Chapter 58. Development of The Lower Respiratory System: Larynx and Trachea - Review of Medical Embryology Book

Introduction: the first indication of the future lower respiratory system appears in the 4 mm embryo, in the primitive pharyngeal floor just behind the pharyngeal pouches, early in week 4 of embryonic life, as a longitudinal groove, the laryngotracheal groove. Externally it is seen as a ridge. The entodermal lining of the laryngotracheal groove forms the epithelium and glands of the larynx, trachea, bronchi, and pulmonary lining epithelium. The splanchnic mesenchyme ventral to the foregut will give rise to the connective tissue, the cartilage, and smooth muscle accompanying these structures. When the groove appears, optic vesicle and auditory placode are already present, pharyngeal pouches are forming, oropharyngeal membrane (between stomodeum and foregut) is disintegrating, and the vessels that form heart have fused into a single tube

THE LARYNGOTRACHEAL GROOVE deepens and, with development, the external ridge grows caudally below the pharynx to become a diverticulum, the tubular lung bud

As the diverticulum grows from the pharyngeal floor, it is invested by splanchnic mesenchym The cranial part of the tube becomes the laryngeal epithelium; the caudal part forms the epithelium of the lower respiratory system
As the diverticulum grows, it becomes separated from the pharynx by a partition, the tracheoesophageal septum, which divides the foregut into the laryngotracheal tube and the esophagus

The laryngotracheal tube and surrounding splanchnic mesenchyme give origin to the larynx, the trachea, the bronchi, and the lungs

When the tubular lung bud forms, it develops 2 knoblike enlargements at its distal end, the so-called bronchial buds

The larynx develops from the entodermal lining of the cranial end of the laryngotracheal tube and surrounding mesenchyme (from branchial arches IV, V and VI)

THE MESENCHYME proliferates to produce paired arytenoid swellings, giving the primitive glottis a T-shaped appearance and reducing the laryngeal lumen to a slit

The laryngeal cartilages develop within the arytenoid swellings from the cartilage bars of the branchial arches
The epiglottis develops from the caudal half of the hyopbranchial eminence, a derivative of branchial arches III and IV

THE ENTRANCE TO THE LARYNX ends blindly, between weeks 7 to 10, because of the fusion of epithelium, but as the epithelium breaks down, the laryngeal aditus enlarges and recanalizes. A pair of lateral recesses, the laryngeal ventricles, form which are bound cranially and caudally by anteroposterior folds of mucous membrane, the future vestibular (false) and vocal (true) folds, respectively

The laryngeal muscles develop from muscle elements in branchial arches IV to VI and are innervated by laryngeal branches of the vagus nerve

The trachea: the entodermal lining of the middle segment of the laryngotracheal tube forms the epithelium and glands of the trachea. Mesenchymal cells (from splanchnic mesenchyme) surround the tracheal tube and ultimately form the cartilage, connective tissue, and smooth muscles of its walls

BY WEEK 8 (28-30 mm EMBRYO): mesenchymal rudiments of the 16-20 tracheal cartilages are seen and, in the following 2 weeks, the masses form cartilage beginning cranially and extending caudally. Simultaneously, fibroelastic tissue of the tracheal wall arises from mesenchyme between the cartilage and, posteriorly, between the ends of the embryonic rings smooth muscle (the trachealis) arises

Cilia appear at 10 weeks (51-53 mm embryo)
By week 12, mucosal glands are seen and develop in a craniocaudal direction
By the end of week 20, all major microscopic features of the trachea are visible, but it is short and narrow while the larynx is relatively lon This relationship remains until after birth when the trachea outgrows the larynx to reach its final form