Development of the Respiratory System

Development of the Respiratory System

• 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.

Development of the Larynx

• 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.

Development of the Tracheo-bronchial Tree and the Lungs

• 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.