Technology: Organ transplants prepare for the big chill

 作者:萧独俄     |      日期:2019-03-02 01:18:00
By ANDY COGHLAN A sophisticated chemical which acts like antifreeze is being developed by American researchers who believe it could soon preserve human organs outside the body indefinitely. Greg Fahy of the American Red Cross in Bethesda, Maryland, said last week that he and his colleagues hope to have developed a method for preserving rabbit kidneys – which are very similar in tissue type and structure to human ones – by Christmas. ‘We are very close,’ he told the annual meeting of the International Cryobiology Society in Atlanta, Georgia. The transition from preserving rabbit to human tissue should not require a great leap, he added. At present, organs for transplant only survive for a few days after being removed from donors. According to Jim Southard, president of the society, an organ can last three or so days in orthodox, refrigerated storage, or up to seven days when suffused with a special solution that he developed a few years ago at the University of Wisconsin. This means that transplants are often a race against time, especially if the organ required is delicate or rarely transplanted. If organs could be kept indefinitely then local supply could better match demand. Fahy uses a technique called cryopreservation. This is already widely used to preserve certain kinds of mammalian cells, including blood cells, and will even preserve very early mammalian embryos, including humans, when the cells are all similar and have not yet taken specific functions. The key to the technique is a sudden drop in temperature, which ‘shocks’ the live tissue into a kind of suspended animation. In theory, it can be revived simply by warming. But previously it has proved impossible to preserve whole organs, because ice crystals form in the cells during freezing, literally tearing them apart. For this reason there is little chance that any of the people in the US who have been frozen before death, hoping to be preserved for future revival, can be revived successfully. The key to the success of the Red Cross team is cryopreserving kidneys without freezing them. ‘We can take the organ and maintain the water in it in a liquid state using special cryopreservants,’ says Fahy. ‘They have the same effect on the water as antifreeze in your car – they take the freezing point down so far that you never run into it.’ Instead of freezing, the water in the organ becomes vitrified, at around -125 degreeC, and behaves like glass. Fahy explains: ‘It does not flow, it’s like a supercooled liquid.’ In getting vitrification to occur, Fahy and colleagues have to balance two phenomena that are both potentially harmful, yet necessary – at least for now. The first is the strength and toxicity of the cryopreservant; the other is the pressure applied to maintain the vitrified state. Under sufficient pressure, the freezing point of water is greatly reduced, helping to prevent ice forming in the organs. The cryopreservant consists of a mixture of propylene glycol (the ‘antifreeze’ in the blend), formamide (which is toxic) and dimethylsulphoxide. The propylene glycol is good at inducing the glassy state, while the dimethylsulphoxide appears to limit the toxicity of the formamide. ‘The cryopreservant mixture must be able to get into the kidney cells, and must be gentle towards them,’ Fahy says. He has had most success with very strong solutions of eight moles per litre of the cryopreservant, under pressures of 500 atmospheres. But he and colleagues are confident that they will soon find a cryopreservant which works at atmospheric pressure. ‘We are 95 per cent of the way to a composition where we would not need to apply pressure,’ he says. The Red Cross researchers have also had trouble thawing the kidneys, which has to be done very quickly, but they now have access to an electromagnetic warming technology that they are confident will solve the problem. ‘There are analogies with microwaving, but it’s not quite the same,’ he says. The method is confidential. Fahy says the technique had some success on rat livers and rat hearts. ‘It appears to work across tissue and species barriers,’ he says. If the technique proves successful, it might also be possible to treat kidney tumours outside the body using intensive chemotherapy, while the patient survives on a dialysis machine. Once the tumours have been eradicated,