eli5 Why can human embryo’s be frozen but not human adult?



eli5 Why can human embryo’s be frozen but not human adult?

In: Biology

It’s much harder to flash freeze lots of cells at once. Adults are made up of 30 to 40 million cells. Frozen embryos are 100-120. Freezing a small number of cells lets you control more of the things that could go wrong, like the forming of ice crystals that damage cells.

EDIT: I’d intended to type 30 to 40 trillion, but either blanked on it or had an auto correct. Likely that I had a brain fart moment. That number is the number of cells which are genetically “you”. There are also trillions which are not “you” but are a part of the operation of your body and its biological functions.

Have you ever heard the story of Birdseye? It’s not just a frozen food company that has frozen corn in the grocery store, it is named after a man that revolutionized the food and agriculture industries by developing a way to freeze produce without turning it into mush.

When water freezes it expands, which can rip apart the cells that make up people, or corn, or anything. The result is they are damaged at the time of freezing, and when thawed they are mush. Not good for corn or for people.

Birdseye championed flash freezing after noticing that the speed with which something is frozen, directly affects whether the freezing process will damage it or not. By flash freezing produce, the ice crystals don’t have time to grow large and damage the cells.

That’s step 1, you have to be able to freeze something without the freezing process causing damage. That means it has to be frozen very fast. This is easier to do to something very small as there is not much latent heat to be removed from it. The larger the thing you want to freeze, the longer it will take to freeze. An embryo can be frozen in seconds. A human being might take hours even in extreme cold.

Step 2 is you have to later reverse this and thaw the frozen item without damaging it. In the case of cells, an embryo, some small animals, their small size again helps them thaw quickly so that all of it thaws more or less at the same time. It wouldn’t do well for one part to still be frozen, while the rest had thawed.

For something large like a person, imagine thawing. The limbs would thaw first, even the head, while the main part of the body containing the heart, the lungs, would stay frozen for far longer. So you have thawed limbs and a thawed brain, not getting blood, not getting oxygen, because the heart is still frozen solid. That’s no good. So you would need a way to warm the body at the same rate, so that everything thawed at once. Very tricky to do with something as large as a person.

Finally remember that an embryo is not the same thing as a fetus. It’s just a bundle of cells, not much different from freezing an amoeba or cell cultures. It doesn’t have a heart or a brain to become damaged. Just some cellular walls.

Back in the 80s they were doing scientific experiments on freezing and heating hamsters.
They had problems with the reheating process which left burns on the hamsters.
To solve this, they developed the microwave.

Anything larger than a hamster (ie rabbit) could not be frozen quick enough to survive the freezing and warming process.

Ergo, it won’t work on humans. But hamsters can enjoy the trip.

Because as everybody know, once you defrost something is totally not good to frost it again!

The bigger a living organism is, the harder it is to freeze it quickly enough to not permanently damage it.

This is because, generally speaking, you can only cool the surface of something, but you have to cool its whole volume.

If you have something that you can freeze and you make it twice as big, the new, bigger thing has ~8x the amount of material to cool, but only ~4x the surface area to cool it with. So it’s effectively twice as hard to cool down.

This just gets worse and worse as you get bigger and bigger. Freezing an embryo is thousands of times easier than freezing a grown adult. There were experiments decades ago where hamsters were frozen and revived, but that’s about as large as can be done with living things on Earth.

I’m assuming you’re talking about reanimation.

Tom Scott recently published a very interesting video, where he interviewed a scientist tangentially involved in the invention of the microwave, he was using it to experiment on reanimating mice. The short answer to your question, is that they were only able to get it to work for small things – it worked for some of the mice, but for larger creatures it doesn’t work well, and that goes for both the reanimation process and also for the freezing process.

it’s a function of limiting current technology. the concept is more or less simple, and we’ll get there. there was an ELI5 question the other day asking why freezer burn exists. similar reasoning, and avoiding that is key.

My actual eli5:

Embryos are super tiny with a few simple parts. You can freeze/defrost it fast enough that nothing breaks.

Adults have tons of complex parts that would break if you froze them, and unfreezing them would break more since it takes so long, and they all have to be working at once or it doesn’t work.

I would then take a bit of an ice cube out of a freezer, and sit it on the counter next to some frozen piece of meat. In ten minutes I’d show them the water from the sliver of ice cube that already melted, and then knock the frozen meat off of the counter to get a good thud. Maybe even show them in an hour how only a small part of the outside is even remotely defrosted. No good, you see, because everything in an adult has to be working at once. We can’t magically make it unfreeze all at once. That is one dead adult we’re eating for dinner. (Bonus points if you’re a cannibal and you actually used a frozen adult for this).

Simple small thing, ok. Big complex thing, not ok.

It skips some details, but, it gets the job done.

Anything can be frozen, it’s the unfreezing that becomes the problem. In my lab, I would freeze cells all the time as a kind of record of progress. You add special buffers to prevent ice crystals from tearing the cells apart and then cool them down to eventually -80 C, and then you can cryogenic freeze them in liquid nitrogen. You slowly “unfreeze” them in crushed ice / water slurry which is just at about 0.2 C. You can’t do that with organs or I should say we haven’t figured out how just yet. We do transport chilled organs, but they aren’t frozen. If we could do that organ donations would much, much less of a logistics and timing issue.

I have seen goldfish that where left outside for some reason and freeze solid in the ice. The next day was very warm and the ice melted and the fish started swimming again for a while… yet eventually died, so it is possible. No clue how it happened at the cellular level.

edit: words

I read that they were once successfully able to freeze and unfreeze dogs, half the dogs had brain damage but still.

The larger the organism they harder it is to avoid complications.


There’s a relevant Tom Scott about this, which actually relates to the invention of the microwave. Basically, anything bigger than a small rodent cannot reheat quickly and evenly enough to be able to reanimate.


Size. The freezing and defrosting process of something small like an embryo is vastly quicker and easier than an adult.

The biggest thing they have managed to freeze and defreeze is a hamster.

There’s just noway to get the defreezing agent into the tissue cells quick enough. It would invariably lead to cell death and probably death death.

Either by not entirely defreezing the person or by defreezing them and cooking them in the process.

Isn’t there a company that freezes whole people?