The ultrastructure of low resistance junctions between segments of lateral giant fibers in crayfish is studied in sections from specimens fixed either by conventional methods or by glutaraldehyde-H2O2 or by glutaraldehyde-lanthanum. Cross sections through junctions fixed by conventional glutaraldehyde display the usual trilaminar profile of two parallel membranes separated by a narrow gap. Most of the junctional regions appear covered by 500–800 Å vesicles which lie on both sides of the junction in rows adjacent to the membranes. Gross sections through junctions fixed by glutaraldehyde-H2O2 display, in regions containing vesicles, membranes with a beaded profile. The beads correspond to globules ∼125 Å in width and ∼170 Å in height arranged in a hexagonal pattern with a unit cell of ∼200 Å. The globules of one membrane match precisely with those of the adjacent membrane, and opposite globules seem to come in contact with each other at the center of the junction. The membrane of the vesicles also contains globules. Occasionally the globules of the vesicles seem to join with those of the junctional membranes, apparently forming intracellular junctions. Injunctions negatively stained by lanthanum the globules are seen organized into two arrangements. Areas containing globules in a hexagonal array with a unit cell of ∼200 Å (swollen pattern) are seen adjacent to areas in which the globules are more closely and disorderly packed (close packing), the minimum center-to-center distance between adjacent globules being ∼125 Å. At higher magnification each globule appears composed of six subunits arranged in a circle around a central region occupied by lanthanum (possibly a pit).

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