Expansion
Embryonic, Pseudoglandular, Canalicular, Saccular, Alveolar
Expansion
- Embryonic (weeks 4 to 7) - lung bud, trachea and main bronchi form
- Pseudoglandular (weeks 5 to 17) - branching to the terminal bronchioles; the lung looks like a gland. No respiration possible
- Canalicular (weeks 16 to 26) - respiratory bronchioles form, capillaries invade, type II cells appear. Viability just becomes possible near the end
- Saccular (week 26 to birth) - terminal sacs develop, surfactant production rises
- Alveolar (week 32 to about 8 years) - true alveoli form and multiply
Most alveoli form after birth, which is why lung growth continues through childhood and why early injury has such long-term consequences.
Because surfactant is not adequate until roughly 35 weeks, prematurity below that threshold risks respiratory distress syndrome; the reason antenatal corticosteroids are given to accelerate type II cell maturation.