Stripped sperm more effective for ICSI
Dr. Kirsty Horsey
Progress Educational Trust06 October 2005
Scientists from the University of Hawaii School of Medicine have published results of a study showing that in intracytoplasmic sperm injection (ICSI), using sperm stripped of its acrosome results in fewer deformed embryos. When traditional IVF is unsuccessful, couples may take up the option of ICSI. In this technique a single sperm is first immobilised and then injected directly into an egg cell. While ICSI can sometimes work where IVF has failed, its success rate is still low. Scientists have now investigated the effect that removal of the sperm cap - the acrosome - can have on ICSI success rates. The acrosome contains a number of powerful hydrolysing enzymes that help the sperm to penetrate the egg's outer covering; in natural fertilisation these enzymes do not penetrate the egg itself.
The research team, led by Ryuzo Yanagimachi, conducted experiments intended to assess the potential hazards of sperm cap enzymes on developing mouse embryos. The investigators injected varying number of sperm, with and without the acrosome, into mouse eggs. They found that seven hours after injection using sperm where the acrosome was removed, none of the embryos were deformed, even when injecting up to seven sperm per egg. In contrast to this when only three fully intact sperm were injected per egg, up to one third of the resulting embryos were extensively deformed. Injections of pure acrosome-like enzymes had a similar adverse effect.
ICSI developer Gianpiero Palermo from Cornell University is not convinced by the findings. He points out that, in practice, labs carrying out ICSI only ever inject a single sperm per egg and that acrosomes of humans and mice are relatively small. Further to this, the process of sperm immobilisation, where the sperm tail is mashed prior to injection, acts to disrupt the cell membrane and results in the acrosome bursting. Acrosome enzymes are therefore unlikely to ever penetrate the oocyte and are unlikely to be the reason for low ICSI success rates.
Yanagimachi counters that electron micrographs have shown that the acrosome is sometimes visible inside ICSI embryos and that it takes around thirty minutes after sperm tails have been crushed to ensure that the acrosome is disrupted. As sperm are often injected immediately following tail disruption he contends that acrosome removal may improve the efficiency of ICSI in humans.
Reproduced with permission from BioNews, an email and online sources of news, information and comment on assisted reproduction and genetics.