Freeze-fracture Characterization of Isolated Myelin and
Axolemma Membrane Fractions
The freeze-fracture technique permits the observation of the macromolecular arrangements of membrane components. This technique was used to observe the membranes of central nervous system (CNS) myelinated nerve fibers sampled at each step of the biochemical isolation procedure.
As early as the initial homogenization step, axon and myelin membranes undergo rearrangement and possible loss of components. The paranodal myelin regions appear to vesiculate and separate with the axon membrane. Since these paranodal regions are the areas where new membrane is inserted into the expanding sheath, our results call into question whether previous reports on myelination using myelin membrane fractions are lacking some of the most significant regions of the sheath in their analysis.
The isolated axon membrane contains clustered components in contrast to the dispersed components in the in situ membrane; the isolated compact myelin contains large regions lacking intramembranous particles (presumably integral membrane proteins).
In summary, biochemical enrichment procedures alter components of CNS membranes. The types of changes differ depending upon the composition of the membrane, and the changes occur prior to the opportunity to examine the material biochemically. Data from studies on isolated membrane should be interpreted with an understanding that some components may be lost or associations altered during the isolation procedure.