Normal lung development

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