Figure 8.

Behavior of granules before exocytosis. A standard model for exocytosis is that granules translocate from the cell interior to the plasma membrane and stably bind or dock to the plasma membrane. After subsequent priming steps, the granule is able to fuse with the plasma membrane in respond to elevated Ca²⁺. The present study indicates that granules that undergo exocytosis can have numerous behaviors immediately preceding fusion. Some granules do not undergo detectable motion and may be bound to the plasma membrane. Others are jittering in place within 100 ms of fusion. A few granules are not present in the evanescent field before fusion and may move through it within 100 ms of fusion. Granule and plasma membrane priming events can occur before the final docking step that leads to fusion.