polyspermia

Polyspermia, commonly called polyspermy, is the fertilization of an oocyte by more than one sperm. When multiple sperm contribute chromosomes to a single egg, the resulting zygote typically becomes polyploid (triploid or higher), a state usually incompatible with normal development in mammals.

Prevention of polyspermia relies on fast and slow blocks to fertilization. The fast block is an almost immediate depolarization of the oocyte plasma membrane after the first sperm fuses, which reduces the chance that other sperm fuse. The slow block begins with cortical granule exocytosis, which alters the zona pellucida and prevents subsequent sperm penetration. Failures in these blocks can allow additional sperm to fertilize the egg.

Occurrence and outcomes vary by species. Polyspermia is rare in humans and most resulting triploid zygotes do not develop to term, usually ending in miscarriage. In many non-mammalian species, such as certain fish, amphibians, and birds, polyspermy can occur more readily, and development may follow species-specific patterns.

Clinical and reproductive implications: In assisted reproduction, polyspermia is monitored because it risks abnormal chromosomal complements. Techniques such as intracytoplasmic sperm injection (ICSI) are used to minimize the risk by delivering a single sperm directly into the oocyte. Diagnosis of polyspermia or triploidy involves embryocellular assessment, karyotyping, or chromosomal analysis of fetal tissue when pregnancy is established.

See also: Triploidy, Zygote, Cortical granule, Fertilization.