It's freezing!

In these times of sub-zero temperatures (in both Fahrenheit and Celsius!) in Arkansas, we thought it might be fun to look at why we're not able to freeze ourselves and then thaw out alive at a later date.

One reason is that much of our body's cells is made of water.  When water freezes, it crystalizes, and crystals would puncture our cell walls.  Scientists have found ways to freeze some things like stem cells and embryos by putting them in a fluid that prevents water from crystalizing when it freezes, but they haven't been able to make it work for freezing more complicated life.  The fact that water expands when it freezes also causes irreversable damage to our cell walls (for example, frostbite).

Freezing cells slowly (a.k.a. supercooling) could be a solution as it can prevent intracellular freezing, but it only works in certain specific circumstances, so it isn't a suitable technique for freezing animals.  Another problem with supercooling our cells is that you can still end up with the frozen water crystal and beside it, the salts and everything else that was inside the cell.  This chemical process, purification through crystallization, of course dashes hopes of the cell coming alive again after being thawed.  To get around that, scientists freeze, for example, very small cells such as sperm cells in a special solution very quickly and the cells do come alive again when defrosted.  That seems to be the limit so far since this method doesn't work on cells even a little larger, such as blood cells, let alone a whole human.

Research in this area continues, though, because being able to freeze cells or to induce torpor (state of decreased physiological activity) in a human without causing damage would be very useful.  One example is for astronauts who could travel much greater distances without needing to take enormous amounts of food, oxygen, etc, with them into space.  Another example is being able to freeze organs used for transplants.  As scientists often do when faced with a problem, they look to nature for possible solutions.

As far as animals that don't migrate, we know that some warm-blooded animals survive through winter by hibernating.  Their body temperature drops considerably and they live off their fat reserves.  A black bear loses 15% to 30% of its weight during hibernation!  Research continues on how hibernation works.

Scientists have also found that some cold-blooded animals such as turtles and frogs hibernate (under ponds, mud, or leaves), with an interesting twist at the cellular level.  The fluid around their cells freezes, but not their cells, and they are able to survive sub-freezing temperatures using anaerobic metabolism (explanation below Resources and References at the end of this blog.)  Some fish have another strategy: unique enzymes that allow their bodies to continue functioning at colder temperatures.  Other fish, some teleost in polar regions, have "antifreeze proteins" that bind ice crystals in their blood to prevent widespread crystallization.

"Brown fat" in birds and other animals is another discovery that helps to explain how they survive in cold temperatures.  Brown fat cells are rich in mitochondria, they burn energy and create heat, and they are activated by cold.  Human babies have brown fat (which is why they don't shiver from cold until after about 6 months old), and human adults also have some brown fat.

Despite all these advances in knowledge about cryopreservation and hiberation, scientists haven't yet arrived at the stage where it can be applied to organs or people.  Well, as researchers and scientists continue in their quest for knowledge, we're working on keeping warm!

We wish you much warmth for the season, both physical and in your hearts.

Season's Greetings from Brittany, Nick, and the whole team

Resources and References:

• University of California at Santa Barbara on freezing humans - http://scienceline.ucsb.edu/getkey.php?key=3132
• ACS on hibernation in cold and warm blooded animals - https://www.acs.org/education/resources/highschool/chemmatters/past-issues/archive-2013-2014/animal-survival-in-extreme-temperatures.html
• World Scientific on fish antifreeze proteins - https://www.worldscientific.com/worldscibooks/10.1142/4917#t=aboutBook
• Live Science on what is brown fat - https://www.livescience.com/49652-what-is-brown-fat-facts.html
• Discover Magazine on wildlife surviving in winter - https://www.discovermagazine.com/planet-earth/how-does-wildlife-survive-winters-freezing-temperatures
• The Nature Foundation of Will County on how animals endure winter - https://www.reconnectwithnature.org/news-events/the-buzz/how-animals-endure-worst-winter-weather/
• Science Daily on wood frogs freezing and thawing cycles - https://www.sciencedaily.com/releases/2014/07/140722164359.htm
• National Park Service on wood frogs in Alaska - https://www.nps.gov/gaar/learn/nature/wood-frog-page-2.htm
• Science Direct on supercooling - https://www.sciencedirect.com/topics/earth-and-planetary-sciences/supercooling
• Appalachian History on wood frogs - https://www.appalachianhistory.net/2019/02/the-frog-who-freezes-solid-for-the-winter.html
• Business Insider on why babies don't shiver - https://www.businessinsider.com/babies-dont-need-to-shiver-2015-10
• Science Direct on supercooling - https://www.sciencedirect.com/topics/earth-and-planetary-sciences/supercooling
• December 2022 edition of Wired magazine article about hibernation research for space travel - "The Hibernator's Guide to the Galaxy"
• National Library of Medicine on cryopreservation - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5395684/
  

Cellular activity in cold-blooded hibernators:

Scientists have observed that water in between the cells freeze, but not water in the cells.  How does this work?  As water becomes ice outside the cell, the concentration of water inside the cell becomes greater and the water moves outside the cell.  This is due to osmosis, where water goes from an area where there is less solute (solute is a dissolved substance, such as salts and other things in or around cells) to where there is more solute.  There is more solute in the area around the cells relative to water because the quantity of liquid water there decreases as it turns into crystals.  Thus, water flows out of the cell to equalize the concentrated solution outside the cell.

At the same time, a large quantity of glucose migrates into the cells, and it acts as an antifreeze by lowering the freezing point of water.  Glucose is also the substance used for anaerobic metabolism (which is how the wood frog survives).  Thus, the animal's cells are protected even in sub-freezing temperatures. 

To note, when the wood frog, which uses this glucose (and urea) strategy in winter, defrosts in spring, it's the heart that defrosts first and starts beating.  Then the brain activates, and the legs start to work.