Structural changes underlying exocytosis evoked by the application of endotoxin to Limulus amebocytes were studied at the level of detail afforded by freeze-fracture and freeze-substitution techniques combined with the time resolution of direct rapid-freezing. The results with amebocytes prepared in this manner differed from those with other secretory cells prepared by conventional means. Exocytosis begins within seconds of endotoxin treatment when the plasmalemma invaginates to form pedestallike appositions with peripheral secretory granules. The juxtaposed membranes at these pedestal appositions form several punctate pentalaminar contacts, but examination of freeze-fractured pedestals failed to reveal any corresponding changes in the intramembrane particle distribution. Small secretory granule openings or pores, which are very infrequent, appear within the first 5 s after endotoxin treatment. These pores rapidly widen and this widening is immediately followed by the sequential dissolution of the granule contents, which then move into the surrounding extracellular space. Cytoplasmic filaments connecting the plasmalemma with the granule membrane are suitably deployed to be responsible for the plasmalemma invaginations. How pores begin is not certain, but the appearance of clear spaces between the granule core and the granule membrane at this point in exocytosis supports the possibility of a role of osmotic forces.

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