The head and neck of a human embryo thirty-two days old, seen from the ventral surface. The floor of the mouth and pharynx have been removed. (Laryngo-tracheal tube labeled at lower left, second from bottom.)
The rudiment of the respiratory organs appears as a median longitudinal groove in the ventral wall of the pharynx. The groove deepens and its lips fuse to form a septum which grows from below upward and converts the groove into a tube, the laryngotracheal tube. The cephalic end opens into the pharynx by a slit-like aperture formed by the persistent anterior part of the groove. Initially the cephalic end is in open communication with the foregut but eventually it becomes separated by indentations of mesoderm, the tracheoesophageal folds. When the tracheoesophageal folds fuse in the midline to form the tracheoesophageal septum, the foregut is divided into trachea ventrally and the esophagus dorsally. The tube is lined by endoderm from which the epithelial lining of the respiratory tract is developed. The cephalic part of the tube becomes the larynx, and its next succeeding part the trachea, while from its caudal end two lateral outgrowths, the right and left lung buds, arise, that branch into main (primary), lobar (secondary), segmental (tertiary), and subsegmental bronchi and lead to development of the lungs.The Hox complex, FGF-10 (fibroblast growth factor), BMP-4 (bone morphogenetic protein), N-myc (a proto-oncogene), syndecan (a proteglycan), tenascin (an extracellular matrix protein) and epimorphin (a protein) appear to play a role in development of the respiratory system.