By Zaira Salvador BSc, MSc (embryologist).
Last Update: 11/29/2018

Human fertilization is defined as the union between egg and sperm cells to cause a pregnancy. In humans, fertilization is an internal process, which is to say, it takes place inside the body of females, particularly in the Fallopian tubes, hence the more specific term natural or ‘in vivo’ fertilization.

The egg-sperm binding can be performed in the lab, too. This is possible thanks to the use of Assisted Reproductive Technology (ART), particularly the In Vitro Fertilization (IVF) technique.

Definition of fertilization

Fertilization is defined as the fusion between the male and female gametes, that is, sperm and egg, thereby reestablishing the normal number of chromosomes in humans (46 chromosomes).

For human fertilization to be possible, it is necessary that a man ejaculates inside the vagina of a woman. From that moment on, spermatozoa will start their journey inside the female reproductive tract until they reach the Fallopian tubes, where the egg cell is located.

Out of the millions of sperm released during ejaculation, just about two hundred are able to hit the egg cell in the Fallopian tube. In the end, just a single spermatozoon is able to interact with the egg, resulting in an embryo.

Once sperm reach the Fallopian tube after intercourse, they will be able to meet the egg provided that the woman is on her fertile days and ovulation has taken place. In that case, spermatozoa surround the egg cell in an attempt to fertilize it.

Stages of natural fertilization

As simple as the process whereby egg and sperm become one seems, actually it requires the activation of multiple mechanisms and changes in gamete cells for it to be possible.

The following are the four main stages of fertilization in human beings:

Penetration of the corona radiata

The first stage of human fertilization is the penetration of spermatozoa into the corona radiata of the egg, a coat made of cells that surrounds the egg.

Sperm cells are able to go through this first barrier thanks to the release of the hyaluronidase enzyme, and the motion of their flagellum (the tail).

When they cross this layer, spermatozoa encounter a second barrier: the zona pellucida (ZP). It is an external layer that surrounds oocytes.

Penetration of the zona pellucida

More than a single sperm cell is required to degrade the ZP. Nonetheless, in the end just one of them will be the “winner”, that is, the one who fertilizes the egg.

In order to be able to cross this second barrier, the head of the sperm establishes contact with receptor ZP3 of the ZP. This triggers the acrosome reaction, which involves the release of a series of hydrolytic enzymes (contents of the acrosome). These enzymes dissolve the ZP to allow the passage of the sperm cell.

The acrosome reaction causes a series of modifications of the sperm cell that allow its natural capacitation. Sperm capacitation, at the same time, allows it to get into the cell egg, causing the membranes of both reproductive cells to fuse together.

Fusion of membranes

When the egg cell makes it to the plasma membrane of the oocyte, it triggers three different processes in the female gamete:

  • Formation of the fertilization cone
  • Instant depolarization of the egg membrane
  • Release of cortical granules from the egg

The formation of the fertilization cone enables fusion between the membranes of both the egg and the sperm, allowing passage of the sperm’s head into the egg. Simultaneously, thanks to depolarization and the release of cortical granules, the entrance of multiple sperm is prevented.

Fusion of nuclei & zygote formation

Now that the passage of sperm has taken place, the oocyte activates itself to finish meiosis, the process whereby the number of chromosomes is reduced. With it, the second polar body is released, and chromosomes distribute themselves forming a structure called female pronucleus.

Pronuclei are the nuclei of gametes, which have the particularity of having half the chromosomes in comparison to the remainder of cells in the body, that is, 23 chromosomes.

On the other hand, the sperm continues the fertilization process until its head, which contains the nucleus, reaches the female pronucleus. The sperm will lose its tail at some point, and the nucleus will swell to create the male pronucleus.

When both pronuclei are next to each other, fusion occurs.

The fusion of pronuclei means that the membranes of both end up disappearing so that the chromosomes can fuse together. This allows the cell to reestablish its normal number of chromosomes, that is, 46 chromosomes.

The fertilization process of humans culminates with the formation of the zygote: the first cell of the organism, created after egg and sperm fuse into one.

In addition to all this, fertilization determines the gender of the baby-to-be based on sex chromosomes:

Male zygote
sex chromosomes are XY, so the unborn child is a boy.
Female zygote
sex chromosomes are XX, so the unborn child is a girl.

Egg cells always carry an X-chromosome. Thus, the sex of embryos is determined by the sperm, which can carry either an X or Y chromosome.

This is the entire human fertilization process step by step. However, you can see a diagram of the complete process summarized below:

How do twins happen naturally?

Contrary to common belief, twins do not happen when two spermatozoa fertilize the egg. As explained above, the egg has a natural mechanism to inhibit multiple fertilization, since the resulting embryos would be nonviable.

If two sperm penetrated the egg cell, the total number of chromosomes would be 69, with 23 from one sperm, 23 from the other, and 23 from the egg. This type of embryos are triploid, that is, they have three pairs of chromosomes. Triploid embryos would not survive anyway.

Actually, the stages of fertilization are exactly the same in both cases: a sperm cell penetrates the egg cell. The difference can be found in the cell divisions that occur afterwards. In twin pregnancies, due to causes still unknown, the embryo splits into two, leading to the formation of two genetically identical babies, which means that their gender will be the same, too.

In the case of non-identical twins, the process is different. In this case, two different eggs are fertilized simultaneously, each one by a different sperm. Thus, fertilization and subsequent embryo development would occur as in a singleton pregnancy, although two babies will develop independently inside the maternal womb. As the name suggests, non-identical or fraternal twins are genetically different, and the gender can be different as well.

Further reading: Fraternal Twins vs. Identical Twins – What’s the Difference?

What happens after fertilization?

The fertilized egg forms a new cell called zygote, which starts descending through the Fallopian tube to the uterus. During this journey, the zygote divides to become a two-cell embryo. Actually, the term zygote is used to refer to the first stage of embryo development.

As the embryo continues its pathway inside the tube, it continues dividing until it becomes a blastocyst, a multiple-cell structure that is able to attach to the uterus (i.e. embryo implantation) and cause a pregnancy in females.

To learn more about the embryo implantation process, the stages of embryo development, and the changes that occur in the pregnant woman during pregnancy, visit the following articles:

FAQs from users

Is “conception” the same as “fertilization”?

By Zaira Salvador BSc, MSc (embryologist).

Technically yes. “Conception” refers to the union of sperm and egg and subsequent pregnancy, which is the same as “fertilization”, but “fertilization” just refers to the union of sperm and ovum, which does not mean that implantation occurs in all cases.

Let’s have a look to the definition of each according to the Merriam-Webster Dictionary to see slight differences between them:

Conception: the process of becoming pregnant involving fertilization or implantation or both.

Fertilization: the process of union of two gametes whereby the somatic chromosome number is restored and the development of a new individual is initiated.

Taking this into account, it is important to distinguish both of them from the term implantation, which is defined as follows:

Implantation: (in placental mammals) the process of attachment of the early embryo to the maternal uterine wall.

Where does fertilization usually take place?

By Zaira Salvador BSc, MSc (embryologist).

It is commonly believed that fertilization occurs in the uterus, but it actually does not. In particular, fertilization occurs in the Fallopian tubes, which are the oviducts that connect both ovaries (where egg cells are produced) with the uterus (where pregnancy develops).

How long after intercourse does conception occur?

By Zaira Salvador BSc, MSc (embryologist).

Fertilization can occur only after the egg leaves the ovary and hits the Fallopian tube. Thus, the woman is on day 14 of the menstrual cycle approximately. Even though sperm can stay in the female reproductive system for up to three days, the egg cell has a lifespan of just 24 hours after it is released, which means that fertilization can take place only within the first 24 hours after fertilization. However, intercourse may have occurred between 2 and 3 days before.

To sum up, the fertilization process can take several hours, although the length can vary from case to case, so giving an exact figure is complicated. Generally, it occurs within 19-24 hours.

Can you combine two female eggs for fertilization?

By Zaira Salvador BSc, MSc (embryologist).

No. For human fertilization to give rise to the development of a new human being, there must be an egg and a sperm. To date, fertilization using two egg cells is impossible. Not even Reproductive Medicine has been able to make this possible.

How is three parent fertilization performed?

By Zaira Salvador BSc, MSc (embryologist).

Actually, it is a technically challenging assisted reproduction option that is permitted in the UK for the moment. It is done using the egg of the intended mother, the sperm of the intended father, and a donor egg. It is used to prevent mitochondrial diseases, that is, those that can be inherited by offspring due to defects in the mitochondria of the egg.

Mitochondria are organelles that can be found within cells, and their function is to produce energy. If we look into the inside of an egg cell, we can find genetic material in the nucleus, along with a small proportion of mitochondria in the cytoplasm.

The three parent technique involves replacing the nucleus of the donor egg (free of sick mitochondria), which contains the genetic material of the egg donor, with the nucleus of the mother. Thus, the intended mother will be the genetic mother of the child, but the mitochondria of the egg’s cytoplasma will pertain to the donor. By doing this, we can ensure that mitochondria are healthy, and the baby will not be sick. Finally, the resulting egg is fertilizing using the intended father’s sperm.

What is post-mortem fertilization?

By Zaira Salvador BSc, MSc (embryologist).

It refers to the use of sperm that was cryopreserved prior to the death of the male, with the goal of causing a pregnancy in the widow using In Vitro Fertilization (IVF).

Suggested for you

Confusing words in the field of human reproduction and embryology is normal. For this reason, we have published a comprehensive guide to the main terms to be used after fertilization: What’s the Difference Between Zygote, Embryo & Fetus?

Women who are encountering trouble getting pregnant are likely to need to undergo IVF, which is an artificial technique that emulates the natural fertilization process. Get more info about this technique here: What Is In Vitro Fertilization (IVF)? – Process, Cost & Success Rates.

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References

Edwards RG (1980) Conception in the Human Female. Academic Press, New York.

Johnson, M.H., Everitt, B. J. (2007) Essential reproduction – 6th ed. Published by Blackwell Publishing. Library of Congress Cataloging-in-Publication Data.

Kozlovsky P, Gefen A. Sperm penetration to the zona pellucida of an oocyte: a computational model incorporating acrosome reaction. Comput Methods Biomech Biomed Engin. 2013 Oct;16(10):1106-11.

Veeck L.L., 1999. An Atlas of Human Gametes and Conceptuses. The Parthenon Publishing Group, New York and London.

Authors and contributors

 Zaira Salvador
BSc, MSc
Embryologist
Bachelor's Degree in Biotechnology from the Polytechnic University of Valencia (UPV). Embryologist specializing in Assisted Procreation, with a Master's Degree in Biotechnology of Human Assisted Reproduction from the University of Valencia (UV) and the Valencian Infertility Institute (IVI). More information
License: 3185-CV
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