Freeze-fracture characterization of isolated myelin and axolemma membrane fractions.

The macromolecular organization of membranes isolated from the rabbit optic nerve and tract was analyzed using the freeze-fracture technique. A myelin fraction and two axolemma-enriched fractions were prepared from a preparation of myelinated axons isolated by flotation in a buffered salt-sucrose medium. In the myelinated axon preparation, axolemma and myelin membranes were easily identified. Larger areas of the axon membrane and myelin membrane totally lacked intramembranous particles. The particles remaining on the myelin membrane formed patches of evenly distributed elongated and globular particles. In contrast, the particles remaining on the axolemma were globular in shape and tightly clustered. Particle clustering and particle-free areas were not characteristic of either the axolemma or myelin membrane of whole nerves fixed in situ and processed for freeze-fracture. The isolated myelin membrane fraction contained a large number of vesicles completely lacking intramembranous particles. Of the remaining membrane vesicles, profiles with dispersed elongated and globular particles predominated. A small percentage of vesicles displayed intramembranous particles of the same size, shape and clustering pattern as that seen on the axolemma of the myelinated axon preparation. The two axolemma fractions were enriched in membrane containing tightly clustered globular particles. Particle-free vesicles as well as some myelin membrane vesicles were also seen in the axolemma fractions.