By Dr Liji Thomas, MD
The respiratory system in humans is formed from an outfolding of the primitive gut in the fetus. This evagination appears in the first month, from the ventral side of the esophagus towards the cephalic end.
It is pinched off completely by the growth of the tracheoesophageal folds on each side to meet in the center as the tracheoesophageal septum, except at the most cephalic tip where it opens on the floor of the gut, at the point where the pharynx meets the esophagus.
This end forms the larynx, which is formed from a U-shaped mound facing caudally, situated around the opening into the esophagus. The two side walls are elevated to form the arytenoid ridges, while a pair of secondary elevations form the cuneiform ridges.
The arytenoid ridges grow larger and closer till the laryngeal opening is almost walled off except at the end which is most cephalic. This produces a T-shaped laryngeal aditus. Simultaneously, the furcula starts to develop a transverse fold at this cephalic end, the hyobranchial fold, which becomes the epiglottis. This eventually joins the arytenoid ridges to form the aryepiglottic ridges.
In the fourth month a depression develops linearly in each arytenoid ridge on the medial side. It deepens till there are two folds on each side enclosing the laryngeal opening. The folds on the cephalic side form the external or false vocal cords, while the internal folds become the true vocal cords. At this time, which is the 10th week of fetal development, the laryngeal opening is re-formed to connect the pharynx with the trachea.
Last Updated: Mar 28, 2016