alveolarization

Lung development occurs in five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. Alveolarization specifically refers to the alveolar stage, beginning around 36 weeks gestation and continuing postnatally, during which secondary septa grow into saccular walls, subdividing them into smaller units (alveoli). This process is driven by fibroblast-myofibroblast interactions, elastin deposition at the tips of secondary septa, and coordinated signaling involving platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), retinoic acid, and TGF-beta pathways. Angiogenesis is tightly coupled with alveolarization (the 'vascular hypothesis' of lung development), as capillary growth is essential for septation. After the initial burst of alveolarization (roughly 36 weeks to 2-3 years), microvascular maturation and further alveolar growth continue into childhood, though the classic teaching of a fixed cutoff has been revised—some alveolarization may continue into adolescence. Clinically, premature infants who require supplemental oxygen and mechanical ventilation are at risk for bronchopulmonary dysplasia (BPD), a chronic lung disease characterized by arrested alveolarization, dysmorphic vasculature, and fewer, larger, simplified alveoli rather than the classic fibrotic changes seen in old BPD (pre-surfactant era). Hyperoxia, inflammation, mechanical stretch, and nutritional deficiencies (e.g., vitamin A deficiency) impair septation and vascular growth. Vitamin A (retinoic acid) supplementation has been studied to reduce BPD incidence given its role in alveolar septation. This concept is high-yield for understanding neonatal respiratory distress syndrome complications, BPD pathogenesis, and the long-term pulmonary outcomes of premature infants, including risk for asthma-like symptoms and reduced pulmonary function in later childhood/adulthood.