What is the cortical reaction?

The cortical reaction is a process that occurs during fertilization in the ovum of many animals. It is triggered when a sperm cell fuses with an egg cell, and it serves to prevent other sperm cells from also fertilizing the egg.

Here are the steps of the cortical reaction:

1. Sperm Binding: The process begins when a sperm cell binds to the egg cell. This binding triggers the next step.

2. Calcium Release: The binding of the sperm causes the release of calcium ions in the egg. The calcium ions spread like a wave across the egg.

3. Cortical Granule Exocytosis: The increase in calcium causes tiny sacs, or cortical granules, in the egg's cortex to release their contents outside the egg.

4. Fertilization Envelope Formation: The contents of the cortical granules cause the zona pellucida, a layer around the egg, to harden and form a fertilization envelope. This prevents other sperm cells from entering the egg.

5. Completion of Meiosis II: The egg then completes its second meiotic division, forming a mature ovum and a second polar body.

6. Sperm and Egg Nuclei Fusion: The nuclei of the sperm and egg then fuse to form a zygote, the first cell of a new organism.

So, the cortical reaction is a crucial part of the fertilization process, ensuring that only one sperm fertilizes the egg and that the resulting organism has the correct amount of genetic material.