Don't blame the father

时间:2019-03-08 08:18:13166网络整理admin

By Nell Boyce CHILDREN conceived by an artificial fertilisation technique in which a sperm is injected into an egg have higher rates of genetic abnormalities than children conceived naturally. Now researchers say the technique itself, rather than faulty sperm from infertile men, may be to blame. The finding has emerged from the first studies of the technique, known as intracytoplasmic sperm injection (ICSI), in primates other than humans. “Ironically, the babies are the guinea pigs and the monkeys are the next step,” says Gerald Schatten of the Oregon Health Sciences University in Portland, who led the new investigation. Worldwide, more than 20 000 children have been conceived with the help of ICSI since 1992. The technique was first used for men whose sperm were unable to fertilise an egg, but it is increasingly replacing conventional IVF even when the sperm are healthy, as it guarantees fertilisation. Some large medical centres already use it for more than half of all IVF procedures. One study of ICSI children showed they have twice the incidence of birth defects compared with children conceived naturally (This Week, 22 November 1997, p 5). Such birth defects include missing or extra sex chromosomes. A possible explanation for this is that many men who donated sperm would have been infertile, and so more likely to have damaged sperm. But now Schatten and his team have shown in tests using eggs and sperm from rhesus monkeys that the technique itself might cause the damage. The team found that ICSI can puncture the meiotic spindle, a net that surrounds chromosomes and draws them to the proper place during cell division (Nature Medicine, vol 5, p 431). Most fertility centres use a structure called the polar body to work out where the spindle is so they can avoid hitting it, but Schatten found the polar body can move around, suggesting it is not a reliable guide. The study also showed that with ICSI, sperm retain packaging proteins and protective coats that don’t normally enter the egg, and that cell division is delayed because the sperm DNA remains too condensed. “The egg can sense that `look, we’re not properly set up to continue with DNA synthesis’,” says Schatten, who adds that no one knows if the delay poses any risk. Experts in male fertility welcome Schatten’s work, although they say it only confirms suspicions and is unlikely to affect the popularity of ICSI. Dolores Lamb of Baylor College of Medicine in Houston, Texas, says she is not surprised that sperm retain protective proteins, because the act of injecting a sperm bypasses the processes in which the egg removes some proteins. Gianpiero Palermo, a pioneer of ICSI at the Cornell Medical Center in New York City, says that deviations from natural fertilisation need further investigation. However, he says he has always assumed that ICSI would sometimes hit the meiotic spindle surrounding chromosomes: “We weren’t so naive.” Palermo argues that the high rate of chromosome abnormalities in ICSI children could still be accounted for by abnormal sperm. “In the largest centres, like ours, I think it’s pretty safe,