The alveolar-lining layer in the lung of the axolotl, Ambystoma mexicanum. An electron-microscopic study using heavy...

Lungs of neotenic larvae of Ambystoma mexicanum were prepared for maintaining the air-tissue boundary during aldehyde fixation. Four methods of postfixation were applied: 1) osmium tetroxide followed by en-bloc staining with uranyl acetate and phosphotungstic acid, 2) ruthenium redosmium tetroxide, 3) osmium tetroxide-ferrocyanide, and 4) tannic acid-osmium tetroxide. Three types of cells line the inner surface of the axolotl lung: 1) pneumocytes, covering the capillaries with flat cellular extensions and containing two types of granules: the osmiophilic lamellar bodies, precursors of extracellular membranous material, and apical granules of unknown significance; 2) ciliated cells, also containing osmiophilic lamellar bodies; and 3) goblet cells filled with secretory granules as well as osmiophilic bodies. The extracellular material forms membranous whorls as well as tubular myelin figures, consisting of membranous "backbones" combined with an intensely stained substance. This material strikingly resembles the surfactant of amphibian lungs.