ABSTRACT
Exo- and endocytosis are frequently coupled events, when a cell releases its secretory contents by reversible fusion of a secretory granule membrane with the cell membrane. Since the freeze-fracture and freeze-etching techniques expose preferably membranes, these techniques have frequently been used to study exo-endocytosis. The situation has become more complicated since focally disordered lipids or lipids in a micellar arrangement were also shown by freeze-fracturing to form intramembranous particles-like structures. In most cases exocytosis is coupled with endocytosis, which allows the cell to maintain the area size and the compositional specificity of the cell membrane. Although a real time sequence of exo-endocytosis stages cannot be obtained with a nonsynchronous system, one can subdivide all stages actually observed into subgroups according to size. These stages can then be pooled into different categories of size range.
