Polispermia

Polispermia is a condition in which more than one sperm fertilizes an egg. This typically occurs when multiple sperm penetrate the zona pellucida, the outer layer of the egg, and enter the ooplasm. In most species, including humans, polispermia is a lethal event. The extra sperm nuclei are unable to properly integrate with the egg's chromosomes, leading to a disruption of cell division and embryonic development. This usually results in the death of the embryo shortly after fertilization or implantation fails.

The mechanisms that prevent polispermia in normal fertilization are crucial for ensuring the viability of a zygote. One primary mechanism is the cortical reaction. After the first sperm fuses with the egg membrane, the egg releases cortical granules. These granules modify the zona pellucida, making it impermeable to other sperm. This process is known as the zona reaction. If the cortical reaction fails or is incomplete, polispermia can occur.

While generally detrimental, research has explored the implications of polispermia in specific contexts, such as assisted reproductive technologies. However, in natural conception, the occurrence of polispermia is extremely rare due to these protective biological mechanisms. The consequences of polispermia are profound, highlighting the importance of precise gamete fusion for successful reproduction.