Ultrastructural analysis of the lining layer of the rat pulmonary alveolus.

Using electron microscopy we have examined the lining layer of the rat pulmonary alveolus. This layer appears as a morphological entity 1-3 days after birth: it is composed at first of a filamentous Ruthenium Red-negative material derived from lamellar bodies, and subsequently (4 days after birth) of a homogeneous Ruthenium Red-positive material. This latter material, which corresponds to the epithelia lining of the alveolus typical of adult rats, is presumably derived from a mixture of the filamentous material produced by the lamellar bodies, and a material produced by the alveolar cells 4 days after birth which contains acidic groups which bind Ruthenium Red.