2023-04-15 20:00:00
“When you feel bad, remember that you were the fastest sperm.” Perhaps that is one of the most famous phrases when trying to motivate a friend or person close to her great potential. Used in the first person, it also often represents the way many people brag about their intelligence or excellence in certain activities.
However, nothing is further from reality. Actually, the idea that the fastest spermatozoon is the one that manages to fertilize the ovum and generate offspring It’s just a myth of biology. Now it is known, thanks to the work of various scientists, that not only is this “race” false, but also that the role played by the ovum in the entire fertilization process is much more active and decisive than previously believed.
The exact place and time
The theory is self-evident: if all goes well, once a sperm comes into contact with an egg and penetrates it, the egg shuts down. In this way, the female gamete prevents other spermatozoa from entering it, and ensures the safe development of the embryo. Probably, following this reasoning is how it has been derived in the belief that it is first come first come in and “leave the rest at the gates.”
However, it is not a reliable explanation of reality. In truth, the ovum is surrounded by a translucent outer layer known as pellucid membrane. This acts as a defensive barrier for the cell that protects it from foreign elements and prevents others from entering later once a spermatozoon passes through it. In addition, it has a series of chemical substances in its composition that recognize the spermatozoa themselves and attracts them. In fact, without this attractive action, the male gametes would pass by without detecting the female ones.
This is how spermatozoa really advance
In this way, when the spermatozoa reach the ovum they must go breaking and weakening that membrane, with the aim of entering and fertilizing it. However, the first to arrive will not succeed: they will be left weak and give the job to the next ones. That is, if we take into account that in an ejaculation there are 20 to 150 million sperm per milliliterwe can affirm that the first million of them, “the fastest”, will really be the ones that will sacrifice themselves to gradually weaken the membrane.
THE FELLY MEMBRANE
Thus, once the membrane is sufficiently weakened, the sperm can enter. This means that it will not be the fastest, but the one that is in the right place and at the right time the one that contacts the membrane, passes through it and manages to fertilize it. Once this occurs, what remains of the membrane itself prevents another, later spermatozoon from entering the ovum and the development of the embryo begins.
Shi W, Xu B, Wu L-M, Jin R-T, Luan H-B, Luo L-H, et al (2014)
Zona pellucida of an ovule, indicated as ZP
The pellucid membrane is maintained as a protective element while the zygote divides. This occurs during the first 5 days after fertilization, when the two cells that initiate the formation of the embryo begin to divide and create more. After this process, the embryo has its clearly differentiated cells and is called a blastocyst. It is here when the membrane degenerates and disappearsjust before the deployment phase begins.
THE SELECTION OVUM
However, a study published by geneticist Joseph H. Nadeau could put another twist on the process. And it is that, this scientist from the University of Washington affirmed that, not only is it not the first sperm that gives rise to fertilization, but rather the ovum plays an important selection role. That is, the ovum chooses or rejects the sperm that can enter, once the membrane has weakened, according to the DNA quality that they carry
In fact, already in 2013 Matthew Gage, a researcher at the University of East Anglia, in England, observed this behavior in an experiment carried out with fish. In it, he took eggs from female fish, such as salmon and trout, and made them available to a mixed group of male fish of both species. The result was that 70% of the eggs of each class were fertilized with sperm from fish of the same species. This, Gage proposed, is due to the combination of two phenomena: the attraction that the spermatozoa feel for the ovule of the same species (due to the chemical composition of the pellucid membrane), as well as the selection made by the ovules themselves, allowing only fertilization by complementary DNA.
This discovery had a great importance For the scientific community, then, it not only gave a certain complexity and background to Mendel’s simple law of genetic randomness, but also broke with the image of a passive element projected by the figure of the ovum.
The oldest sperm in the world
A NEW PERSPECTIVE
A good example of a case of “non-random” selection by the ovum is one of the experiments carried out by H. Nadeau with rodents. In a first phase, the scientist added a normal gene and a mutant cancer gene to the female mice, leaving all the male mice with the normal and healthy genes. The result responded exactly to the laws proposed by Mendel: each parent carries two copies of each gene, both normal, and fertilization occurs randomly.
However, in a second phase, he added a mutant gene and a normal one to the male mice, this time keeping those of the normal females. The results were now puzzling by classical laws: only 27% of the mice had the mutant gene. That is, fertilization had not occurred completely randomly, and the healthy eggs had rejected a certain part of the sperm with mutation, minimizing the possibility of offspring carrying this mutation. On the other hand, in the first experiment the spermatozoa did not present any type of selective attitudeand fertilized healthy and diseased eggs in the same way.
Therefore, and contrary to the popular image, it was confirmed that the ovule presents such an important role during fertilization like that of the spermatozoon. Although it is the latter that performs the dynamic and movement part, it will apparently be the ovum that has the last decision during fertilization, selecting the DNA that gives rise to the best combination.
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