Table 1.

Developmental Stages of the Human Lung

Developmental Stage	Gestational Age (approximate)	Processes
Embryonic period / Organogenesis	Week 1 through 7	- Lungs appear at ~day 26 - Early lung lobes form - Dichotomous divisions form the bronchial tree (branching morphogenesis) [27] - Pulmonary arteries [28-31] and veins [32] form via angiogenesis, vasculogenesis or both. - Epithelial-mesenchymal “cross-talk” is important [33-35]
Pseudoglandular	Week 5 through 17	- Bronchial tree forms to terminal bronchiole via branching morphogenesis [36] - Epithelial-based acinus/alveolus forms via cell differentiation [37, 38] - Arterial and venous system develop in parallel to the airways [31, 32] - Capillary plexus forms connecting arteries and veins [39, 40] - Epithelial-mesenchymal “cross-talk” is important [33-35]
Canalicular	Week 16 through 26	- Respiratory epithelial growth and differentiation - Respiratory bronchiole formation - Further development of acinus/alveolus - Lung parenchyma “canalization” by capillaries [24, 26] -Air-blood barrier is formed
Saccular	Week 24 through term	- Distal saccule formation (lung size increases) - Alteration of capillary network (inter-air-space wall capillaries increase in number) - Interstitial elastic fibers are laid and differentiation of cells further advances -Surfactant production by type II epithelial cells begins - Larger vessels muscularize [41]
Alveolarization	Week 36 through 18 months post-natally	- Alveoli form via septation of the saccules (~90% post-natally in humans) [24, 25, 42]
Microvascular Maturation	Starts shortly after birth and continues through the first few years of life	- Capillary network morphs from bi-layered to single-layered through capillary fusion [24, 25, 43]