Early spring, if you're willing to plod through snowbanks in northern woods, you just might get to watch a miracle unfold. As shallow ponds melt and then quickly freeze again, a tiny brown wood frog may get locked in the ice, just a few inches beneath the surface. Crouched as if contemplating a jump it will never get to make, the frog becomes frozen stiff. Its heart stops; its breathing ceases. But when the sun thaws the pond again, the frog can thaw, too. Ice melts in its body, its heartbeat returns, and its blood begins to circulate anew. The creature gulps for air and then, as if hours or days of suspended animation were just a small inconvenience, it hops away.
Turning into a block of ice might seem an odd way to cope with frigid temperatures, but for the wood frog and a number of other cold- blooded animals, it's an effective one. And it may one day prove effective for humans, too--or at least for human organs. By co-opting the physiological strategies frogs have evolved to survive freezing, a number of researchers think they can find ways to preserve human hearts, livers, and other organs for transplantation.
Right now a kidney can be kept alive for only three days, in a bath of proteins, enzymes, and ions at 4 degrees above freezing; a liver can be kept no more than 36 hours, and hearts and lungs rarely survive more than 6. Those numbers might be extended from hours and days to months and years if the bath's temperature could be dropped, but to attempt to freeze and then thaw a human organ is to drag it through a physiological minefield. As an organ freezes, ice crystals stab and rip into blood vessels, cells dehydrate, and their membranes collapse and leak, destroying the vital proteins inside. Thawing can be even more dangerous. As the temperature creeps up toward the melting point, the ice crystals fuse, squeezing the cells into tighter and tighter spaces and deforming them. That's why, despite researchers' success in freezing single cells like sperm and blood cells, and even simple tissues like corneas and immature embryos, freezing and restoring whole organs, with their complicated and vulnerable architecture, remains largely beyond reach.
But with an engineer's conviction, Boris Rubinsky, a Romanian- born scientist, insists that the minefield can be cleared if we can just learn enough about the terrain. To that end, ten years ago this bioengineer invented a microscope to study in exquisite detail how cells freeze and thaw. With his sensitive instrument he's discovered a new way to protect mammalian organs from the deadly damage that can occur at cold temperatures. And last year, with recipes picked up from a range of cold- tolerant animals, he and his colleagues at the University of California at Berkeley managed to freeze mammalian livers for six hours and revive them. That's not exactly stopping the hands of time, but it's certainly slowing them down.
