esophagus on the other hand. This study was performed in chicken embryos in accordance to the developmental stages described. As the main results from these illustrations show, we found no evidence for lateral foregut ridges inside the undivided foregut chamber and no fusion of lateral foregut components to form a trachea-esophageal septum.