What is human fertilization and what are its stages?

Fecundation is the union of the ovule and the sperm so that pregnancy can occur. In the human species, fertilization is internal, that is, it takes place inside the body of the woman, specifically in the fallopian tubes. This is the so-called natural or in vivo fertilization.

It is also possible to perform fertilization in an assisted reproduction laboratory, a process known as artificial fertilization or in vitro fertilization.

Below you have an index with all the points that we will discuss in this article.

Index

What is human fertilization and what are its stages?

1. Definition of fertilization
2. Stages of natural fertilization
2.1. Penetration of the radiated crown
2.2. Penetration of the zona pellucida
2.3. Membrane fusion
2.4. Fusion of nuclei and zygote formation
3. The fertilization of twins and twins
4. What happens after fertilization?
5. Questions from users
5.1. "Fertilization" is the same as "fertilization"?
5.2. Is fertilization intrauterine or occurs elsewhere?
5.3. How long after intercourse does fertilization take place?
5.4. Can an egg be fertilized with another egg?
5.5 How is the fertilization of three parents?
5.6. What is "post mortem" fertilization?
6. Recommended reading

Definition of fertilization

Fecundation is the fusion of the male and female gametes, that is, the sperm and the ovule, so that the normal chromosomal endowment of the human being is restored (46 chromosomes).

For the phenomenon of fertilization to occur, the man must ejaculate inside the vagina of the woman. At this time, the sperm can ascend through the female genital tract and reach the fallopian tubes, where they will meet the egg.

Of the millions of sperm released in the ejaculation, only about two hundred will get to their destination in the trunk. Finally, only one sperm will interact with the ovule and fertilization of the embryo will take place.

Once the sperm reach the fallopian tubes after intercourse, they can only find the egg if the woman is in her fertile days and there has been ovulation. In that case, the sperm will be placed around the egg and will try to fertilize it.

Stages of natural fertilization

Although the process of union between ovum and sperm may seem very simple, the truth is that there must be several mechanisms and changes in both gametes so that fertilization can occur.

Below we explain step by step the different stages of fertilization in humans:

Penetration of the radiated crown

The process of fertilization begins with the penetration of sperm through the layer of cells that surrounds the ovule: the corona radiated.

The sperm get through this layer thanks to the release of the enzyme hyaluronidase and the movement of its flagellum (the tail).

Penetration of the radiated crown

Once they pass through this layer, the sperm meet with a second barrier: the zona pellucida, the outer layer that surrounds the ovule.

Penetration of the zona pellucida

It takes more than one sperm to achieve degradation of the zona pellucida, but finally only one of them can enter the ovule.

In order to cross this second barrier, the sperm head contacts the ZP3 receptor in the zona pellucida of the ovule. This triggers the acrosomal reaction, which consists of the release of hydrolytic enzymes called spermiolysins. These enzymes dissolve the zona pellucida to allow the passage of sperm.

Penetration of the zona pellucida

Likewise, the acrosome reaction provokes a series of changes in the sperm that allow its final training to be able to penetrate inside the ovum, fusing its membranes.

Membrane fusion

When the sperm comes in contact with the plasma membrane of the ovum, 3 different processes are triggered in the female gamete:

The formation of the fecundation cone
The instantaneous depolarization of its membrane
The release of cortical granules into the perivitelline space

The formation of the fertilization cone allows the fusion of the membrane of the ovule with that of the sperm so that the head of the sperm can enter. In turn, thanks to the depolarization of the ovule membrane and the release of cortical granules, the entry of another sperm is prevented.

Membrane fusion

Fusion of nuclei and zygote formation

With the entry of the sperm, the ovum is activated to end meiosis, a process that allows the reduction of the number of chromosomes. Thus, the second polar corpuscle is released and the chromosomes are placed forming a structure called female pronucleus.

The pronuclei are the nuclei of the gametes, which have the peculiarity of having half of chromosomes with respect to the rest of the cells of the body, that is, 23 chromosomes.

For its part, the sperm advances until its head, which contains the sperm nucleus, is next to the female pronucleus. The tail comes off to end up degenerating and the nucleus swells to form the male pronucleus.

Once both pronuclei are next to each other, the fusion of both occurs.

This supposes that the membranes of both pronuclei disappear so that their chromosomes can join and that the cell reestablishes its chromosomal endowment, that is, 46 chromosomes in total.

All this process of fertilization culminates with the formation of the human zygote: first cell of the organism fruit of the union of the ovule and the sperm.

Fusion of nuclei

In addition to all this, in fertilization is established if the future baby will be a boy or a girl based on their sex chromosomes:

Cigoto masculinosus sex chromosomes are XY and the future baby will be a boy.Cigoto femeninosus sex chromosomes are XX and the future baby will be a girl.

The ovum is always a carrier of the X chromosome. Therefore, the sex of the embryo will be defined according to whether the sperm is a carrier of an X chromosome or a Y chromosome.

Although we have seen each stage of fertilization in detail, in the following image you can see a diagram of the entire process.

Process of fertilization of the ovule

The fertilization of twins and twins

Contrary to popular belief, twins do not arise from the fertilization of an egg by two sperm. As we have already indicated, the ovule has a mechanism to avoid double and multiple fertilization, since the resulting embryos would not be viable.

If 2 sperm penetrated the egg, in total there would be 69 chromosomes: 23 chromosomes of one sperm, 23 of the other and 23 of the ovule. This type of embryos would be triploid, that is, they would have 3 sets of chromosomes, and could not continue with their development.

So that twins can originate, the fertilization is identical to the one that gives rise to a single baby: a sperm penetrates inside the ovule. The difference lies in the cell divisions that occur next. In this case, for reasons still unknown, the embryo is divided into two and two genetically identical babies will be born, which implies that they will be of the same sex.

The origin of the twins is different. In this case, the fertilization of two different ovules takes place, each of them by a spermatozoon. Therefore, the processes of fertilization and embryonic development would be the usual, with the particularity that the two babies would develop at the same time in the maternal womb. Babies would not be genetically identical nor need to be of the same sex.

You can check this link for more information about the pregnancy of twins and twins: What differentiates twins pregnancy from twins pregnancy?

What happens after fertilization?

The fertilized ovum is a new cell called a zygote, which begins to descend through the fallopian tube into the uterus. During this journey, the zygote divides to give rise to the embryo of two cells. The term zygote is only used to define the first embryonic stage of a single cell.

As it moves through the tube, the embryo will continue to divide to allow the formation of the blastocyst, a structure with many cells that begin to differentiate and that has the ability to implant in the uterus and lead to pregnancy.

If you are interested in knowing in detail the process of implantation, the stages of embryonic development and the changes in the mother throughout pregnancy, we recommend you read these two articles: What is the implantation of the fertilized ovule? and Pregnancy month by month.

Questions from users

"Fertilization" is the same as "fertilization"?

Although both terms are often used for different things, they are synonymous. On the one hand, the word fertilization is used to describe the process by which the male gamete (spermatozoon) and the female gamete (ovule) come together to form an embryo. As for the term fertilization, it is used to name the process through which the land is prepared to make it more fertile.

Is fertilization intrauterine or occurs elsewhere?

Fertilization does not occur in the uterus, so it is not intrauterine. This process takes place inside the fallopian tubes, which are the ducts that connect the ovaries, where the eggs are produced, and the uterus, where the pregnancy develops.

How long after intercourse does fertilization take place?

The fertilization must necessarily occur once the ovum leaves the ovary and reaches the tube. Therefore, the woman should be approximately on her 14th day of the menstrual cycle, at which time ovulation usually occurs. Although sperm can live up to 3 days in the female genital tract, the egg only lives 24 hours, so fertilization must occur during the first 24 hours after ovulation. However, it is possible that intercourse occurred between 2 and 3 days before.

The process of fertilization itself can take several hours, but the times can vary so much between some cases and others that it is not possible to give a specific time. In general, it is considered to occur within a period of 19-24 hours.

Can an egg be fertilized with another egg?

No. For fertilization and the development of a new being to take place, an egg and a sperm are essential. For now, not even advances in assisted reproduction have made it possible for the laboratory to produce fertilization between two human ovules of two women.

How is the fertilization of three parents?

It is a complex process of assisted reproduction that, for the moment, is only allowed in the United Kingdom. It consists of using the mother's ovule, the sperm of the father and, in addition, the donor's egg. It allows to avoid mitochondrial diseases, that is, those that appear in children due to defects in the mitochondria of the ovule.

Mitochondria are organelles that are found inside cells and are responsible for producing energy. Inside the ovum, it is possible to find the genetic material contained in the nucleus but, in addition, there is a small part of mitochondria in its cytoplasm.

In the fertilization of three parents, the nucleus is replaced with the genetic material of the donor's ovum (free of diseased mitochondria) by the mother's nucleus. Thus, genetically it will be a child of the future mother, but the mitochondria of the cytoplasm of the ovum will be from the donor, they will be healthy and, therefore, the child will not suffer from the disease. Finally, this resulting ovule is fertilized by the father's sperm.

What is "post mortem" fertilization?

This term refers to the use of frozen semen after the death of the male in order to impregnate the woman with an assisted reproduction treatment. According to the Law 14/2006 on Techniques of Assisted Human Reproduction, this is possible provided that this semen is used within 12 months after the husband's death and with his consent signed previously, through a specific document, in writing public, in will or in a document of previous instructions.

For more information on this topic, you can continue reading in the following section: Law 14/2006 of assisted human reproduction in Spain.

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