Lung phospholipids in the embryonic and immature chicken: Changes in lipid composition and biosynthesis during maturation of the surfactant system

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Abstract
Total phospholipid and its components were measured in lungs of the embryonic chick, and in tissue, lamellar bodies, and lavage fluid after hatching. Phosphatidylcholine (PC), the major phospholipid in surfactant, was fractionated and the amount and fatty acid composition of its disaturated (DSPC) component was determined. The synthetic rates of PC, DSPC, and other choline phospholipids were determined on days 14 and 19 of incubation from incorporation of 14C-choline. Choline pool size was also estimated at these times. Phospholipid content of the lung increased most rapidly after day 18 of incubation with the greatest increase in PC. The PC synthesized just prior to breathing was enriched in DSPC, which constituted 45% of PC on day 19 and only 30% on day 14. DSPC content was even lower in embryonic liver and yolk. Incorporation of 14C-choline in vitro into pulmonary PC, Sphingomyelin (Sphingo), and DSPC was greater in birds before breathing (day 19) than in the younger embryos. Choline pool size decreased between days 14 and 19 of incubation, but the synthetic rate of DSPC doubled in this interval. Increased accumulation and biosynthesis of pulmonary PC, Sphingo, and DSPC in lung tissue of the embryo paralleled appearance of lamellar bodies. In 5–6-week-old chickens the major pulmonary phospholipid was also DSPC. In lavage, 70% of PC was DSPC, predominantly dipalmitoyl PC. As in the mammalian lung, PC, especially its DSPC component, forms the major phospholipid of avian surfactant, and the synthesis of DSPC is specifically stimulated prior to breathing.