DEVELOPMMENT OF RESPIRATORY SYSTEM

1. DEVELOPMMENT OF RESPIRATORY SYSTEM Dr. Ahmed Fathalla Ibrahim

2. LOWER RESPIRATORY ORGANS • LARYNX • TRACHEA • BRONCHI • LUNGS

3. PHARYNGEAL ARCHES

4. RESPIRATORY PRIMORDIUM

5. RESPIRATORY PRIMORDIUM

6. RESPIRATORY PRIMORDIUM 6

7. RESPIRATORY PRIMORDIUM • Begins to form in the middle of 4th week • Laryngotracheal groove: a median outgrowth from the caudal end of the floor (ventral wall) of primordial pharynx, caudal to 4th pharyngeal pouch • Respiratory diverticulum: produced by deepening (evagination) of the groove, located ventral to caudal part of foregut

8. RESPIRATORY PRIMORDIUM • Tracheal bud: it is the enlarged distal end of diverticulum • Longitudinal tracheoesophageal folds develop in diverticulum, approach each other & fuse to form tracheoesophageal septum dividing cranial part of foregut into: • Ventral part: laryngotracheal tube: primordium of larynx, trachea, bronchi & lungs • Dorsal part: pharynx, esophagus • Both parts maintain communication through primordial laryngeal inlet

9. EMBRYOLOGICAL ORIGIN • Endoderm lining laryngotracheal tube: epithelium & glands of all respiratory organs • Fourth & sixth pharyngeal arch cartilages: all laryngeal cartilages EXCEPT epiglottis • Sixth pharyngeal arch muscles: intrinsic muscles of larynx • Mesenchyme of hypopharyngeal eminence: epiglottis • Splanchnic mesoderm surrounding foregut: connective tissue, cartilage, smooth muscles, blood & lymphatic vessels of trachea, bronchi & lungs

10. DEVELOPMENT OF BRONCHI & LUNGS

11. DEVELOPMENT OF BRONCHI & LUNGS • During 5th week, tracheal bud divides into 2 primary bronchial buds • Main bronchus: formed by primary bronchial bud together with its surrounding splanchnic mesoderm, divided into: secondary (lobar), tertiary (segmental) & intrasegmental branches • By 24 weeks, 17 orders of branches have formed & respiratory bronchioles have developed • After birth, 24 orders of branches are present

12. DEVELOPMENT OF BRONCHI & LUNGS • As the lungs expand: • They invaginate pleura & acquire a visceral layer of pleura (derived from splanchnic mesoderm) • They grow caudally into body wall & lie close to heart. The thoracic body wall becomes lined by a parietal layer of pleura(derived from somatic mesoderm)

13. MATURATION OF LUNGS • Pseudoglandular period • Canalicular period • Terminal saccular period • Alveolar period

14. MATURATION OF LUNGS

15. PSEUDOGLANDULAR PERIOD (6-12 WEEKS) • Lung resembles an exocrine gland • Onlybronchial & primordial terminal bronchioles are formed • Respiration is not possible

16. CANALICULAR PERIOD (16-26 WEEKS) • Lumina of bronchi & terminal bronchioles enlarge • Respiratory bronchioles & alveolar ducts develop • Lung tissue becomes highly vascular • Respiration is possible • Fetus born at this period often dies because of immaturity of systems

17. TERMINAL SACCULAR PERIOD (26 – BIRTH) • Alveolar ducts give rise to terminal saccules (primordial alveoli) • Terminal saccules: cuboidal epithelium begins to change into squamous • Capillaries begin to bulge into alveoli to establish blood-air barrier that permits adequate gas exchange • Terminal saccules are lined with: • Type I pneumocytes(across which gas exchange occurs): main type of cells • Type II pneumocytes(secrete surfactant)

18. TERMINAL SACCULAR PERIOD (26 – BIRTH) Surfactant: • Role: counteracts surface tension forces & facilitates expansion of alveoli • Production: begins by 20 weeks & increases during last 2 weeks of pregnancy • By 26-28 weeks: surfactant is produced in sufficient amount for survival of prematurely born fetus

19. ALVEOLAR PERIOD (32 WEEKS – 8 YEARS) • Number of respiratory bronchioles and terminal saccules (primordial alveoli) increases • Type I pneumocytes of terminal saccules becomes extremely thin squamous so that adjacent capillaries bulge into it • Alveolocapillary membrane is sufficiently thin to allow gas exchange

20. ALVEOLAR PERIOD (32 WEEKS – 8 YEARS) • Lungs begin to work after birth • Transition from dependence on placenta for gas exchange to lungs requires: • Production of sufficient amount of surfactant • Development of alveolocapillary membrane that is sufficiently thin

21. ALVEOLAR PERIOD (32 WEEKS – 8 YEARS) • Maturation of alveoli (about 95%) occurs after birth • From birth to third year: Increase in size of lungs is mostly due to increase in number of respiratory bronchioles & primordial alveoli • Immature alveoli have the potential for forming additional primordial alveoli • Immature alveoli increases in size & become mature

22. TRACHEOESOPHAGEAL FISTULA

23. TRACHEOESOPHAGEAL FISTULA • Most common anomaly • Associated with esophageal atresiain more than 85% of cases • Often associated with polyhydramnios • Cause: incomplete fusion of treacheoesophageal folds resulting in a defective tracheoesophageal septum & faulty partitioning of foregut into trachea & esophagus

24. TRACHEOESOPHAGEAL FISTULA Clinical manifesttaions: • Infants cough & choke when swallowing (due to accumulation of saliva in upper respiratory tract) • Infants regurgitate when swallowing milk (milk fills esophageal pouch & is regurgitated) • Pneumonitis

25. RESPIRATORY DISTRESS SYNDROME • Also known as hyaline membrane disease • Affects about 2% of newborn (especially premature newborn) • Cause: deficiency of surfactant • Manifestation: lungs are underinflated, alveoli contain a fluid with a high protein content that resembles a hyaline membrane

26. RESPIRATORY DISTRESS SYNDROME • Predisposing factors: • Prolonged intrauterine asphyxia • Deficiency of thyroxine • Treatment:glucocorticoid

27. OTHER ANOMALIES • Stenosis (narrowing) or atresia (obstruction) of trachea: due to unequal partitioning of foregut into esophagus & trachea • Congenital lung cysts: disturbance in bronchial development leads to dilation of terminal bronchi & formation of cysts filled with fluid or air • Agenesis of lung: due to failure of bronchial bud to develop