Polyspermy

Polyspermy

Diagram illustrating physiological polyspermy.

Polyspermy is a phenomenon that occurs during the fertilization process when an egg is fertilized by more than one sperm. This can lead to abnormal development and is generally prevented by various mechanisms in sexually reproducing organisms.

Mechanisms of Polyspermy Prevention

In most species, polyspermy is prevented by a series of mechanisms that ensure only one sperm fertilizes the egg. These mechanisms can be broadly categorized into two types: fast block and slow block.

Fast Block to Polyspermy

The fast block to polyspermy occurs immediately after the first sperm fuses with the egg. This involves a rapid depolarization of the egg's plasma membrane, which prevents additional sperm from fusing with the egg.

Slow Block to Polyspermy

The slow block to polyspermy involves the cortical reaction, where cortical granules release their contents into the space between the plasma membrane and the zona pellucida. This leads to changes in the zona pellucida that prevent further sperm from penetrating the egg.

Types of Polyspermy

Polyspermy can be classified into two main types: pathological polyspermy and physiological polyspermy.

Pathological Polyspermy

Pathological polyspermy occurs when multiple sperm fertilize an egg, leading to an abnormal zygote with too many chromosomes. This often results in developmental failure and is typically lethal.

Physiological Polyspermy

Physiological polyspermy is a controlled process observed in some species, such as birds and reptiles, where multiple sperm enter the egg, but only one sperm's genetic material fuses with the egg's nucleus. The additional sperm contribute to the activation of the egg but do not contribute genetically.

Consequences of Polyspermy

The primary consequence of polyspermy is the formation of a polyploid zygote, which usually cannot develop properly. In most cases, the resulting embryo is non-viable due to the imbalance in genetic material.

Related Pages

• Fertilization
• Cortical reaction
• Zona pellucida
• Zygote