This question is courtesy of my ten year old daughter who meant to search “cornception” on YouTube but got autocorrected to “conception.” The videos that came up were fortunately educational in nature. Thanks for reading this far!
In: Biology
The cortical reaction!
To ELI5 it:
First, you need to know that egg cells are surrounded by a thick layer of jelly-like goop (Zona Pellucida) Sperm cells have to burrow through that goop to actually reach the egg itself. The first sperm to make it through the goop to the egg sends its genetic information into the egg cell. This triggers a kind of intruder alarm, which releases chemicals (enzymes) that turn the jelly-goop into a hard, thick shell. Any sperm that was still in the goop is now trapped, and any sperm that is late to the party has found the door closed. This makes sure that only one sperm can actually make it to the egg.
Edit: Because a lot of people are asking about twins and how they come about:
Identical twins are the result of 1 egg + 1 sperm- very early on in development the ball of cells that will become a baby (these are stem cells, cells that can become any part of the baby or the placenta, they haven’t ‘chosen’ a job yet) splits into two balls of cells, each of which develop into their own baby. Since these babies started from the same fertilized egg, they will be genetically identical.
Fraternal twins are the results of 2 eggs + 2 sperm, each fertilized egg will develop into its own unique baby. This can happen because Mom ovulated two eggs at the same time (the tendency to do this runs in families!). This also means that you can have fraternal twins with different dads, if Mom had more than one set of sperm around when the eggs were available. Fraternal twins are no different than any other set of siblings (genetically) except that they happened to share a womb.
Bigger sets of multiples are usually a combination of both processes, or the result of fertility treatments like IVF. For example, triplets may be caused by 2 eggs + 2 sperm then one of them splits (so 2 identical babies, plus a 3rd fraternal triplet), 3 eggs + 3 sperm, or, most rare, 1 egg + 1 sperm that then splits, and splits again to make three identical embryos.
Once the first sperm cell hits the egg it creates a reaction from the egg to block other sperm. This is an almost instant reaction where the mucus lining gets thicker and creates a “fertilization envelope” that’s basically like a giant barrier that no other sperm can get through.
Not a biologist but I recall a line from sex egg that may or may not be true.
But multiple sperm attempt to penetrate the egg at once. When one makes it through, it releases a chemical which tells the other sperm to stop attempting penetration.
Then they wait around to be devoured by the immune system or flushed out…I think.
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I just want to add to the other great specific answers that these biological don’t “know” anything, as your quotes imply.
You can think of biology as a chemical Rube Goldberg machine, where everything is a consequence of an upstream event, resulting in a domino effect.
So a chemical in the sperm reacts with a chemical in the egg, that causes it to release a chemical that reacts with the outer layer in a way that causes it to harden.
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A human egg is surrounded by a protein matrix “shell” called a zona pellucida. This layer has special docking sites called ZP2 proteins that a sperm can “see” and attach to. Once one sperm gets in to the egg and fertilization occurs these docking sites fall off, and the matrix shell changes and hardens, making it difficult for more sperm to try and enter the egg. Secondly, an egg, like all cells has a membrane around it that selectively allows some things in, After fertilization, it “depolarizes” which means it changes its electrical charge so it is difficult for more sperm to get inside, even if they made it past the first barrier. It is not a foolproof system and sometimes extra sperm get in, creating a condition called polyspermia. Around 10% of spontaneous abortions are due to triploidy , the presence of an extra set of chromosomes in a fetus, usually caused by polyspermy
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