Fertilization and Zygote Formation: Definition and Processes

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  • 0:05 What Is Fertilization?
  • 1:24 Key Players: Sperm vs. Egg
  • 3:55 Sperm Meets Egg
  • 5:21 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.

Follow the male's sperm and the female's egg on their way to meet each other in this lesson that takes you through the process of fertilization in humans. Learn about the obstacles the sperm must get through before it can fertilize the egg.

What Is Fertilization?

Life... while you may have seen the popular Discovery series on the topic, or you may have pondered the meaning of life, have you ever thought about where it begins or about the events that lead up to the birth of a baby? What exactly is the miracle of life?

Well, it all starts with two small cells called gametes: a mature sperm (from the male) and a mature egg, or oocyte (from the female). In humans, each of these gametes has 23 chromosomes - that's half the amount of DNA required for development, so these cells are called haploid cells.

The fusion of these two cells is called fertilization and it produces a diploid cell with 46 chromosomes - twice the amount found in each gamete. The fertilized egg is now called a zygote and has just the right amount of DNA needed for normal human development.

Sounds simple, right? Well, like many things in life, it's not quite as simple as it looks.

While the process of fertilization isn't completely understood by scientists, we do know a fair bit about what happens. In this lesson, we will cover some of the basics of that fated meeting between egg and sperm.

Key Players: Sperm vs. Egg

Sperm is about 2,000 times smaller than the egg it fertilizes
Egg Size vs Sperm

First up: our two key players, the egg and the sperm. The first thing you may notice here is a drastic difference in size. Did you know that the egg is, on average, almost 2,000 times larger in volume than the sperm? That's because the egg has to provide all the cellular organelles and nourishment to support the developing embryo until it reaches the uterus. The sperm, on the other hand, simply delivers his DNA and then his job is done!

This delivery, or fertilization, occurs within the female's uterine tubes, usually within 24 hours after ovulation. By that time, the oocyte has traveled a few centimeters down the uterine tube towards the uterus, while the sperm have made the long trek from the vagina, through the uterus and into the uterine tube.

Did you know that sperm move about 12.5 centimeters an hour? That may not seem like a lot to you, but to a cell that's so tiny it's not visible to the naked eye, that's a lot of ground to cover! And it's not an easy trip. Of the almost 200 million sperm that are released by the average male, only a few thousand actually reach the uterine tube, and from that only a few hundred actually reach the egg. Talk about survival of the fittest!

This trip through the female's reproductive tract can take as long as a few hours or as little as 30 minutes, depending on the environment within the female's uterus. That's a lot of swimming for those tiny sperm! And, after all that, they still have a lot of work to do if they want to fertilize the egg.

The three layers of the oocyte
Oocyte Layers

You see, when the oocyte leaves the ovary it is surrounded by a couple of different layers. On the outside is a layer of cells called the corona radiata. Underneath that is the zona pellucid, and underneath that is the oocyte membrane. The sperm have to get through each one of those layers before they can reach the oocyte. That's a lot of work!

The corona radiata protects the oocyte as it leaves the ovary and travels down the uterine tube. The first job of the sperm is to first penetrate this barrier.

On top of the sperm's head is a cap called the acrosome; inside this cap are enzymes the sperm use to penetrate the corona radiata and the zona pellucida.

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