Figure 4.

Schematic diagram of sequential steps in phospholipid membrane fusion. As a first step the outer monolayer of a phospholipid vesicle coalesces with the contacting monolayer of the planar phospholipid bilayer membrane forming a trilaminar structure (hemifusion). A small fusion pore is formed in this area. Once formed, the fusion pore can close again or expand to some critical size when further expansion becomes irreversible.