Hi all- I work with artificial insemination and have recently wondered how sperm/ eggs/ embryos be frozen in liquid nitrogen (for years!) and then later thawed (in about 60 seconds) where it’s then back to functioning like normal, but you can’t freeze a living person and then unfreeze them again? At what developmental point can you stop freezing biological material before it won’t work again? (No political motive here, just questioning the science behind it, please please no arguing)
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Well the biggest problem is water. Our cells contain it. It is a material that expands and forms ice crystals when frozen. These ice crystals will rupture the cell membrane, so that after the cell is thawed, its insides are all spilled out.
This is still a problem with egg/embryos. But the material is small enough that sometimes it’s enough to use a rapid freezing process, whereby the temperature is dropped quickly enough not to allow ice crystals to form. This is called vitrification.
If this began working on a larger scale, then we might see some realistic cryopreservation.
In an article I read about freezing hamsters, and then thawing them with a microwave, the scientist explained that freezing is a process we have down to bring well executed and understood. Thawing is the problem. There’s a limit to how quickly you can thaw a large object, and if you can’t do it quickly enough then the subject dies.
To your question, sperm are so small that this process is relatively simple and easy because there’s not a lot of mass to worry about. They are also very simple cells without a lot of complex machinery
When water freezes it forms ice crystals. Large ice crystals poke holes in cell walls, which kills them (permanently). If you freeze tissue fast enough, the ice crystals don’t have time to grow, so they don’t pose a problem. It’s easy to flash-freeze a small tissue sample like a small volume of sperm. It’s much harder (impossible, with current technology) to quickly freeze something as large as a human body, or even a whole organ. The problem being that you can only “apply cold” (take away heat) from the outside of the tissue, and it takes time for the heat to dissipate out of the center of the tissue. So this center cools down too slowly, (large) ice crystals form, and the tissue is destroyed.
To add to the other answers, sperm cells are one of the smallest cells in a human organism and don’t have much water in them. Also normally a sperm sample has millions of live sperm cells, of which you really need only a few for in vitro fertilization, so there is plenty of room for error. This makes it easy to freeze sperm more or less however you can and still have some usable sperm cells after thawing.
Egg cells are one of the largest cells in the human body, but they are still single cells. Early stage embryos are of the same size as egg cells (they initially divide “inside”, without actually growing). Freezing them uses a process called vitrification. It prevents ice crystals from forming by basically lowering the temperature extremely fast. It can’t really be done with larger objects, like whole human bodies, because there is no way to lower the temperature so fast everywhere at the same time.
Typically the are frozen slowly with a cryoprotectant in the media, DMSO is the most commonly used in cell culture in research. The DMSO can hydrogen bond with water molecules preventing large ice crystals from forming. Ice crystals will rupture membranes killing cells.
Freezing (and successfully thawing) a single cell is just easy compared to trying to do that for an entire tissue of thousands of connected cells, let alone an entire organ of multiple tissues, not even an entire organism of many organs.
And sperm and eggs (but especially sperm) are, notably, extremely simple sorts of cells, having no other functional role in the organism that generated them other than being gametes. Granted, that is a complex task requiring specialized cellular machinery, but it is a once-and-done task rather than an ongoing metabolic process.
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