Cortical reaction

A process in fertilization that prevents polyspermy

Cortical Reaction

The cortical reaction is a crucial event in the process of fertilization that occurs after a sperm cell successfully penetrates an egg cell. This reaction is essential for preventing polyspermy, which is the fertilization of an egg by more than one sperm.

Diagram of a human egg cell showing cortical granules

Mechanism

Upon the entry of a sperm into the egg, a series of biochemical changes are triggered within the egg's cytoplasm. One of the first responses is the release of calcium ions from the egg's internal stores. This increase in calcium concentration initiates the cortical reaction.

The cortical reaction involves the exocytosis of cortical granules, which are specialized secretory vesicles located just beneath the plasma membrane of the egg. These granules release their contents into the space between the plasma membrane and the zona pellucida, a glycoprotein layer surrounding the egg.

Function

The substances released from the cortical granules modify the structure of the zona pellucida, making it impermeable to additional sperm. This modification is known as the zona reaction. The enzymes released by the cortical granules cleave sperm receptor proteins on the zona pellucida, preventing further sperm binding and penetration.

Significance

The cortical reaction is vital for ensuring that only one sperm fertilizes the egg, thus maintaining the correct ploidy of the resulting zygote. Failure of the cortical reaction can lead to polyspermy, which usually results in an inviable embryo due to abnormal chromosome numbers.

Related Pages

• Fertilization
• Sperm
• Egg cell
• Zona pellucida
• Calcium signaling