Polyspermiä

Polyspermiä refers to the fertilization of an egg by more than one sperm. This phenomenon occurs in many species, but is generally considered abnormal and detrimental to development in most vertebrates, particularly mammals. In most cases where polyspermiä occurs, the resulting zygote will not be viable or will develop with severe abnormalities.

In some organisms, such as sea urchins and some amphibians, mechanisms exist to prevent polyspermiä. The fast block to polyspermy involves a rapid change in the electrical potential of the egg membrane upon fusion with the first sperm, which repels subsequent sperm. The slow block to polyspermy, or the cortical reaction, involves the release of enzymes from cortical granules in the egg's cytoplasm. These enzymes modify the egg's outer layers, making it impenetrable to further sperm penetration.

In mammals, the prevention of polyspermiä is crucial for proper embryonic development. The zona pellucida, a glycoprotein layer surrounding the egg, plays a significant role. Upon fusion with the first sperm, enzymes are released from the sperm's acrosome that modify the zona pellucida, preventing other sperm from binding and penetrating. Despite these mechanisms, polyspermy can still occur due to various factors, including high sperm concentration or abnormalities in the egg or sperm. Polyspermic human embryos are typically not viable.