The intermediate dense line of the myelin sheath is preferentially accessible to cations and is stabilized by cations.
Biophysical studies have shown that the narrow slit between the turns of the myelin leaflet includes a water space lined by strongly negative, fixed charges on the faces of the myelin leaflet. The accessibility of this slit to a marker should depend largely on the interaction between the marker charges and the surface charges on the myelin leaflet. This premise was explored in vitro by comparing the redistribution of anionic ferritin with highly cationized ferritin under a variety of experimental conditions. Cationized ferritin stained the basal lamina and penetrated it. It also bound to Schwann cell membranes, and it entered mesaxons and lodged between myelin lamellae. There was evidence of facilitated particle redistribution due to attractive forces between the cationized ferritin particles and the membrane surfaces. Anionic ferritin did not enter sheaths under identical experimental conditions. Additional experiments reconfirmed X-ray spectrographic data on a loosening of lamellar coherence upon elution of Ca2+ and recompaction of myelin by small amounts of Ca2+. If cationic ferritin was substituted for Ca2+ in these experiments, it also caused recompaction of myelin which had been loosened by previous Ca2+ elution. The cationic ferritin particles sandwiched between the recompacted myelin lamellae. These observations show that the slit between the turns of the myelin leaflet is preferentially accessible to cations, that cations can redistribute along it and that their presence is important for maintaining myelin periodicity. They also throw light on the significance of wide-spaced myelin in pathological conditions.