saccular period

Lung development is divided into five stages: embryonic (weeks 4-7), pseudoglandular (weeks 5-17), canalicular (weeks 16-26), saccular (weeks 26-38), and alveolar (week 36-8 years postnatal). During the saccular period, terminal sacs (saccules) form as the terminal bronchioles subdivide further, and the interstitial tissue thins, bringing capillaries into closer approximation with the developing airspaces to establish the initial blood-air barrier necessary for gas exchange. Type II pneumocytes proliferate and increase surfactant synthesis (surfactant-associated proteins and phospholipids like dipalmitoylphosphatidylcholine), which is essential for reducing surface tension and preventing alveolar collapse. Clinically, this period is critical for premature infants: those born before adequate surfactant production (typically before 34-36 weeks, though variable) are at risk for neonatal respiratory distress syndrome (NRDS) due to insufficient surfactant, leading to alveolar collapse, hypoxemia, and increased work of breathing. Antenatal corticosteroids (e.g., betamethasone) are administered to mothers at risk of preterm delivery to accelerate fetal lung maturity by stimulating type II pneumocyte surfactant production. Understanding this stage is high-yield for embryology questions on USMLE Step 1, particularly for correlating gestational age with lung maturity and predicting complications of prematurity.