Why does only one sperm cell get to fertilize the egg? How is multi-sperm fertilization prevented and does this prevention ever fail?
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So, there are 2 main ways this prevention of polyspermy happens.
1st, there is a fast reaction by the egg upon sperm entry. It depolarizes the membrane, which prevents other sperm from being able to bind to it. Kinda like a magnet.
2nd, is a slow reaction. It is the hardening of this egg membrane, which is called cortical reaction. This hardening of the membrane makes the egg impenetrable to other sperm.
Many, many sperm are required to break down the enzymes of the egg membrane, not just one. Once the first sperm penetrates the egg it undergoes a near instantaneous transformation of the membrane that allows no other sperm to enter.
In case the prevention ever fails, the result wouldn’t be viable and be discarded in the same way an unfertilized egg is. Period.
It does fail. My first son died because of this.
Usually, the body will spontaneously abort (miscarry) but…apparently that doesn’t always work which is why I have a prob with how abortion laws are changing.
In my case, I had some spotting during the first trimester but no other issues. At 16 weeks, they did the typical sonogram and blood tests. The next day, the doctors called and told me they didn’t know WHAT was wrong but that I had little amniotic fluid and very low HcG (preg hormones). In the sonogram, he appeared perfect. All his organs, limbs, brain…just too small for his gestational age..
I spent the next 4 weeks going to various hospitals and specialists and no one could figure out what was wrong. All they could tell me was that he was perfect but he would die either in utero around 7-8 mo or at birth.
This was absolute hell on me psychologically. Fortunately I live in MD and have multiple teaching hospitals. I was 20 weeks by the time I decided I could not go through the pregnancy. Because so few states allow “late term abortions” I had to fight and pull strings to get an appointment before I entered my 3rd trimester.
At this point, they have to induce labor and you have to give birth. Contrary to what the pro-life nut jobs claim…the doctors give you medication to stop the baby’s heart so that you have a still birth.
It was hell psychologically and would have been worse to go a full 40 weeks.
We had an autopsy done at his birth. He had “triploody” which is having 3 full sets of chromosomes. This is caused when 2 sperm fertilize one egg.
The first way the egg makes sure only one sperm enters is by slowing down the sperm with the cumulus oophorus. Think of it like a thick wall that the sperm has to break through. Almost instantaneously when a sperm enters the egg the egg undergoes a dramatic change where the membrane becomes impenetrable no longer letting anymore sperm to enter. It’s VERY rare but sometimes two sperm meet the egg at the exact same time and they both enter. Having 3 sets of chromosomes isn’t viable for life so eventually the cells will die from the error. There is one case I could find from [2019](https://www.bbc.com/news/health-47371431) where two sperm fertilized one egg but the twins got very lucky that the egg was able to divide into two embryos each with 2 sets of chromosomes. I cannot emphasize how rare this is! Most of the time the body’s process for conception goes by perfectly and one egg and one sperm create the perfect 2 sets of chromosomes.
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