Figure 1: Picture showing the formation of the respiratory diverticulum ventral to the foregut. Available at http://www.ultratwistersgym.com/Resources/Respiratory/Respiratory.html
The larynx, trachea, bronchi, and lungs form the lower respiratory tract organs. They develop at the 4th week of gestation.
The laryngotracheal groove is the primordial of the lower respiratory organs and develops caudal to the 4th pharyngeal pouches (D).
The epithelium and glands of the larynx, trachea, bronchi and pulmonary epithelium are derived from the endodermal lining of the laryngotracheal groove.
The splanchnic mesoderm of the surrounding foregut will give derives to the connective tissue, cartilage and smooth muscle of the lower respiratory organs.
The laryngotracheal diverticulum is a pouch like structure that found at the ventral to the caudal part of the foregut and is formed at the laryngotracheal groove (A, B).
Globular respiratory bud is formed at distal end of the laryngotracheal diverticulum as it elongates. The tracheoesophageal ridges separate the diverticulum with the foregut as it elongates and later will fuse and become the tracheoesophageal septum (C).
This septum divides the foregut into dorsal and ventral part. The dorsal part is esophagus and the ventral part is the laryngotracheal tube.
The primordial pharynx maintains communication with the respiratory diverticulum through the primordial laryngeal inlet.