Filipin-sterol complexes at nodes of Ranvier

Using the filipin-sterol technique, regional heterogeneity in the axonal and Schwann cell plasma membranes was investigated at the node of Ranvier and paranodes. Filipin-sterol complexes were abundant at the nodal axolemma but infrequent throughout the paranodal axolemma. The paranodal Schwann cell plasma membrane was rich in complexes which extended over the nodal Schwann cell microvilli. There were no regional differences in filipin labelling of the nodal-paranodal Schwann cell plasma membrane in relation to features such as paranodal cytoplasmic columns or mesaxonal furrows. However, the paranodes of adjacent Schwann cells were sometimes markedly different from each other in the amount of filipin labelling. The extent to which filipin labelling is indicative of cholesterol membrane content is discussed and the findings are related to current concepts of distribution, mobility and interaction of protein and lipid in biomembranes, with particular reference to the nodal axolemma.