Polyspermy

Polyspermy is the fertilization of an oocyte by more than one sperm, resulting in a zygote with extra paternal genomes and often abnormal chromosome numbers. In many animals, polyspermy is normally prevented by rapid cellular responses at the moment of fertilization and, if it occurs, the zygote typically fails to develop properly.

The prevention of polyspermy involves two main blocks. The fast block to polyspermy is a rapid change in the egg’s plasma membrane potential after the first sperm fuses, which reduces the likelihood that additional sperm can fuse with the egg membrane. The slow block to polyspermy involves cortical granule exocytosis, which releases enzymes that modify the surrounding extracellular matrix (such as the vitelline envelope or zona pellucida) and create a fertilization envelope that alters or removes binding sites for sperm, preventing further entry.

Occurrence and consequences vary by organism. In externally fertilizing species such as sea urchins, polyspermy can occur under high sperm concentrations, but the fast and slow blocks typically prevent it; if these blocks fail, the resulting zygote often cannot develop. In mammals, blocks to polyspermy are highly efficient, and polyspermy is rare. When it does occur in humans or other animals, the zygote is usually triploid or otherwise abnormal and is not viable, commonly leading to early pregnancy loss.

Polyspermy has been a key area of study in developmental biology, helping to elucidate the cellular and molecular mechanisms by which eggs enforce monospermy and ensure proper embryonic development.