Ultrastructural study of the alveolar lining and the bronchial mucus layer by block staining with oolong tea extract: the role of various surfactant materials

To investigate the roles of various surfactant materials in the lung, we examined rat lung fixed with a mixture of 0.2% oolong tea extract (which contains various polyphenols) and 2.5% glutaraldehyde by electron microscopy. We also measured the surface tension of various isolated surfactant fractions, with a Wilhelm balance. A fraction containing lamellar bodies and a fraction containing lattice-like structures were obtained by discontinuous sucrose gradient centrifugation. From the 15 000 g supernatant, a fraction containing electron-dense amorphous materials was obtained as the 105 g precipitate. The fraction containing lamellar bodies and the fraction containing electron-dense amorphous materials displayed surfactant activity, but the fraction containing lattice-like structures did not. The lamellar bodies were found to be gradually secreted from type II epithelial cells while self-decomposition occurred. The alveolar lining layer had the form of a thin film consisting of electron-dense amorphous materials. These electron-dense amorphous materials may be precursors of the phospholipid film, which exhibits surfactant activity, on the alveolar surface. Lattice-like structures and lamellar bodies were found to be located in the interalveolar pores. The interalveolar pores were filled with surfactant, and this indicated that they do not play a role in the collateral ventilation of the alveoli. It may be that the lattice-like structures serve to connect lung epithelial cells in the interalveolar pores, as well as serving as the basis for the formation of the alveolar ducts. The bronchial mucus layer, which consisted of fibrillar mucous materials, was not divided into an epiphase and a hypophase. A surfactant, in the form of an osmiophilic surface film and some trilaminar materials, was found to cover the mucus layer. Thus, it is possible that the lamellar bodies could be transformed into various surfactant materials, which then serve bronchial or alveolar mechanical and physiological functions.