Figure 3.

Fertilization requires two independent fusions. Intact spermatozoa have a plasma membrane (blue) and an acrosomal membrane (orange). After the acrosome reaction, these two membranes fuse and form a new sperm membrane (pink). The first fusion takes place between the pink membrane and egg plasma membrane (black). After the first fusion, egg and sperm membrane form a new consecutive membrane (green). If fusion is accomplished in this step, Izumo1 on the acrosomal cap of the inner acrosomal membrane (indicated by red) should spread on the newly-formed egg surface (green). However, the second fusion (invagination) follows the first fusion that separates the acrosomal cap and acrosomal sheath areas (light blue) from the fused membrane (green). Thus, Izumo1 on the inner acrosomal membrane is invaginated into the cytoplasm of the eggs. From live imaging, Izumo1 seems to be required for the first fusion. The nature of the second fusion remains totally unknown.