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.
During migration, sperm undergo capacitation, which removes glycoproteins from their surface, enhancing their ability to bind to the egg’s outer layer (zona pellucida).
2. Acrosome reaction:
Upon reaching the zona pellucida, the sperm’s acrosome (a vesicle at the tip of the sperm head) releases enzymes that digest this outer layer, allowing the sperm to penetrate the egg.
3. Sperm-egg binding and fusion:
The sperm binds to receptors on the egg’s plasma membrane, leading to fusion. The sperm’s nucleus enters the egg’s cytoplasm, leaving behind its tail and mitochondria.
4. Cortical reaction:
Once the sperm enters the egg, cortical granules release enzymes that harden the zona pellucida, preventing additional sperm from entering (polyspermy).
5. Pronuclear fusion and zygote formation:
The male and female pronuclei fuse, combining genetic material to form a zygote, marking the completion of fertilization.
6. Activation of the egg's metabolic processes:
The egg’s metabolic processes activate, preparing the zygote for cell division and embryonic development.
7. Implantation
A few days after fertilization, the multicellular blastocyst implants into the uterine wall, beginning the next stage of development.