1. DEVELOPMENT OF LARYNX, TRACHEA AND BRONCHI

2. 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

3. 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

5. 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

7. 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

9. 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

10. 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

11. 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

13. 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

15. 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

17. 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

19. 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

21. 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

22. 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

23. 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

24. The End