The alveolar lining layer: a review of studies on its role in pulmonary mechanics and in the pathogenesis of atelectasis, by Mary Ellen Avery, MD, Pediatrics, 1962:30:324-330

The alveoli of the normal lung are lined by a substance that exerts surface tension at the air–liquid interface. In the expanded lung, the tension is high and operates to increase the elastic recoil of the lung. In the lung at low volumes, the surface tension becomes extremely low. This confers stability on the air spaces and thus prevents atelectasis. This lining layer is a lipoprotein film, which is not found where alveoli still are lined by cuboidal epithelium. Its appearance coincides with the appearance of alveolar lining cells. Electron microscopic evidence of secretory activity in alveolar cells suggests that they may be the source of the surface-active film. The normal alveolar lining layer is not present in lungs of infants who die from profound atelectasis and hyaline membrane disease. Whether its absence is a failure of development or attributable to inactivation is not established.