26 days to 6 weeks (embryonic phase)
- lung bud develops from primitive foregut
- bud divides to form early tracheobronchial tree
6 to 16 weeks (pseudoglandular phase)
- airway development up to terminal bronchioles
- deficient alveolar saccules
16 to 28 weeks (canalicular or acinar phase)
- multiple alveolar ducts develop from respiratory bronchioles
- ducts lined by type II alveolar cells that produce surfactant
- type I alveolar epithelial cells
- primitive alveoli form end of this phase
- progressive thinning of pulmonary interstitium allows gas exchange by approximating proliferative capillaries with type I cells
28 to 34 weeks (saccular phase)
- increase in number of terminal sacs
- further thinning of interstitium
- continuing proliferation of capillary bed
- early development of true alveoli
36 weeks to 18 months of age (alveolar phase)
- most alveoli forms by 5-6 months of age but continues well into early adulthood