AM REV RESPIR DIS 1985; 131: 439-460 nary surfactant is a complex material that consists mainly of phospholipids but also contains specific proteins (1). Dipalmitoylphosphatidylcholine is the major phospholipid component and the one largely responsible for the surface activity (1). The major clinical condition involving a defect in the surfactant system is respiratory distress syndrome (RDS) of the newborn, which is due to lung immaturity with consequent insufficient surfactant. Surfactant deficiency may also be a contributory factor in certain adult lung diseases. The objective of this article is to review recent developments in lung and surfactant phospholipid biochemistry, with emphasis on mechanisms and control of phospholipid synthesis, developmental changes in the neonatal lung, and the influence of hormones and other physiologic agents on surfactant synthesis and secretion. Because the surfactant literature has mushroomed in the past several years, the choice of topics and literature cited is of necessity selective and reflective of the author's bias. Previous STATE OF THE ART articles in these pages (2, 3) may be consulted for more complete coverage of the older literature on surfactant biochemistry. Other recent reviews examine surfactant and lung development from a variety of different perspectives (4-10).

Experimental Approach Two major difficulties must be considered in biochemical studies on lung surfactant. First, although surfactant can be described functionally as material that lowers surface tension at an airaqueous interface, it is difficult to define chemically. Extracellular surfactant is readily obtained by bronchoalveolar lavage. After removal of cellular material by brief low-speed centrifugation, the surfactant-enriched material can be pelleted by centrifugation at 13,000 x g for 1 h (11) and may be further purified by density gradient centrifugation (12).