Fertilization: Oocyte Activation & Cell Cleavage

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  • 0:07 Oocyte Activation
  • 2:29 Cell Cleavage
  • 3:36 Lesson Summary
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Lesson Transcript
Instructor: Heather Adewale

Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.

When is the exact moment of fertilization? Learn the answer in this lesson. We'll look at what happens after the sperm penetrates the outer layers of the oocyte during the process of fertilization.

Oocyte Activation

As you may already know, fertilization is the fusion of the male and female gametes: the sperm and the egg. But what happens to the egg, also called an oocyte, once the sperm penetrates its outer defenses? It's all a race, a race to be the first sperm of hundreds to get through and bind to the oocyte's membrane. But only one can win!

Once the first sperm gets to the oocyte's membrane, the sperm and oocyte membranes fuse. This fusion triggers oocyte activation. Now, oocyte activation involves a series of chemical changes in the oocyte, including a polyspermy block and the completion of meiosis II.

Oocyte activation is triggered when the sperm and oocyte membranes fuse.
Oocyte Activation

The polyspermy block is what prevents fertilization by more than one sperm by releasing enzymes that inactivate the sperm receptors and harden the zona pellucida. That way, sperm can no longer bind to the sperm receptors, and any sperm already bound are prevented from getting through the zona pellucida. Additionally, oocyte activation includes the completion of meiosis II in the oocyte. You may remember from other lessons that the oocyte leaves the ovary paused at metaphase of meiosis II.

Once the sperm fuses with the oocyte's membrane, it triggers the completion of meiosis II. Once meiosis II is completed, the oocyte's nuclear material reorganizes to form a structure called a pronucleus. The oocyte is now called an ovum and is ready to combine its DNA with that of the sperm.

Now, while all this is going on and the female pronucleus is forming, the sperm's DNA is also reorganizing into the male pronucleus. Both pronuclei move toward the center of the ovum, where they find each other and fuse together. This fusion allows their DNA to combine, and fertilization is complete! The ovum is now called a zygote and will begin the process of cell cleavage.

When the male and female pronuclei fuse, a zygote is formed and fertilization is complete.

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