Monisikiöisyys

Monisikiöisyys, also known as polyspermy, is a biological phenomenon where more than one sperm fertilizes an egg. In most sexually reproducing organisms, including humans, a natural block exists to prevent this from happening. This block ensures that the resulting zygote has the correct number of chromosomes. If polyspermy occurs, the resulting embryo will have an abnormal number of chromosomes, which can lead to developmental issues or be non-viable.

The process typically involves two main blocks against polyspermy. The first is the fast block, which is a rapid electrical depolarization of the egg's plasma membrane immediately after the first sperm fuses with it. This change in electrical potential prevents other sperm from binding to the egg. The second, and more permanent, block is the slow block, also known as the cortical reaction. This involves the release of enzymes from cortical granules located just beneath the egg's plasma membrane. These enzymes modify the egg's outer layers, preventing further sperm penetration.

Despite these protective mechanisms, polyspermy can still occur, albeit rarely, and is often associated with assisted reproductive technologies like in vitro fertilization (IVF). In IVF, techniques used to facilitate fertilization might inadvertently increase the risk of multiple sperm entering the egg. When polyspermy is detected in human embryos, it usually leads to the discarding of the embryo due to the high probability of developmental failure or genetic abnormalities.