DEVELOPMENT OF LARYNX, TRACHEA AND BRONCHI

Lower Respiratory Organs The lower respiratory organs i.e. larynx, trachea, bronchi and lungs begin to form during the fourth week The respiratory primordium is indicated by a median outgrowth from the caudal end of the ventral wall of the primordial pharynx

Laryngotracheal Groove Is a primordium of the tracheobronchial tree Develops caudal to the fourth pair of pharyngeal pouches The endoderm lining the laryngotracheal groove gives rise to bronchi and pulmonary epithelium

Respiratory Diverticulum By the end of fourth week, the laryngotracheal groove has evaginated to form a pouch -like respiratory diverticulum or lung bud Is located ventral to the caudal part of the foregut This diverticulum elongates and is invested with splanchnic mesenchyme and its distal end enlarges to form a globular tracheal bud

Tracheoesophageal folds & Septum Longitudinal tracheoesophageal folds develop in the laryngotracheal diverticulum These folds approach each other and fuse to form a partition called tracheoesophageal septum This septum divides the cranial part of the foregut into a ventral part, the laryngotracheal tube and a dorsal part, oropharynx and esophagus

Development of Larynx The epithelial lining of the larynx develops from endoderm of the cranial end of laryngotracheal tube The cartilages of the larynx develop from the cartilages in the fourth and sixth pairs of pharyngeal arches The laryngeal cartilages develop from the mesenchyme that is derived from neural crest cells

Development of Larynx The laryngeal epithelium proliferates rapidly resulting in temporary occlusion of the laryngeal lumen Recanalization of larynx normally occurs by the tenth week 1-Laryngeal ventricles form during this recanalization 2-These recesses are bounded by folds of mucous membrane that become the vocal folds and vestibular folds

Epiglottis It develops from the caudal part of the hypopharyngeal eminence The rostral part of this eminence forms the posterior third or pharyngeal part of the tongue Growth of the larynx and epiglottis is rapid during the first three years after birth By this time the epiglottis has reached its adult form

Development of Trachea The endodermal lining of the laryngotracheal tube distal to the (larynx differentiates into the epithelium and glands of the trachea and pulmonary epithelium The cartilages, connective tissue, and muscles of the trachea are derived from the splanchnic mesenchyme surrounding the laryngotracheal tube

Tracheoesophageal Fistula It is an abnormal passage between the trachea and esophagus Occurs once in 3000 to 4500 live births Most affected infants are males  In more than 85% of cases, the fistula is associated with esophageal atresia It results from incomplete division of the cranial part of the foregut into respiratory and esophageal parts

Development of Bronchi and Lungs Tracheal bud divides into two outpouchings, called primary bronchial buds These buds grow laterally into the pericardioperitoneal canals, the primordia of pleural cavities Bronchial buds differentiate into the bronchi

Development of Bronchi and Lungs Connection of each bronchial bud with the trachea enlarges to form the primordium of a main bronchus The right main bronchus is slightly larger than the left one and is oriented more vertically The embryonic relationship persists in the adult The main bronchi subdivide into secondary bronchi that form lobar, segmental and intersegmental branches

Development of Bronchi and Lungs The segmental bronchi, ten in right lung and eight or nine in the left lung begin to form by the seventh week The surrounding mesenchyme also divides Each segmental bronchus with its surrounding mass of mesenchyme is the primordium of a bronchopulmonary segment

Development of Bronchi and Lungs By 24 weeks, about 17 orders of branches have formed and respiratory bronchioles have developed An additional seven orders of airways develop after birth As the bronchi develop, cartilaginous plates develop from the surrounding splanchnic mesenchyme

Development of Bronchi and Lungs The bronchial muscle and pulmonary connective tissue and capillaries are also derived from this mesenchyme As the lungs develop they acquire a layer of visceral pleura from splanchnic mesenchyme The thoracic body wall becomes lined by a layer of parietal pleura derived from the somatic mesoderm

The End