Fertilization is the process by which a sperm cell from a male combines with an egg cell from a female, resulting in a single-celled zygote, the first stage of embryonic development.
Fertilization can be divided into several phases:
1. Sperm migration and capacitation:
After ejaculation, sperm cells enter the female reproductive tract and swim towards the fallopian tube where the egg is located.
While migrating, sperm undergo capacitation, a process that removes certain glycoproteins from the sperm's surface, enhancing its ability to bind to the egg's zona pellucida.
2. Acrosome reaction:
When the sperm reaches the egg, it encounters the zona pellucida, a glycoprotein matrix that surrounds the egg.
The acrosome, a specialized vesicle at the tip of the sperm head, releases enzymes that break down the zona pellucida, allowing the sperm to penetrate.
3. Sperm-egg binding and fusion:
The sperm binds to specific receptors on the egg's plasma membrane, facilitating membrane fusion between the sperm and the egg.
The sperm's nucleus enters the cytoplasm of the egg, leaving behind the sperm's tail and mitochondria.
4. Cortical reaction:
After the sperm enters the egg, cortical granules within the egg's cytoplasm release their contents into the space between the plasma membrane and the zona pellucida.
This causes the zona pellucida to harden, preventing polyspermy (entry of multiple sperm).
5. Pronuclear fusion and zygote formation:
The male and female pronuclei, which contain the respective genetic material, approach each other and eventually fuse to form the zygote.
This marks the completion of fertilization and the beginning of embryonic development.
6. Activation of the egg's metabolic processes:
Following fertilization, the egg's metabolic processes are activated, preparing the zygote for subsequent cell division and embryonic development.
7. Implantation
Several days after fertilization, the now multicellular organism, called a blastocyst, reaches the uterus and implants itself into the uterine wall, where it continues to grow and develop into an embryo.