ORGANIZATION OF THE CORTICAL ENDOPLASMIC-RETICULUM IN THE SQUID GIANT-AXON

The organization of the cortical endoplasmic reticulum in the squid gi ant axon was investigated by rapid freeze and freeze-substitution elec tron microscopy, thereby eliminating the effects of fixatives on this potentially labile structure. Juvenile squid, which have thinner Schwa nn sheaths, were used in order to achieve freezing deep enough to incl ude the entire axonal cortex. The smooth endoplasmic reticulum is comp osed of subaxolemmal and deeper cisternae, tubules, tethers and vesicl es. The subaxolemmal cisternae make junctional contacts with the axole mma which are characterized by filamentous-granular bridging structure s approximately 3 nm in diameter. The subaxolemmal junctions with the axolemma resemble the coupling junctions between the sarcoplasmic reti culum and the T-tubules in muscle. Reconstruction of short series of s ections showed that a number of the elements of the endoplasmic reticu lum were continuous but numerous separate vesicles were present as wel l. The morphology of endoplasmic reticulum as described here suggests that it is a highly dynamic entity as well as a Ca2+ sequestering orga nelle.