The larynx develops from the IV pharyngeal pouch. Despite past claims, more recent studies place this level at the supraglottic region.
Human development is divided an embryonic period or the first 8 weeks of gestation and a following fetal period.
The embryonic period consists of 23 developmental stages (crf. Carnagie). Each stage has its own characteristics present in the previous stage.
Laryngeal development first appears in stage 11 (approximately at the fourth week of gestation).
The first sign of the respiratory system is seen as an epithelial lining along the ventral margin of the cephalic portion of the primitive intestine, and known as the respiratory primordium.
The respiratory primordium is separated from the hepatic primordium by the transverse septum, a structure that will develop into the central diaphragmatic tendon.
The respiratory diverticulum then is a ventral pocket of the opening of this upper portion of the primitive intestine that extends into the respiratory primordium.
The place of origin of the respiratory diverticulum is called the primitive pharyngeal floor and will develop in the glottic region of the adult larynx.
The cephalic portion of the respiratory diverticulum will develop in the infraglottic region of the adult larynx.
The primordium of the pharyngeal floor is separated from the pharyngeal floor , at the level of the IV pharyngeal pouch, by a segment of the primitive gut originally classified as the primitive laryngopharynx; this will become in the adult supraglottic larynx.
The respiratory diverticulum originates bilateral projections called bronchopulmonary sprouts; these will develop in the lower respiratory tract. The bronchopulmonary sprouts are attached to the upper margin of the transverse septum.
Dynamic changes to the forming primitive gut occur at these sequential stages.
For example, the heart and hepatic primordium rapidly proliferate on opposite surfaces of the transverse septum. These forces are exerted on the adjacent region of the primitive intestine, which can result in a significant lengthening of the same on a cranio-caudal plane.
The result can be seen as a widening distance between the respiratory primordium and the hepatic primordium. All this happens at stage 12. In stage 13 the bronchopulmonary buds are dragged causally and inferiorly because they are linked to the transverse septum and the cephalic margin of the primitive intestine, and the respiratory diverticulum migrates superiorly.
In this way, two main bronchial nuclei develop and will elongate at stage 14.
The elongation of this primitive intestine will give rise to the trachea. At this level of development significant lengthening of the trachea and esophagus occurs.
Anatomically the esophagus is in close proximity to the keel region.
Vascular compromise of the developing esophagus can create esophageal atresia, or tracheoesophageal abnormalities.
Esophageal atresia in a newborn presents clinically with increased salivation requiring frequent swallowing, with the pulmonary triad of cough, choking, cyanosis.
These symptoms are the result of accumulation of saliva in the loop of the proximal esophageal pouch resulting in overflow into the airways.
Aspiration is greater in children who have a direct connection to the airway due to an associated tracheoesophageal fistula.
A tracheoesophageal fistula with a proximal esophageal pouch and distal tracheoesophageal fistula occurs in 80 to 85% of cases with resulting gastric distension caused by air swallowed with each breath.
Gastric distention associated with increased gastric output leads to respiratory symptoms caused by direct tracheal aspiration of the reflux mixture of air and gastric acid and decreased diaphragmatic excursion.
If the diagnosis is not made promptly and breastfeeding is started, choking episodes occur.
Developing vascular compromise of the trachea at that stage may result in complete tracheal agency, or tracheal stenosis with complete tracheal rings.
Usually, both of these anomalies are associated with a normal larynx and normal lung development since the anomaly is limited to the region of tracheal development.
A consequent elevation of the median pharyngeal floor leads to inflammation of the arytenoids.
This occurs at the same height as the fourth pharyngeal pouch.
The laryngeal caecum originates as a triangular space along the ventral margin of the arytenoid inflammation and continues causally along the ventral margin of the primitive laryngopharynx until reaching the level of the glottis at stage 18.
In stage 19 the epithelial lamina begins to reanalyse from a dorsalcephalic to a ventrocaudal direction.
During this process, communication between the ventral laryngeal cecum and the dorsal pharyngoglottis is reestablished.
It is an incomplete recanalization of the epithelial lamina which can give rise to a full spectrum of glottic and supraglottic atresia.
In stage 21 the laryngeal cecum gives rise to the laryngeal ventricles bilaterally.
The development of the laryngeal cartilage and its muscles is visible starting from stage 14 in the form of triangular laryngeal mesoderm adjacent to the primitive laryngopharynx.
Subsequently, consolidation occurs in two distinct regions, the hyoid and the thyrocricoid.
Fusion of the laryngeal mesoderm occurs dorsally in the cricoid region during stage 18.
Cartilaginous consolidation begins along the ventral margin of the cricoid at stage 17 and continues dorsally until fusion occurs at the level of the posterior lamina of the cricoid at stage 20.
An incomplete fusion at this level gives rise to the complete clinical picture of a laryngotracheal fissure.
Four types of crack can be found:
- Type 1: in the arytenoid area
- Type 2: partial cricoid
- Type 3: Complete cricoid (remains above the thoracic outlet)
- Type 4: laryngotracheoesophageal fissure