Sperm alteration may enhance fertility

The findings from an animal study suggest that the common practice of using intact sperm in assisted reproduction procedures actually increases the risk that the egg will become damaged or destroyed.

Acrosomes are structures that cover the head of the sperm and contain a variety of enzymes that help the sperm penetrate the outer membrane of the egg. During normal fertilization, the acrosome never actually enters the egg, whereas in an assisted reproduction procedure called intracytoplasmic sperm injection (ICSI), it is directly injected into the egg along with the rest of the sperm.

There has been some concern that including the acrosome during ICSI might adversely affect the egg, according to the report in the early edition of the Proceedings of the National Academy of Sciences. In fact, in some animals, such as the hamster, ICSI is only successful when sperm without acrosomes is used.

Drs. Ryuzo Yanagimachi and Kazuto Morozumi, from the University of Hawaii School of Medicine in Honolulu, evaluated ICSI using mouse sperm with or without intact acrosomes.

They found that the target eggs became deformed and were destroyed when three or more sperm with intact acrosomes were injected. By contrast, this did not occur with acrosome-free sperm, regardless of how many were used. Further analysis suggested that enzymes were causing the adverse effects seen when sperm with intact acrosomes were used.

Unlike hamsters, humans and mice do not require acrosome-free sperm for ICSI to work, but using such sperm may still improve the effectiveness of the procedure, the authors conclude.

Proc Natl Acad Sci USA 2005.

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