During fertilisation, only one sperm can penetrate the oocyte. The entry of additional sperms is blocked by

During fertilisation, a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Thus, it ensures that only one sperm can fertilise an ovum.

Within a few seconds after the fusion of a sperm cell with a secondary oocyte, the cell membrane of the oocyte depolarises, which acts as a fast block to polyspermy. This depolarisation triggers the intracellular release of calcium ions, which stimulate exocytosis of secretory vesicles from the oocyte. The molecules released by exocytosis inactivate ZP3 (sperm receptor) and harden the entire zona pellucida (slow block to polyspermy).

Capacitation causes the sperm’s tail to beat more vigorously and prepare its plasma membrane to fuse with the oocyte’s plasma membrane.