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Monday, September 8, 2008

Wood Frogs - freeze, thaw back to life

Some frogs (Rana Sylvatica) are able to be frozen and then thawed, and continue living.

Kingdom: Animalia
Phylum: Chordata
Class: Amphibia
Order: Anura
Family: Ranidae
Genus: Rana
Species: R. sylvatica

Frogs can survive multiple freeze/thaw events during winter if not more than about 65% of the total body water freezes. The terrestrial wood frog, Rana sylvatica, can tolerate sub-freezing environmental temperatures because of biochemical adaptations that protect cells from freezing.

the ability to endure the actual formation of ice within the body. Ice forming in body tissues can do a lot of harm. Ice crystals can puncture small blood vessels, squeeze and deform cells to the point of breaking, and scramble the micro- architecture inside cells so that upon thawing, organs are severely damaged. Freezing of the blood also interrupts the delivery of oxygen and nutrients to organs and so also causes severe metabolic damage. Any animal species that adopts a freeze tolerance strategy for winter survival must, therefore, find ways to overcome all of these types of injuries that can be caused by ice. Few animals like the Wood Frog can deal with horrendous structural damage that is caused by ice formation inside of cells, so freeze tolerant animals typically confine ice growth to extracellular spaces of their bodies while using protective mechanisms to keep the water inside their cells from freezing (extracellular freeze tolerance plus intracellular freeze avoidance). Their soft, water-permeable skin is no barrier to ice and so, whenever frost penetrates into their winter home, they freeze. Ice penetrates though all of the fluid compartments of the animal and within just a few hours a mass of ice fills the abdominal cavity encasing all the internal organs. Large flat ice crystals run between the layers of skin and muscle, and the eyes turn white because the lens freezes. Their blood stops flowing and as much as 65% of the frog's total body water is converted to ice. Breathing, heart beat, and muscle movements all stop and the frozen "frog-sicle" exists in a virtual state of suspended animation until it thaws.

Firstly, wood frogs and other freeze tolerant animals take active control over freezing. Rather than wait for spontaneous ice formation to begin, they employ special ice nucleators that actively seed ice formation in their bodies. In this way the animals can start freezing just below 0·C.

Second part of freezing survival is the same as that for freeze avoidance - the animals build up high concentrations of sugars or sugar alcohols in their tissues. In this case, however, the sugars are used to keep just the insides of cells from freezing, rather than the whole organism. So ice forms all around the outsides of the internal organs, sucking water out of them, but leaving behind a thick syrupy solution inside cells which can't freeze.

Third part to freeze tolerance is the need for all organs to survive through the freeze without any deliveries of oxygen or nutrients via the blood, which is frozen. The low body temperature during freezing helps, for metabolic rate is also very low, but all freeze tolerant animals have also enhanced the abilities of their organs to survive without oxygen.

Most freeze tolerant insects use the sugar alcohol, glycerol, for antifreeze as do their freeze avoiding insect cousins, but the wood frog, and other frogs, use glucose which is the normal blood sugar of all vertebrate animals. What is unique for wood frogs, however, is that they easily tolerate blood sugar levels that are 100-fold or higher than normal and show none of massive injuries that are suffered by human diabetics when their blood sugar rises by only 2-10 fold. Hence, these frogs may have some important lessons to tell us about how an animal can manage hugely high sugar levels in its tissues without ill effect.

Interesting aspect to freeze tolerance of animals which has not yet been explored is the fact that, while frozen, organ functions cease - heart beat stops, breathing halts: what signals these organs to shut off and how they are reactivated after thawing.

Finally, freeze tolerant animals also appear to enhance their body's damage repair mechanisms in order to deal with any physical injuries to organs that may be caused by ice while they are frozen. In wood frogs, for example, the levels of clotting proteins rise in the blood so that any bleeding that is detected during thawing can be quickly halted.


krishna kashyap av said...

Survival with evolution.
Great information..
Thanks a lot for the post.
Make website india

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