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【medical-news】卵子冷冻：生殖辅助技术的革命
21 March 2007
NewScientist.com news service
Rachel Nowak
It is a dilemma that haunts many 30-something, child-free women in the west: either have a baby soon or risk years of invasive fertility treatment later with no guarantee of a child in the end.
Yet a technique that allows some women to extend their fertility is making progress. That technique is egg-freezing, and in theory it could allow a woman to stop her biological clock for a decade or more. When she finds a partner, say, or is in a position to take a career break, she can have her eggs thawed and use routine IVF procedures to get pregnant.
However, egg-freezing is not being widely promoted for healthy women - in part, because fertility experts are in a quandary about which women should have access to it.
Everyone agrees that egg-freezing is a comparatively untested technology that has yet to be fully developed. Some fertility specialists believe that the responsible thing to do is to restrict its use to women with a clear medical need - for example, those about to have cancer treatment that will destroy their ovaries. Others say that egg-freezing offers women in their 30s who have to delay childbearing their best chances of success and that it should be made widely available.
"There are no guarantees. But a woman is more likely to get pregnant at 40 with one of her eggs frozen in her early 30s than with her 40-year-old eggs," says Gillian Lockwood, medical director of Midland Fertility Services in Aldridge, West Midlands, in the UK.
Peter Illingworth, medical director of IVF Australia in Sydney, is less certain. "I'm not sure this helps women. There's a lot of emotional attachment to this piece of frozen tissue, and six or seven years later it's going to be a blow when it doesn't thaw out," he says. Others worry that egg-freezing will give a false sense of security, discouraging women from finding ways to have children during their peak reproductive years.
Voting with their eggs
While debate continues, healthy women who are aware of the technology - and have around $10,000 to spare - are already stepping up to put their eggs on ice. One company - Extend Fertility, based in Woburn, Massachusetts - markets "insurance" egg-freezing to professional women and recently announced that seven babies have been born during the past 12 months. Other fertility clinics in the UK, Australia and the US discreetly offer egg-freezing to women who ask about it - despite a recommendation by the American Society for Reproductive Medicine that egg-freezing should still be considered an experimental technique and not generally offered to patients to extend their fertility.
"All the clinics I work for offer it, a lot of women enquire, and somewhere between 15 and 30 [women] a year do it," says Barry Behr of Stanford University, who is also consulting lab director of Huntington Reproductive Center based in Pasadena, California, and Conceptions Reproductive Associates in Denver, Colorado. No public records exist of the numbers of women who have used insurance freezing, but fertility specialists estimate that it is in the thousands worldwide.
The first live human births from previously frozen eggs occurred during the late 1980s. High failure rates, and concerns that freezing would damage chromosomes and cause birth defects, kept egg freezing on the sidelines for a decade, however. Those fears were reduced in the early 1990s when studies by Debra Gook at the Royal Women's Hospital in Melbourne, Australia, showed that the internal structure of the egg and the chromosomes looked normal in eggs that survived slow freezing - then the prevailing technique.
Relatively few eggs did survive freezing, though. "These are the biggest cells in the body. They contain huge amounts of water that form massive ice crystals. These are what burst the cells," says Gook.
However, recent tweaks to slow-freezing methods, such as altering the composition of the fluid the egg is suspended in before freezing, have meant that several teams around the world are now reporting thaw survival rates approaching 80 per cent and fertilisation rates equivalent to fresh eggs. Even so, for reasons that have yet to be fully explained, more embryos are lost before implantation than with conventional IVF.
Even better survival rates - and fewer losses at implantation - are being reported with a newer technique called vitrification, in which eggs are frozen so quickly that ice crystals cannot form.
To get an overview of egg-freezing success, a team led by Kutluk Oktay of the Center for Reproductive Medicine and Infertility in New York contacted every centre in the world that reported results of egg freezing using conventional methods between January 1997 and June 2005, and all but one returned data. The team found that the overall live birth rate for each embryo successfully created using a thawed egg and placed in a woman's uterus was 22 per cent, with a marked rise to 32 per cent between 2002 and 2004 (Fertility and Sterility, vol 86, p 70). Compared with fresh eggs, with which live birth rates in the US average 43 per cent for women under 35, that might not sound great, but ideally a woman would have multiple eggs on ice to compensate for the lower efficiency of thawed eggs.
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作者:admin@医学,生命科学 2011-01-09 05:14
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