FROM FROZEN ICE BLOCK TO LIVELY AMPHIBIAN:
THE EMERGENCE OF THE WOOD FROG AFTER WINTER
by Anita Caveney
Reprinted from the April 2005 Cardinal
The Wood Frog in its frozen state is referred to as a “frogsicle” by biochemical researchers Ken and Janet Storey at Carleton University, who have studied the frog for more than 20 years (Svoboda 2005). Rana sylvatica spends its winters in a state of subzero sleep, with its tissues becoming so steel-rigid that “the legs are as easily broken off as a pipe-stem”, a description used by British explorer Samuel Hearne in the Canadian Arctic in the late eighteenth century.
However, the frogs are actually partially liquid and syrupy inside, thanks to some remarkable biochemistry, which is also a characteristic of Box Turtles. As the external temperature drops, the frog draws its legs in and tucks its digits under the body to keep them from desiccating over the winter. Below freezing, the frog’s metabolism comes to a near halt, allowing its cells to survive on negligible amounts of oxygen and energy. The liver starts to pump out glucose, raising concentrations in the bloodstream to more than 50 times those found in a human diabetic! As ice crystals form in the frog’s body cavities, water is drawn from the cells in the flesh and organs. This further concentrates the glucose inside the cells, causing it to function as an antifreeze that keeps the remaining water from solidifying. This allows the frog to remain in a torpid state until spring, when its metabolism kicks into high gear again. The Storeys think that this freeze-thaw cycle probably evolved in Wood Frogs during an ice age about 15,000 years ago. The cells in the moist, delicate skin were already optimized to prevent dehydration, and glacial conditions enhanced the process.
In other organisms, such high blood-sugar levels usually trigger glycation, a harmful process in which glucose molecules bind to the body’s structural proteins, among other things, causing cellular damage. This is not the case in Wood Frogs. The Storeys recently isolated a gene that short-circuits glycation. They have also identified genes that turn off metabolic processes, control cellular volume during freezing, and limit the damage that oxygen can do to cells when it flows into them again in spring. The researchers found high levels of messenger RNA coding for fibrinogen, a clotting factor, when they compared the livers of the frozen frogs to those of control frogs in a normal state. On activation by an enzyme in the bloodstream, fibrinogen forms fibrin fragments that mesh together into a sturdy lattice to seal any leaks induced by the stress of the freeze-thaw cycle in a blood vessel wall.
Svoboda, E. 2005, February. Waking from a dead sleep. Discover 26(2): 20–1.