Polyspermian

Polyspermian is a term used in biology to describe the process where more than one sperm fertilizes an egg. This is a condition that occurs in some species, although in most animals, it is prevented by mechanisms that block the entry of additional sperm after the first one has successfully penetrated the egg. The term specifically refers to the event of fertilization itself, rather than any resulting developmental abnormalities, although polyspermic fertilization often leads to inviable embryos in many species.

In the animal kingdom, polyspermy is generally considered abnormal. Following the fusion of the first sperm with the egg's plasma membrane, a rapid electrical or cortical reaction is triggered. This reaction alters the egg's outer layers, making it impermeable to subsequent sperm. This ensures that the resulting zygote has the correct chromosome number, a process known as monospermy. If these blocks fail and multiple sperm fuse with the egg, the resulting embryo will have an abnormal number of chromosomes, which is usually lethal.

However, there are exceptions. In some invertebrates, such as certain species of sea urchins, polyspermy can occur at a higher frequency without necessarily leading to developmental failure in all cases. The study of polyspermian in these organisms has provided valuable insights into the mechanisms of fertilization control and the importance of maintaining proper ploidy for embryonic development. The term itself is derived from Greek roots, with 'poly' meaning many and 'sperma' meaning seed or sperm.