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Sperm Cells Compete with Each Other to Fertilize Eggs

The proteins and behaviors that make sperm competition possible, discussed.

March 22, 2021
Macie Gelb

Did you know that sperm cells compete with each other to end up being the one to fertilize the egg and become the embryo? Every sperm cell wants to be the one to fertilize the egg, and they go after it with intensity. Which sperm cells are able to succeed in the end is not all about luck, however. It turns out that there are certain proteins that can impact which sperms are stronger and more competitive. Favorable amounts of active protein improve the competitiveness of individual sperm, whereas abnormal activity can cause male infertility. It is all a very delicate and difficult balance that depends entirely on the presence and amount of a specific protein called RAC1. 

Research has shown that certain amounts and types of proteins can impact which sperm cells are able to reach the egg and fertilize it faster. 

The Research

A team of researchers at the Max Planck Institute for Molecular Genetics in Berlin discovered and described how a genetic factor called “t-haplotype” can promote the fertilization success of the sperm carrying it. 

Sperms with the t-haplotype protein are more progressive and are therefore able to move faster forward than sperm without the t-haplotype protein. This gives them an advantage when it comes to fertilization because these sperms are more likely to make it to the egg faster than their competitors. The researchers found that all the slowest moving sperms did not have t-haplotype, while the faster ones did. 

The differences in sperm speed, motility, and mobility also center around the molecule RAC1. RAC1 is a molecular switch that transmits signals from the outside of the cell to the inside by helping to activate other proteins inside the cell. RAC1 has been known to be involved in directing white blood cells or cancer cells towards cells that exude chemical signals. It is now suggested to play a part in directing sperm cells towards the egg through the same method of “sniffing” their way to their target. Sperms are able to be most competitive and productive when there is the exact right level of RAC1. Too much or too little RAC1 interferes with the sperm’s ability to move forward. 

Sperms with the t-haplotype proteins are able to poison the other sperms without it, putting those without it at an even higher disadvantage. 

How Sperms with t-haplotype Poison their Competition

Sperms that have the t-haplotype protein are also referred to as t-sperms. T-haplotype poisons all sperm that it encounters, but it provides an antidote for t-sperms, while completely disabling the regular sperms. Scientist and researcher Bernhard Herrmann compares the phenomenon to a marathon: it can be helpful to imagine this in a marathon metaphor. Imagine that all participants in the marathon are being poisoned by their drinking water, but some of the participants are able to access an antidote. The participant’s able to access the antidote will be able to complete the marathon and finish quickly and without much difficulty. This is basically what happens with t-haplotype and sperm, the sperms with t-haplotype are able to reach the goal: the egg. 

The t-haplotype protein contains certain gene variants that serve the purpose of confusing regulatory signals. These confusions create distortions that impact the early phase of spermatogenesis, and thus get passed on to all the sperm in the body, not just those carrying the t-haplotype. The antidote becomes relevant and important after the chromosomes are split evenly between the sperm during the sperm’s maturation period. After the maturation period, each sperm cell contains half the chromosomes they will need to merge with and fertilize the egg. However, only the half of the sperm with the t-haplotype will be able to reverse the negative effects of the distortions caused by the original t-haplotype invasion. 

While t-sperms appear to have somewhat of an advantage over “normal” sperms, that advantage is eliminated when the sperms are treated with a RAC1 inhibitor. When the RAC1 inhibitor comes in, all sperms are equally disabled in their rush to the egg. This proves that deviant RAC1 activity is the main reason for sperm immotility. Essentially, RAC1 is the true evil behind sperm competition. However, if RAC1 levels become too low, sperms are also disabled, proving that RAC1 levels need to be just right for sperm to effectively find and fertilize the woman’s egg. 

The fact that sperms are able to poison and sabotage each other is an incredibly important scientific finding that can give a lot of insight to male infertility and help scientists and doctors understand and mitigate the occurrence of male infertility. With this information, scientists should be able to monitor levels of RAC1 and t-haplotype in hopeful fathers, and help them manage their levels to create the best possible chance of fertilization success. 

As always, it is important to remember that issues with fertility are extremely common across genders, ages, cultures, and lifestyles, and scientific findings such as this one prove that. When it comes to trying to have a baby, it is normal for things to take time, and sometimes to require the help of a doctor or medical professional. The best thing you can do is be informed about possible issues that could come up in your fertility journey, and be ready to treat and manage those potential issues. Infertility is no one’s fault, and as science progresses, it is becoming more and more manageable.


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