Fertilization

• 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.

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