Comparative analysis of junctions in the myelin sheath of central and peripheral axons of fish, amphibians and mammals: a freeze-fracture study using complementary replicas.

Tight junction-like structures are found in freeze-fractured myelin of the peripheral and central nervous system of the electric eel and trout, but not in the electric ray, Torpedo californicus. Myelin junctions are more difficult to find in the peripheral myelin of Xenopus frogs and of mice and rats, except in the myelinating fibres of young animals. Junctions are most frequently seen at the outer mesaxons, but are also observed at the inner mesaxons, paranodal loops, incisures of Schmidt-Lanterman and in compact myelin. The junctional strands vary considerably in appearance; instead of displaying the extensive strands and grooves of typical tight junctions, they are usually particulate with single or fused particles, short strands, and corresponding pits and grooves. After fixation, elements of the junctions partition between P- and E-faces with the majority of particles remaining with the P-face. In unfixed fish myelin, strands often remain with the E-face but fixation does not greatly alter the appearance of rat myelin. Complementary replicas show that junctions are often discontinuous. The significance of the distribution and morphology of junctions among the different specimens examined is discussed with regard to the possible functions and maintenance of myelin junctions.