A small frog found in the Alaskan boreal performs a miracle every spring, when it thaws from the totally frozen state in which it spends the winter. Scientists are beginning to understand how this miracle occurs and are excited by potential medical applications of the frog’s survival technique, which could revolutionise the science of organ transplants.
Campaigners for tropical forests, such as those of the Amazon basin, regularly cite the importance of retaining forest biodiversity for their many benefits to medical science. Tropical herbs are proclaimed as potential cures for everything from cancer to acquired immune deficiency syndrome (AIDS). Now boreal forest campaigners can point to the North American wood frog as the medical miracle of the boreal.
The wood frog (Rana sylvatica) is the most widely distributed frog in Alaska and it inhabits forest, musteg and tundra. To look at, it is an ordinary frog. It is smooth-skinned, up to 7 cm long, light brown or grey with a light stripe and often dark spots down its back. What makes it extraordinary is its incredible ability for deep hibernation. It can survive its body temperature dropping to –6oC and spends 2–3 months frozen each winter, which it spends in shallow pockets of dead vegetation. Its bodily functions cease, it does not breathe and its heart stops beating. The water in its body forms ice crystals. Then, in spring, the frog melts, and the seemingly impossible happens: its heart starts beating, it starts breathing again, and eventually it warms up and hops off to find a mate and breed in the pools of springtime.
Scientists who carry out organ transplants, such as life-saving kidney, liver and heart transplants, struggle to keep chilled organs in a usable state for more than a few hours. The science of ‘cryopreservation’ would be transformed if a human organ could be frozen for weeks and thawed out when it needed to be used. However, the received wisdom has always been that a living organ can only survive if the formation of ice crystals is totally avoided. The wood frog defies this, and annually survives crystalisation. How?
The secret to freezing is to do it very slowly. This gives time to try to avoid freeze-drying or dehydration of the body’s cells by flooding them with a ‘cryoprotectant’. The wood frog uses glucose, which prevents too much dehydration by binding strongly to the water molecules within each cell. As its body temperature drops the frog produces up to 100 times its normal level of glucose. The glucose keeps the cell structures intact while the freezing process draws up to 60% of the water from the cells to form ice crystals in the frog’s blood stream and body cavities.
Dr Kenneth Storey, from Carleton University, Ontario, Canada, has studied this process in wood frogs. Now Dr Boris Rubinsky at Berkeley University, California, USA, is developing a technique to mimic it to enable long term storage of donor organs. This involves flooding them with a cryoprotectant ‘cocktail’, to enable them to survive a long, slow freeze. He is currently perfecting the technique for the liver which has extensive networks of blood vessels for distributing the cocktail. Human trials of the technique have not yet begun but we will await the results of his experiments with interest.
Although the medical benefits from biodiversity may be used to make a potent argument for forest conservation, the fact that this research has involved animal experiments could pose a real moral dilemma for campaigners. |
