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Lung development is a complex process that occurs in five distinct stages. Initial development begins with the lung bud formation from the distal end of the respiratory diverticulum during week 4. The mnemonic “Every Pulmonologist Can See Alveoli” can help recall the stages: Embryonic, Pseudoglandular, Canalicular, Saccular, and Alveolar.
Stage | Structural Development | Notes |
---|---|---|
Embryonic (weeks 4–7) | Lung bud → trachea → bronchial buds → mainstem bronchi → secondary (lobar) bronchi → tertiary (segmental) bronchi. | Errors at this stage can lead to tracheoesophageal fistula. |
Pseudoglandular (weeks 5–17) | Endodermal tubules → terminal bronchioles. A modest capillary network surrounds it. | Respiration impossible at this stage; development is incompatible with life. |
Canalicular (weeks 16–25) | Terminal bronchioles → respiratory bronchioles → alveolar ducts. Surrounded by a prominent capillary network. | Airways increase in diameter. Respiration is possible starting at 25 weeks. Pneumocytes develop at 20 weeks. |
Saccular (week 26–birth) | Alveolar ducts → terminal sacs. Terminal sacs are separated by primary septae. | |
Alveolar (week 36–8 years) | Terminal sacs → adult alveoli (due to secondary septation). | In utero, “breathing” occurs via aspiration and expulsion of amniotic fluid, reducing vascular resistance through gestation. At birth, fluid is replaced with air, leading to a decrease in pulmonary vascular resistance. At birth, there are 20–70 million alveoli; by 8 years, there are 300–400 million alveoli. |
For more detailed information, you can refer to this comprehensive resource on lung development or explore lung development stages in medical textbooks.