A freeze-fracture study of tubular myelin in fetal rabbit lung.

A freeze-fracture replication study was undertaken to clarify the relationship between intracellular lamellar bodies of type II alveolar epithelial cells and intra-alveolar contents in late gestation rabbit fetuses. The interior of the inclusion bodies was composed of multiple stacks and/or whorls of membranes devoid of membrane-associated particles, while the limiting membrane of intracellular lamellar inclusion bodies was studied with membrane-associated particles of 150A diameter. The intra-alveolar contents were comprised of two components; spherical bodies, and tubular elements. Spherical bodies were identical to the internal contents of the lamellar bodies and also were devoid of membrane-associated particles. Tubular elements mostly appeared rectangular on cross-fractured faces, but triangular and hexagonal forms could also be seen. These tubules rested on the surfaces of the spherical bodies and appeared to be formed of the outer lipid monolayer of spherical bodies. The present observations suggest that the tubular element of the alveolar contents is formed through the interaction between the discharged lamellar body content and the alveolar fluid.