[Electron microscopy of giant nerve fibers]

The present paper is a brief review on the ultrastructural details of giant nerve fibers and their possible correlation to some physiological findings which have been reported in the squid giant axon. In both, thin sections and freeze-fracture replicas, the most striking feature is the exceeding amount of paired membranes populating the Schwann cell layer. These membranes represent permeable intercellular clefts connecting the axon surface to the endoneurial extracellular space, thus leaving the axolemma as the only continuous barrier between the axoplasm and the neuron exterior. Close apposition of the axon and Schwann cell at the level of structural complexes involving both cells plasma membranes are observed in sections and replicas. These zones could represent the morphologic expression of the functional coupling reported in the same preparation. Besides, the Schwann cell appears to be very active according to the amount of exo-endocytotic profiles seen in all its fracture faces. Finally, the endoneurial cells are different in the various giant fibers studied: in the squid they appear as spongy cells, whereas in the lobster they exhibit an extraordinary amount of exo-endocytosis mixed with some gap and a few incomplete tight junctions, and in the crayfish they present the same features as the adaxonal glia.