Extramembranous particles in tubular myelin from rat lung

Abstract

Tubular myelin is one morphologic form taken by extracellular pulmonary surfactant. We used electron microscopy after fixation in a mixture of tannic acid and glutaraldehyde followed by osmication to examine tubular myelin fixed in situ , isolated from lavage fluid by isopyknic centrifugation in sucrose gradients, and prepared in vitro from isolated lamellar bodies by the addition of 5 m M Ca 2+ . In each case, the bilayered membranes of tubular myelin were seen as parallel arrays or as a lattice with a spacing of approximately 5 nm. The use of tannic acid produced good visualization of rows of rod-like particles 5 nm in diamter and 14–16 nm in length. The particles were regularly spaced at intervals of 14–16 nm in rows along the angels formed by intersecting membranes. Particles on opposite sides of the bilayer were in register. Since these particles were observed on tubular myelin formed by the addition of calcium to purified lamellar bodies as well as on tubular myelin formed in vivo , they appear to form from material prepackaged in the lamellar bodies of the type II cells. We suggest that these particles are protein and are involved in the formation or maintenance of the structure of tubular myelin.