Figure 1.
Electron micrographs reveal multigranular exocytic structures. (a) Electron micrographs were generated from lactotrophs stimulated with 100 mM KCl for 2 min, washed in iced phosphate buffer, fixed, and processed for electron microscopy. There are numerous omega figures in various stages of dilation visible in the micrographs. Examples of compound exocytosis are plentiful. The panels in this figure demonstrate increasing complexity of exocytic events. (b) To determine whether granules in the electron micrographs were continuous with the extracellular space, ruthenium red was applied during fixation. This dye is fixed to the extracellular surface of the cell, and is electron dense so that any portion of the cellular membrane exposed to the extracellular surface during fixation contains an electron-dense coating visible in electron micrographs. Both of these micrographs show two cells directly opposed. The membranes delimiting the two cells are stained with ruthenium red (left, membranes are on the left side; right, membranes are at the top). Many granules that are located away from the surface membrane were stained with ruthenium red (arrowheads), indicating that they were contiguous with the extracellular surface at fixation. Presumably, the connection to the surface was via chains of connected granules that had fused together and were fused to the plasma membrane.