Most creatures in the world have no choice but to let their body temperatures drop to low levels during the cold seasons. They have evolved various strategies to allow them to survive these ordeals and strategies are described by Mr Kenneth and Janet Storey in the May/June 1999 edition of The Sciences.
Only mammals, birds and a few other creatures generate enough heat from their body chemistry to enjoy a high body temperature. Other creatures must live with the cold. Normal human body temperature is 36 degrees, and you would probably die of hypothermia if your body temperature dropped below 28 degrees.
On the other hand, many creatures happily live in the deep ocean at temperatures between 2 and 4 degrees and others live on the surface of polar seas where winter temperatures drop to minus 2 degrees. Water normally freezes at 0 degrees.
The biological organisms and tissues we are most familiar with are badly damaged by freezing - even a mild frost can wreak havoc in the garden. The basic reason for the damage is that biological tissue is 75 per cent water and water expands when it freezes, bursting the delicate architecture of biological cells in the process.
To survive periods of extreme cold animals adopt one of two strategies - they either avoid freezing at all costs or they tolerate freezing. Most creatures opt for the former strategy and achieve this by manipulating some basic properties of water. Water freezes at o degrees, but this can be lowered considerably either by dissolving chemicals in the water, or by ensuring the water contains no particulate impurities. Employment of these devices can ensure water stays liquid down to temperatures as low as minus 4 degrees.
Impurities in water act as seeds that help ice crystals to grow rapidly, and so, removing impurities makes it more difficult for water to freeze. As hard winter approaches, creatures that need to avoid freezing, carefully clean their bodies of potential ice-seeders and also synthesise chemicals that dissolve in their body fluids and dramatically lower the temperature at which they will freeze.
We are all used to adding anti-freeze to our car radiators in winter in order to prevent the water from freezing - ethylene glycol is a common radiator antifreeze. Some insects synthesise ethylene glycol as antifreeze, but most make another sugar alcohol called glycerol. As the cold weather develops, insects gradually convert the carbohydrate built up over summer into glycerol, until as much as 20 per cent of total body mass may be made of glycerol.
Another type of antifreeze chemical synthesised by many species is antifreeze protein. This works by binding to small ice crystals and preventing further growth. These antifreeze proteins, supplemented by glycerol, can be powerful enough to protect many terrestrial invertebrates from freezing even though temperatures drop to minus 4 degrees or lower.
The second strategy employed to survive extreme cold is to tolerate freezing. This device is used, for example, by Canadian wood frogs and other animals that pass the winter frozen as solid as ice-pops, thaw out in spring and immediately begin to engage in frenzied mating.
I have already described the damaging effects of ice formation on biological cells. Animals that survive freezing use several devices to protect themselves. They are able to control how fast and where ice crystals form in their bodies, and they have developed an effective repair mechanism to deal with damage to the blood circulating system caused by freezing.
Animals that freeze adjust the freezing process so ice forms only outside cells and not inside them. Formation of the ice crystals outside the cells also has the effect of sucking water out of the cells and partially dehydrating them.
When ice crystals form, all non-water chemicals are automatically expelled from the ice and these enter the cells and build up in concentration. These chemicals act as antifreeze in the cells and prevent them from freezing. Up to 65 per cent of total body water in such animals will freeze, but the partially dehydrated cells remain fluid.
When such animals thaw out in spring some of the blood circulation is inevitably damaged leading to leakage of blood. However, these animals have enhanced blood-clotting abilities designed to plug and repair this damage.
Cryopreservation of human organs for extended periods while awaiting transplantation into recipients would be extremely useful, but, with the exception of thin tissues such as skin and cornea, this procedure is not possible at present. No whole organ has ever been frozen without losing its viability on thawing.
Organ transplants are now a race against the clock. For example, a heart remains viable only for four to eight hours after removal from the donor. In the US, 12 people die each day for the want of transplant organs and world wide, demand for organs grows by 15 per cent annually.
If organs could be frozen in such a manner as to retain viability on thawing, the organ transplant field would be revolutionised. It is to be hoped further study of how animals like the Canadian wood frog successfully freeze and thaw will allow this revolution to be achieved.
William Reville is a senior lecturer in Biochemistry and Director of Microscopy at UCC.