Debunking Myths #1: You are not the fastest sperm

Today begins a new category of the contents of the Science Driven medicine blog, the section "Dismantling myths" where erroneous popular beliefs will be detailed and explaining where they are wrong and where they are right, since every myth has a part of TRUE.

The myth

Since time immemorial, popular belief has held that the fertilization process is a frenetic race in which millions of sperm compete to be the first to reach the egg. This idea, rooted both in culture and in the collective imagination, suggests that the fastest sperm, and therefore the "strongest", is the one that achieves fertilization.

This narrative has been perpetuated through various representations in media, education, and everyday conversations, painting a picture of quasi-athletic competition in the microscopic world of human reproduction.

What is true in this myth?

The fertilization process, in which the sperm (male gametes) reach the egg (female gamete), could be compared to a competition between them , in which (almost always) there is a single winner, but it is not a race. Rather, it would be more correct to define it as an Olympics, in which all sperm compete by passing multiple tests, and among all those who pass all the tests, a single judge (the egg) decides who is the winner (the most handsome, but to be chosen as handsome you have to go to the end).

What is the reality?

Once the sperm are released into the vagina, their carousel of chemical signals, resistance and adaptation begins.

Test #1 - Resistance - The acidic environment of the vagina

Initially, sperm encounter the acidic environment of the vagina, which is hostile to many of them. This environment is designed to protect against pathogens, but can also be harmful to less strong sperm. Only the most resistant survive this first stage.

Test #2 - Viscosity - Passage through the cervix

Next, they must pass through the cervix, the entrance to the uterus. The cervix contains cervical mucus, the consistency of which varies throughout the menstrual cycle. Around ovulation, this mucus becomes thinner, making it easier for sperm to pass through. Outside the ovulation period it is thicker and acts as a barrier. Second screening.

Test #3 - Orientation - Navigation in the womb

Once in the uterus, sperm face a new challenge: navigate a wider space and find the right direction toward the fallopian tubes, where the egg may be. This navigation is guided in part by chemical signals, as if it were a GPS, created with some substances such as progesterone or cytokines. This guidance process, technically known as chemotaxis, is currently an active field of study since there are many factors of it that are still unknown.

Test #4 - Combat - The Immune Challenge

The female immune system can see sperm as potential invaders, which means that they must evade or survive the immune responses of the woman's body, which if the presence of sperm is noticed, are treated as a foreign agent and then The immune system sends lymphocytes to fight the invaders.

Test #5 - Orientation - Arrival at the Fallopian Tubes

Once in the fallopian tubes, the sperm must locate the egg again based on chemical signals that it emits, in order to orient themselves and go towards it.

Test #6 - Secret Agent - The protective layer of the egg

The sperm that manage to reach the penultimate test face crossing the wall that protects the egg. The egg is surrounded by a layer of cells called the corona radiata and an outer layer, the zona pellucida. Sperm must first penetrate the corona radiata and then the zona pellucida to reach the egg.

The mechanism of passing the corona radiata is worthy of James Bond. The sperm releases on the corona radiata some enzymes, like hyaluronidase, which is stored in its head (called acrosome). These enzymes digest the corona radiata cells and the extracellular matrix components that surround them, forcing their way through this barrier.

Test #7 - All or nothing - Fusion with the Egg

Of the millions of sperm that started these Olympics, only a few remain, the rest have died or have fallen by the wayside and will die shortly, devoured by the female immune system, annihilated by some bacteria or melted by the acidity of the vagina.

Once the sperm passes through the corona radiata, its next target is the zona pellucida, a gelatinous layer that surrounds the egg and contains key proteins (ZP1, ZP2, ZP3, ZP4) for sperm interaction. The sperm adheres, especially to the ZP3 protein, triggering the acrosome reaction, which releases enzymes to penetrate this area. Using its exposed head and the movement of its tail, the sperm passes through the zona pellucida. Then, the membranes of the sperm and egg fuse, allowing the genetic material from the sperm to enter the egg.

Finally, the nuclei of the sperm and egg fuse, forming the zygote, the first cell of the new organism.

In the event that several sperm manage to reach the zona pellucida simultaneously, the egg must prevent polyspermy to prevent more than one from releasing its genetic material. Once the strongest one manages to release its genetic material into the cytoplasm of the egg, it has two fascinating mechanisms to avoid the next one:

• Rapid block of polyspermy: An electrical reaction occurs in the egg membrane almost immediately after the first sperm fuses with it. This reaction changes the electrical potential of the membrane, making it less receptive to other sperm. It's like an electrified door that kills anyone who tries to walk through it.
• Zonal lock: Slower but more durable mechanism. Once the first sperm penetrates the zona pellucida, chemical changes are triggered in it. These changes alter the properties of the zona pellucida, causing it to harden and become impermeable to other sperm. This process prevents more sperm from attaching to or penetrating the zona pellucida. As if he were putting a brick wall at the entrance, so that no one can enter again.

How are twins produced?

In no case is it produced by fertilization of the egg by two sperm, since if polyspermy occurs, this fertilization will not be viable.

In the case of identical (monozygotic) twins, they occur when a single fertilized egg divides into two separate embryos during the early stages of development. Each twin has exactly the same genetic information, since they come from the same egg and sperm.

In the case of fraternal twins (dizygotic), also known as fraternal twins, they occur when two different eggs are fertilized by two different sperm in the same menstrual cycle. Fraternal twins are genetically distinct from each other, just like any other pair of siblings, sharing approximately 50% of their DNA.