Fertilization is when a sperm cell and egg cell unite to form a single cell called a zygote.
This article is pending medical review.
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Written by Lea Dörner
Reviewed by Carolin Becker, Marjan Naghdi, Sophie Oppelt
Edited by Juliëtte Gossens
Fertilization is when a sperm cell and egg cell unite to form a single cell called a zygote. This cell contains genetic information from both parents. It's the first step in the development of an embryo. This article will provide a detailed explanation of the mechanisms involved in fertilization.
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(3) Fusion
What Is Fertilization?
We need two cells for the process of fertilization. We need a sperm cell from the male reproductive system, specifically from the testicles. We also need an egg (called an ovum) from the female reproductive system, specifically from the ovaries. These two cells, the sperm cell and the ovum, merge and unite to form a single cell called a zygote. This zygote is the first cell that could grow into an embryo (which becomes a baby). The zygote contains the genetic information from both sperm and egg. (1, 2)
In this article, we are going to explain the process and mechanisms of fertilization in more detail.
Gametes: What Makes Sperm and Eggs So Special?
Sperm cells are produced by the male reproductive system. Ova (the word for the plural of ovum, also known as egg cell) are produced by the female reproductive system. These cells are referred to as gametes. They are highly specialized for reproduction, which is needed to create a new organism (a new human!).
The Sperm Cell
The male reproductive system produces sperm cells in the testicles. These cells are very small and streamlined. They have a long tail known as a flagellum that propels them forward. This makes them specialized for traveling through the female reproductive tract to reach the egg cell. Sperm cells produce lots of energy so they can move forward. The genetic material, necessary for fertilization, is located only in the head of the sperm. The rest of the sperm is used for movement. (3, 4)
The Egg Cell
Egg cells, also known as ova, are the female reproductive cells and are designed for the development into a fetus. Egg cells are stored in the ovaries. Each month, one egg is released into one of the fallopian tubes during ovulation. If you want to know more about the monthly menstrual cycle, you can read about it here. If you want to learn more about the biology of egg cells, you can check out this article.
Unlike sperm cells, the ovum is a large cell. In fact, it’s the largest in the human body! This is very important, because the egg needs to store many of the nutrients and proteins that a growing embryo needs. Furthermore, the ovulated egg is surrounded by a protective layer called the zona pellucida. The zona pellucida prevents multiple sperm from fertilizing the egg. It makes sure that only one sperm penetrates the layer and fertilizes the ovum. (3, 4)
What Are Gametes?
Even though sperm and egg cells look different, they share one very important feature that is essential to their reproductive abilities. They are both gametes and contain only half the amount of DNA compared to other cells in the human body. Our DNA is a molecule that contains genes which carry our genetic information. Our body needs this information to know what to do. Because gametes only contain half the amount of DNA, they are referred to as haploid cells. Cells that contain the “complete amount” of DNA are referred to as diploid cells. Nearly all cells in your body that aren’t gametes, are diploid cells. (4, 5)
As the sperm and egg cell merge during fertilization, the zygote receives half of its DNA from the egg cell and half from the sperm cell. This way, it ends up with a “complete” set of DNA and it becomes a diploid cell. This feature of gametes and fertilization explains why children inherit characteristics from both of their parents. (4, 6)
The Stages of Fertilization
After learning about the unique characteristics of sperm and egg cells, let's discuss the actual process of fertilization, and what happens when the sperm and egg “meet”.
Fertilization can be divided into 3 stages: capacitation, acrosome reaction, and sperm-egg fusion, all of which we are going to explain in the following.
The Capacitation: Preparation of Sperm Cells
During unprotected sex between somebody with a vagina and somebody with a penis, millions of sperm cells are released into the vagina. From this moment on, the sperm embark on a journey through the reproductive tract of the person with the vagina. They swim through the cervix, towards the uterus, and then into the fallopian tubes. Once a month, during ovulation, an egg cell is released from the ovaries into one of the fallopian tubes (7).
The egg in the fallopian tube is the “target” of the sperm cells. They are guided to their destination by molecules that are secreted by the egg cell (8). This journey is necessary for fertilization. Only the fastest sperm cell can fertilize the egg. But right after ejaculation, sperm can’t fertilize an egg even if they reached it immediately. During their journey through the reproductive tract, the sperm cells are modified by the vaginal environment in many ways. This is known as capacitation. This process involves chemical changes caused by small molecules, proteins, and the acid level in the vagina. The end result is a sperm cell that can fertilize an egg.
In summary, capacitation can be described as the activation of sperm cells by the vaginal environment. Next, the sperm cell and egg cell meet. (1, 3)
The Acrosome Reaction: Interaction of Sperm and Egg
Only about 200 out of the 300 million sperm ejaculated during sex manage to reach their desired destination: the ovum (egg cell). The sperm that managed to reach the ovum now have to break through the protective layer called zona pellucida, which surrounds the ovum. The reaction by which the sperm break through this barrier is called the acrosome reaction. We’ll explain how it works.
When a sperm encounters the egg, it binds to its surface: the zona pellucida. The zona pellucida is a protective layer that surrounds the egg cell. It’s composed of special proteins. This layer is very important in the process of fertilization (9). For example, it serves as a barrier between different animal species. This means that a hamster's sperm can’t fertilize a human egg, for instance.
When a sperm cell reaches the zona pellucida, the head of the sperm releases a substance called the acrosome. This substance contains enzymes (special proteins) that help the sperm break through the zona pellucida to reach the egg. The acrosome can be thought of as a “key” for the “door” that is the zona pellucida.
Once a sperm has reacted with the zona pellucida, the layer “locks” for all other sperm that come after it. That’s why typically only the first sperm to reach an egg fertilizes the egg. The sperm cells that failed to fertilize the egg simply break down. Eventually, they are naturally expelled from the body. Now, the fastest sperm is ready to fuse with the egg! (1, 2, 4, 9, 10)
The Fusion of Sperm and Egg
Once a sperm has undergone the acrosome reaction and penetrated the zona pellucida, it can merge with the egg. Both cells are surrounded by a thin layer of proteins called a membrane. These membranes open up a little bit to let the contents of the sperm cell and the egg cell mix. During the fusion process, the entire contents of the sperm are sucked into the egg. The two cells merge to form a single cell, called the zygote. This zygote contains the combined DNA (genes) from the sperm and egg cell and is the first cell of the developing embryo. After fertilization, the zygote travels down the fallopian tube and starts to divide and multiply, to finally implant into the wall of the womb. That's where the embryo can grow bigger and bigger until it's born as a baby. (4, 11)
Timing of Fertilization
Timing is an important factor when it comes to fertilization. The fertile window, or the time frame when a person with a uterus can become pregnant, is limited to a few days during each menstrual cycle. This is because when an egg is released during ovulation, it has a short lifespan and is destined to die within hours to days if not fertilized.
On the other hand, sperm can survive inside the female reproductive system for up to five days. This means that if somebody ovulates within five days of having unprotected vaginal sex with a man or somebody with a penis, the sperm is able to fertilize the egg and cause a pregnancy (4, 12).
Remember that anybody with the right parts can get pregnant! If you have testicles and a penis, you can get somebody pregnant who has a vagina, uterus and ovaries, even if they’re taking gender-affirming hormones such as testosterone, for example.
In summary, sperm cells have to travel a long way and undergo many changes before they can fertilize an egg cell. The egg cell is protected from all these sperm cells by a special layer. Only the fastest sperm cell that can break through this layer can fuse with the egg cell. The result is one cell, the zygote, that contains all nutrients and genetic information that the sperm cell and egg cell carried. The wonder of life starts small!
From there, the zygote can grow into an embryo, then a fetus, and is finally born as a baby.
References
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