Prevention of Polyspermy in Human

In biology, polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the egg and one each from multiple sperm. Usually, the result is an inviable zygote. This may occur because sperm have become too efficient at reaching and fertilizing eggs due to the selective pressures of sperm competition. This is often deleterious to the female, in other words the male-male competition among sperm spills over to create ***ual conflict.

In mammals, in which fertilization occurs internally, fewer sperm reach the fertilization site in the oviduct. This may be the result of the female genital tract being adapted to minimize the number of sperm reaching the egg. Nevertheless, polyspermy preventing mechanisms are essential in mammals; a secretion reaction, the "cortical reaction" modifies the extracellular coat of the egg (the zona pellucida), and additional mechanisms that are not well understood modify the egg's plasma membrane.

Evolutionary advantage
Female defenses select for ever more aggressive male sperm however, leading to an evolutionary arms race. On the one hand, polyspermy creates inviable zygotes and lowers female fitness, but on the other, defenses may prevent fertilization altogether. This leads to a delicate compromise between the two, and has been suggested as one possible cause for the relatively high infertility rates seen in mammalian species.

Plasma membrane block to polyspermy in human oocytes and preimplantation embryos

The existence and time course of the human plasma membrane block to polyspermy were investigated by an in vitro fertilization assay using zona pellucida-free unfertilized oocytes, pronuclear oocytes and embryos. In the time course study using a high sperm concentration (105 spermatozoa ml–1), the number of penetrating spermatozoa at 30 and 60 min after insemination were 1.3 ± 0.3 and 2.9 ± 0.4, respectively. By 2 h after insemination, spermatozoa penetration reached a maximum. A lower maximum number of penetrating spermatozoa was observed at a low sperm concentration (104 spermatozoa ml–1), but the number of penetrating spermatozoa still reached a maximum by 2 h after insemination. A reinsemination experiment demonstrated that the number of penetrating spermatozoa was not significantly different between control and reinseminated oocytes, while sperm penetration was not observed in the oocyte beyond the two-cell stage. Furthermore, the number of binding spermatozoa decreased after fertilization and most of the four-cell stage embryos displayed no sperm binding. These results suggest that the plasma membrane block plays an important role in the prevention of polyspermy in the human oocyte, and that the plasma membrane block may involve permanent changes in the binding or fusion ability of spermatozoa in the oolemma after fertilization.