Excitable membrane ultrastructure. I. Freeze fracture of crayfish axons.

Cross-sectioned and cross-fractured crayfish axons display regions in which axon and Schwann cell surface membranes are regularly curved and project into the axoplasm. At these regions (projections) the two membranes run precisely parallel, separated by a gap of 130-140 A. Longitudinal fractures through the axons expose the inner fractured surface of either the internal (face A) or the external (face B) leaflet of axon and adjacent Schwann cell surface membranes. On both membranes the projections appear as elongated structures oriented with the long axis parallel to the long axis of the nerve fiber. On face A of the axon surface membrane they are seen as elongated indentations 0.5-1.2-microm long, 0.12-0.15-microm wide. The indentations contain parallel chains of globules. The chains repeat every 120-125 A and are oriented obliquely in such a way that if one looks at the axon surface from the extracellular space, the axis of the chains is skewed counterclockwise to the long axis of the indentations by an acute angle (most often 55-60 degrees ). The globules repeat along the chain every 80-85 A. Globules of adjacent chains are in register in such a way that the axis on which globules of neighboring chains are aligned forms an angle of 75-85 degrees with the axis of the chains. The complex structure can be defined as a globular array with a rhomboidal unit cell of 80-85 x 120-125 A. On face B of the axon surface membrane the complementary image of these structures is seen. The projections of the Schwann cell surface membrane also contain groupings of globules; however, these differ from those in the axonal projections in size, pattern of aggregation, and fracture properties. Several possible interpretations of the meaning of these membrane specializations could be proposed. They could be: (a) structures involved in the mechanism of excitation, (b) regions of presumed metabolic couplings, and (c) areas of cell-to-cell adhesion.