The ruthenium red staining technique of Luft ('71b) was utilized in an electron microscopic investigation of developing and adult rat lung. Electron-dense deposits of ruthenium red-positive material were observed on all exposed surfaces of the tissue block, regardless of the stage of development. In the more central areas of the block, sites of ruthenium red binding changed with age. In early prenatal lung (days 16 to 20) dense accumulations of ruthenium red-positive material were found in association with the basement membranes of endodermal epithelial cells. Ruthenium red binding was also observed between adjacent epithelial cells; however, their luminal surfaces were negative. The main intracellular site of ruthenium red binding in intact cells was the lamellar body of the developing type I1 pulmonary epithelial cells. By day 21 of development, accumulations of granular product were observed in association with most lamellar bodies, as well as on epithelial cell luminal surfaces. Ruthenium red binding in postnatal tissue decreases with increasing age. By the second postnatal week, the predominant site of binding is the luminal surface of the type I and type I1 pulmonary epithelial cells. When compared to fetal and early neonatal stages, adult rat lung has a still more limited distribution of ruthenium red-positive material. Changes in the distribution of ruthenium red-positive material correlate with numerous morphologic and biochemical events in rat lung development .The lungs of mammals and some inframammalian vertebrates contain a potent surface-active material which presumably coats the alveolar surface, decreases the surface tension, and thus stabilizes the alveoli against collapse (for review see Scarpelli, '68; Gluck, '72). The presence of this pulmonary "surfactant" is essential to normal pulmonary function, and its absence or diminution has been suggested as an important etiologic factor in such diseases as the respiratory distress syndrome (for review see Scarpelli, '68). Biochemical analyses indicate that the surfactant system is a complex mixture of neutral lipids, polysaccharides, acid mucopolysaccharides and lipoproteins; the major phospholipid present is dipalmitoyl lecithin (Klaus et al., '61; Pattle and Thomas, '61; Finley et al,, '68; Scarpelli, '68; '71). Electron microscopic studies have shown that the surfactant complex of the lung has two phases -a superficial, ANAT. REC., 178: 267-288.osmiophilic layer at the air-liquid interface, and a homogeneous, less dense basal layer (Weibel and Gil, '68; Kikkawa, '70; Kapanci et al., '72; Kuhn, '72). Scarpelli ('68) has proposed that the superficial, interfacial layer (epiphase) represents the more potent surf ace-active phospholipid molecules, whereas the subjacent aqueous phase (hypophase) contains ions, protein, phospholipid and carbohydrate. He further suggests that this "hypophase" may represent a reservoir of components necessary for the synthesis and/or stabilization of the surfaceactive interfacial layer.The phospholipid content o...
