Membrane junctions in the myelin sheath of goldfish lateral nerve.

The myelin sheath of the goldfish lateral line nerve has been studied by means of the freeze-etch technique. Fracture faces of the myelin lamellae reveal linear structural elements which in some respects resemble those of tight junctions. In unfixed myelin rows of particles on the ef face are matched by narrow grooves on the pf face. Fixation with glutaraldehyde produces a partitioning of junctional particles between the 2 fracture faces. These elements are observed most frequently in the outer layers of the sheath where their arrangement is predominantly more or less parallel and longitudinal, merging in places into a more reticulate organization. They also occur at other levels in the myelin and in association with Schmidt-Lantermann clefts. Orientation in these cases appears to be predominantly longitudinal, though circumferentially running elements have been observed. The possible functional significance of these structures and their implications for tissue development, maintenance and growth are discussed.