Question

What are the two main safeguards protecting eggs against Polyspermy?

Answer 1

Sexually reproducing organisms' eggs are designed to avoid this situation. The defences are particularly well characterized in the sea urchin, which response to the acceptance of one sperm by preventing subsequent sperm from successfully penetrating the egg. Other eukaryotes have similar defences. Polyspermy prevention in sea urchins is dependent on a change in the electrical charge across the surface of the egg caused by the fusion of the first sperm with the egg.

Complete answer:
Polyspermy is a biological term that refers to an egg that has been fertilized by more than one sperm. Diploid organisms have two copies of every chromosome, one from each parent. The polyspermy cell, on the other hand, has three or more copies of each chromosome—one from the egg and one from each of the sperm. The result is usually an unviable zygote. Because of the selective pressures of sperm competition, sperm may be too efficient at reaching and fertilizing eggs. Such a situation is frequently harmful to the female: in other words, male-male sperm competition spills over into sexual conflict.

The two primary safeguards that protect the eggs from polyspermy are –
- Polyspermy is prevented at the plasma membrane level, where the union of the first sperm causes membrane depolarization and the release of the egg's sperm receptors – Folr4 – thereby preventing the union of other sperms.
- Fertilization causes cortical granule exocytosis – a cortical reaction. These granules contain enzymes that change the zona pellucida and thus stiffen it. The cleavage of the zona pellucida protein ZP2 reduces the sperm's binding affinity, rendering the zona impermeable to other sperms.

Note: Polyspermy is extremely uncommon in human reproduction. One of two methods for preventing polyspermy in humans is to reduce the number of sperm that swim to the oviduct. The blocking of sperm in the fertilized egg is the other mechanism.

According to Developmental Biology Interactive, when an egg is fertilized by multiple sperm, the embryo develops a variety of paternal centrioles. When this occurs, there is a competition for extra chromosomes. This competition disrupts cleavage furrow formation, and the zygote usually dies as a result. Only two cases of human polyspermy resulting in childbirth have been reported.