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
Hey, this sounds like a question for me! I’m a researcher studying cell division mechanisms, including meiosis (the process of getting egg or sperm cells ready for fertilisation).
There are several ways to prevent polyspermy (when an egg gets fertilised by multiple sperm cells). The first is that although millions of sperm cells are ejaculated, they need to make their way to the egg, which is not easy. In the end, only a few hundred sperm cells will be able to reach the egg.
Second, there is the cortical reaction. There is a thick, gelatinous coat around the egg called the zona pellucida, which sperm cells have to get through before being able to fertilise the egg. This slows down the sperm a lot. As soon as one sperm has managed to make its way through the zona pellucida and fertilise the egg, the egg immediately triggers the release of an enzyme that causes the zona pellucida to harden. This stops any further sperm cells from making their way though it. This is considered the “slow block”, as the cortical reaction can take a little while to occur.
Third, there is depolerisation of the egg’s plasma membrane. This is considered the “fast block”. It is a little more complicated to explain, but once a sperm cell has made its way through the zona pellucida, it must still get through the membrane of the egg. This membrane has properties that allow for a sperm cell to bind it and get through when the egg is unfertilised. As soon as a sperm cell has got through the membrane, it “depolerises”, which (in simple terms) changes the properties of the membrane. Even if other sperm cells had managed to make their way through the zona pellucida, they would no longer be able to bind the membrane and penetrate it. This happens on a very short time scale, so it is considered the fast block to polyspermy.
I happen to have studied these very mechanisms. In humans, they are extremely important because if an egg is fertilised by multiple sperm, the outcome will be a spontaneous abortion. The fertilised egg would simply not be viable.
This backstory is great; better than whatever I found on “cornception.” I like that you asked for some real answers for her. In the meantime, did you tell her to hold on, let me ask the reddit?
If you don’t mind me tacking on another question: If eggs are locked in with one sperm’s code, how is it possible for cats to have split liters with two mates?
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