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.
Sperm is about 2,000 times smaller than the egg it fertilizes
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
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.
Sperm Meets Egg
Now, this isn't an easy feat! It takes dozens of sperm to break through the corona radiata, but only one can fertilize the oocyte. Once sperm get through, they bind to sperm receptors on the zona pellucida, the jellylike membrane underneath the corona radiata. Here's the tricky part: only sperm with an intact acrosome can bind to the sperm receptors. That means that all those other sperm who did all that work breaking down the corona don't even get a chance to fertilize the oocyte!
Once a sperm is bound to the sperm receptor, its acrosome cap bursts open and releases its enzymes. The enzymes digest a pathway through the zona pellucida towards the surface membrane of the oocyte. The sperm follows this pathway and the first sperm to come in contact with the oocyte membrane is the winner - it's the one who gets to fertilize the oocyte.
Sperm breaking through the corona radiata
But how do we make sure there is only one winner? I mean, only one sperm can fertilize the oocyte, so it has to have a way to prevent other sperm from getting through. The answer to that can be found out in the series of events called oocyte activation. Oocyte activation follows the binding of the sperm to the oocyte membrane, but that is a whole other lesson on its own.
And there you have it! The first steps leading up to fertilization - the fated meeting of egg and sperm. Next stop: fertilization and oocyte activation.
Fertilization is the fusion of two haploid gametes to produce a diploid zygote. Each haploid gamete in humans has 23 chromosomes, while the diploid zygote has 46. This fusion usually occurs within 24 hours after the oocyte has been released from the ovary. But before that can happen, our egg and sperm have to meet, and it's not an easy process.
First, the sperm have to survive the long trek up the female's reproductive tract. Then, they have to break through the oocyte's defenses. The corona radiata and the zona pellucida surround the egg and protect it from the external environment.
Once the sperm break through these layers, only one lucky sperm gets to fertilize the egg. The first sperm to get through the zona pellucida binds to the oocyte membrane. This binding initiates the process of oocyte activation and prevents other sperm from entering the egg.
To learn about oocyte activation and what happens once the sperm breaks through the egg's defenses, be sure to watch the other fertilization lesson on our site.
Finishing this lesson should prepare you to:
- Define gamete and zygote
- Summarize the process of fertilization
- Identify the three layers of an oocyte and describe how the sperm can break through them