Why does frozen sperm not die?

I’ve always wondered when people donate male sperm to these clinics that the sperm is able to come alive after being frozen -390F for several years.
How is it that the sperm don’t die?

The main reason organisms die from freezing is cell damage caused by ice crystals puncturing cell walls. Slow freezing allows the crystals to get large relative to the cells, but donated sperm is frozen in liquid nitrogen quickly enough that only very small crystals have time to form, so the cell walls aren’t too badly hurt. I don’t know what the survival rate is, but it’s certainly high enough to do the job.

While I’ve never worked with sperm, I freeze mammalian cells in the lab all the time, and rjk’s got it pretty much right. When we freeze back cells, first we make sure that they’re in the best possible shape (log-phase growth, high viability). We also prepare special freezing medium that contains a final concentration of 10% DMSO (dimethyl sulfoxide), which helps to prevent the formation of ice crystals, though it has a trade-off in that you need to work quickly once you’ve got the DMSO in with the cells as it’s toxic. Once the cells are in freezing medium, the vials are frozen at -70C for 24 hours, and after that we transfer them to liquid nitrogen for long-term storage.

The procedure of thawing cells out of liquid nitrogen is also important for assuring their viability - if you aren’t careful, you can disrupt the cell membranes and they’ll die. The most common procedure is to pull the vial of cells out of liquid nitrogen, thaw it in a 37C waterbath until almost all of the ice is gone, transfer the cells to a fresh tube, and then add cold culture medium (just out of the fridge ~4C) dropwise. Depending on how delicate the particular cells you’re working with are, you can either centrifuge the cells at this point and resuspend in fresh medium to get rid of the DMSO completely, or simply add enough medium to get the DMSO down to a non-toxic concentration if the cells can’t take the added stress of a run through the centrifuge. Again, some cells are more fragile than others, but you generally get pretty good recovery from a thaw, as long as you know how to treat the cells properly.

so why can’t you do this w/ a whole person/animal?

For starters, I imagine that the survival rate on the sperm cells is not a hundred percent. If half the cells in a sample die, it’s still perfectly useful, whereas if half the cells in your body died, you wouldn’t be doing so hot.

Secondly, did you read their description of the process? How would a chemical be able to quickly flood every cell in your body before you died from its toxicity? And how would they centrifuge it out of you afterwards? Semen is a liquid, and so they can easily do both of those things. How would you manage to do that to an animal?

many thanks…