LOW RESISTANCE JUNCTIONS IN CRAYFISH II. Structural Details and Further Evidence for Intercellular Channels by Freeze-Fracture and Negative Staining

Low resistance junctions between axons of crayfish ganglia are studied by freeze-fracture and negative staining . In freeze-fracture, fracture planes that go through a junctional membrane expose two faces, both internal, called face A and face B . Face A belongs to the internal membrane leaflet and faces the gap . Face B belongs to the external membrane leaflet and faces the axoplasm. Face A displays pits, 60-100 A in diameter, arranged in a hexagonal array with a unit cell of -200 A . An -25 A bump is frequently seen at the center of each pit. Some pits are occupied by a globule X125 A in diameter, which displays a central depression -25 A in size . Face B contains globules also arranged in a fairly regular hexagonal pattern . The center-to-center distance between adjacent globules is most frequently -200 A; however, occasionally certain globules are seen separated by a distance as short as X125 A. The top surface of the globules occasionally displays a starlike profile and seems to contain a central depression -25A in diameter . In negatively stained preparations of membranes from the nerve cord, two types of membranes are seen containing a fairly regular pattern. In one, globules -95 A in diameter form a hexagonal close packing with a unit cell of -95 A . In the other, globules of the same size are organized in a larger hexagonal array with a unit cell of X155 A (swollen arrangement) . Some of the globules forming the swollen arrangement are seen containing six subunits . The six subunits form a hexagon which is skewed with respect to the main rows of hexagons in such a way that the subunits lie on rows which make an angle of X37° with the main rows .