Eli5: How come sperm can survive being frozen?
In: Biology
So, the main reason most large living things can’t survive freezing is because extremely sharp ice crystals form in their cells and rupture their membranes – that’s basically what frostbite is. With that in mind there’s two big reasons sperm (as well as eggs and entire human embryos provided they’re small enough) can be frozen.
1. They’re frozen extremely fast. This results in smaller ice crystals which are less likely to damage the cell – this is the same reason store bought ice cream is smooth and creamy but if you just put milk in your freezer it’ll be icy and slushy. The store bought ice cream was frozen faster and thus the ice crystals are small enough that you don’t notice them.
2. Certain compounds, called cryoprotectants, are used in that freezing process. These typically do many things, including drawing water out of the cell (less water = less ice crystals) as well as directly interfering with the formation of ice crystals and stabilizing the cell membrane.
Worth mentioning is that there’s nothing unique about sperm cells in this regard – nearly any type of cell can be successfully frozen using similar techniques.
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