Fertilization is the process by which a sperm cell from a male combine 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 swim towards the egg in the fallopian tube.
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.
It prevents 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 fertilizing.
6. Egg Activation:
The egg’s metabolic processes activate, preparing the zygote for cell division and embryonic development.
7. Implantation:
A few days after fertilizing, the multicellular blastocyst implants into the uterine wall, beginning the next stage of development.