Formation of The Larynx
The larynx is first seen as an outgrowth from the foregut during week 4 of fetal development. The outgrowth of tissue is called the lung bud or the respiratory diverticulum, which is a ventral diverticulum of endoderm that arises from the floor of the foregut caudal to the pharynx.The lung bud forms a groove in the floor of the pharynx called the laryngotracheal groove. The lung bud mesenchyme gives rise to the smooth muscles of the lower respiratory system. The lung bud initially is in open communication with the foregut, but eventually they become separated by indentations of mesoderm called the tracheoesopheageal (or esophagotracheal) folds. In the 4th week of development, the tracheoesophageal folds fuse in the midline to form the tracheoesopheageal septum, and it is here where the foregut divides into the trachea ventrally and esophagus dorsally.The opening of the respiratory diverticulum into the foregut becomes the laryngeal orifice.
Figure 1. (A-C) Successive stages in the development of the lung buds. Note the esophagotracheal ridges and the splitting of the foregut into the esophagus and trachea with lung buds. (D) The ventral portion of the pharynx seen from above. Showing the laryngeal orifice and swellings. (Sadler, 2010)
The internal lining of the larynx originates from endoderm, but the cartilages and muscles originate from mesenchyme of the fourth and sixth pharyngeal arches. The laryngeal orifice can be recognized when mesenchyme of the two arches transforms into the thyroid, cricoid, and arytenoid cartilages. Since musculature of the larynx is derived from mesenchyme of the fourth and sixth pharyngeal arches, all laryngeal muscles are innervated by branches of the vagus nerve, that is, the superior laryngeal nerve and recurrent laryngeal nerve.