Eli5: How come sperm can survive being frozen?



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.

Freezing cells pretty much stops all chemical reactions. Therefore the cell doesn’t degrade or use up any energy.

If you warm the cell up quickly the reactions start up again and the cell works again.

First off not all sperm will necessarily survive, its not the end of the world if 10% or even 50% of the cells die (I don’t know exact numbers). In a complete organism, even a small proportion of dead cells would be bad.

Secondly the size of a cell means you can freeze and thaw it extremely quickly. Quick freezing ensures no large ice crystals form (which rip cells apart). This is very difficult in multicellular organisms.