Development of the Respiratory System

Study Notes

The respiratory system develops from a ventral diverticulum of the foregut, called the respiratory diverticulum (lung bud).
This diverticulum appears as an esophagotracheal groove at the middle of the fourth week.
The margins of this groove fuse to form the esophagotracheal septum.
This septum separates the primitive pharynx into the dorsal part (pharynx and esophagus) and the ventral part (laryngo-tracheal tube).
Fusion of the laryngo-tracheal folds leaves a communication between the laryngo-tracheal tube and the pharynx called the laryngeal inlet.
The cartilages, muscles, connective tissues, blood vessels, and lymphatics of the respiratory tract develop from visceral or splanchnic mesoderm.
The epithelial and glandular anlage of the respiratory tract develop from endoderm.
As the groove closes, the diverticulum elongates caudally and divides to form the endodermal lung (bronchial) buds.
These lung buds develop into the right and left primary or principal bronchi of the corresponding lung buds.
The part proximal to the first division becomes the larynx and trachea.

The internal lining of the larynx originates from endoderm.
The cartilages and muscles originate from mesenchyme of the fourth and sixth pharyngeal arches.
The primitive laryngeal opening (aditus) is cranially bounded by the hypobranchial eminence, which later becomes the epiglottis.
Caudal to the opening, on the ventral wall, two arytenoid swellings appear, narrowing the aditus.
These swellings fuse laterally with the epiglottis forming the aryepiglottic fold.
The laryngeal orifice changes from a sagittal slit to a T-shaped opening due to rapid proliferation of mesenchyme.
When the mesenchyme of the fourth and sixth pharyngeal arches transforms into the thyroid, cricoid, and arytenoid cartilages, the characteristic adult shape of the laryngeal orifice is recognizable.
The laryngeal epithelium proliferates rapidly, resulting in a temporary occlusion of the lumen.
Vacuolization and recanalization produce a pair of lateral recesses called the laryngeal ventricles.
These recesses are bounded by folds of tissue that differentiate into the vocal cords.
The laryngeal cartilages start to develop in the second month from the visceral mesoderm, but a plate of cartilage appears in the epiglottis later in foetal life.

The trachea is a short but rapidly growing epithelial tube at the beginning.
Cartilaginous rings appear in the trachea by the end of the 7th week.
These cartilaginous rings extend to the level of the lobar and segmental bronchi.
The two lung buds give rise to the tissue of the corresponding bronchial tree.
These buds divide into daughter buds, forming three on the right side and two on the left side.
The continuous dichotomic division leads to the formation of the following parts: Trachea → Primary (principal) bronchi → Secondary (lobar) bronchi → Tertiary (segmental/lobular) bronchi → Terminal bronchi → Bronchioles → Terminal bronchioles → Respiratory bronchioles → Alveolar ducts → Alveolar sacs → Alveoli.
The surrounding splanchnic mesoderm forms cartilaginous plates, smooth muscles, connective tissue, and blood capillaries of a bronchopulmonary segment.