The permeability of the extracellular spaces at the Schmidt-Lanterman clefts and paranodes in peripheral myelin sheaths.

Electron dense material was observed in the extracellular spaces between adjacent components of the Schmidt-Lanterman clefts and between lateral loops at the paranodes in some of the heavily myelinated fibers of the anterior roots and peripheral nerves of a patient with amyotrophic lateral sclerosis. Similar deposits were seen in tissue derived from patients who died with no neurological disease. These deposits were also present in the extracellular spaces of the endoneurium and beneath the outer cytoplasmic collar of the sheath, but did not penetrate the intraperiod line of the compact myelin or the periaxonal space and the lateral loops tended to remain in contact with the axon. The findings suggest that a pathway exists between the extracellular spaces outside the sheath to the innermost portion of the sheath but that the periaxonal space is resistant to the penetration of the extraneous material.