alveolar period

Lung development is divided into five stages: embryonic, pseudoglandular, canalicular, saccular, and alveolar. The alveolar period begins around 36 weeks gestation and continues through early childhood (historically thought to end around age 8, though some alveolarization may continue into adolescence). During this period, type I and type II pneumocytes are well established, and secondary septa grow into the saccular walls formed in the preceding saccular stage, subdividing them into smaller units called alveoli. This process, termed septation, is driven by elastin deposition at the tips of secondary crests and involves complex signaling including PDGF-A, retinoic acid, and elastin/collagen matrix remodeling. The pulmonary capillary network also undergoes microvascular maturation, transitioning from a double to single capillary layer, improving efficiency of gas exchange. At birth, only about 20-50 million alveoli are present; this number increases to the adult count of approximately 300-500 million by around 8 years of age. Clinical relevance: infants born prematurely (before 36 weeks) miss significant alveolarization, predisposing them to bronchopulmonary dysplasia (BPD), a chronic lung disease characterized by impaired alveolar and vascular development, often exacerbated by oxygen toxicity and mechanical ventilation. Antenatal corticosteroids are given to mothers at risk of preterm delivery to accelerate surfactant production (a saccular period process) but understanding the alveolar period timeline explains why extreme prematurity leads to simplified, dysplastic alveolar architecture. This concept is tested on USMLE Step 1 in the context of lung embryology, prematurity complications, and BPD pathogenesis.