NEW YORK (Reuters Health) - Findings from an animal study suggest that the common practice of using sperm with intact acrosomes during in vitro fertilization actually increases the risk that the oocyte will become damaged or destroyed.

The acrosome, which covers the head of the sperm, contains a variety of enzymes that help the sperm penetrate the outer membrane of the oocyte. With normal in vivo fertilization, the acrosome never actually enters the oocyte, whereas with intracytoplasmic sperm injection (ICSI), it is directly injected into the oocyte along with the rest of the sperm.

There has been some concern that including the acrosome during ICSI might adversely affect the oocyte, according to the report in the September 19th 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 an acrosome-less sperm is used.

In the present study, Dr. Ryuzo Yanagimachi and Dr. Kazuto Morozumi, from the University of Hawaii School of Medicine in Honolulu, evaluated ICSI using murine sperm with and without intact acrosomes.

The target oocytes became deformed and lysed when three or more sperm with intact acrosomes were injected. By contrast, this adverse effect did not occur with acrosome-less sperm, regardless of how many were used.

Further analysis suggested that it was enzymes, like trypsin and hyaluronidase, that were causing the adverse effects seen when sperm with intact acrosomes were used.

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

Proc Natl Acad Sci USA 2005

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