Filipin-cholesterol binding in CNS axons prior to myelination: evidence for microheterogeneity in premyelinated axolemma

The distribution of cholesterol in axonal membrane of developing rat optic nerves prior to myelination was studied by freeze-fracture cytochemistry. Binding of the cholesterol-specific probe, filipin, to the axolemma of premyelinated axons was heterogeneous; this suggests the presence of microdomains of axolemma with different membrane composition and/or cytoskeletal/extracellular matrix association. Although the reasons for this binding pattern have not yet been determined, heterogeneity occurs prior to association of glia with the axon, and may reflect regional differences in lipid/sterol composition of the axonal membrane bilayer, or distribution of membrane-associated cytoskeleton. The distribution of intramembranous particles was not obviously associated with the pattern of filipin binding in early developing axons, however, as might have been expected from the attending differences in fluidity of the membrane microdomains. Microheterogeneity in axonal membranes of developing axons could have an influence on several membrane properties, and may be associated with processes important for growth and differentiation of axons.