Japanese scientists have discovered a protein in mouse sperm that is necessary for the fertilisation of eggs. Reporting in the journal Nature, the scientists, from the Genome Information Research Centre at Osaka University, say the protein is also present in human sperm, so their discovery could lead to new fertility treatments.

The research team, led by Masuru Okabe, bred male mice that had been genetically engineered to lack an immunoglobin protein - named Izumo after a Japanese shrine to marriage - that is normally found in sperm. The mice, although healthy and still able to produce sperm, were unable to fertilise female mice.

To fertilise an egg, a sperm cell must pass through an external layer of follicle cells, then penetrate the zona pellucida (the outer 'wall' of the egg), before binding to a plasma membrane. Sperm from the genetically engineered mice was able to overcome the first two obstacles, but was unable to fuse with the plasma membrane.

According to the research team, the discovery of the protein 'promises benefits in the clinical treatment of fertility and the potential development of new contraceptive strategies'. In an editorial accompanying the researchers' letter in Nature, Richard Schultz and Carmen Williams, from the Center for Research on Reproduction and Women's Health at Pennsylvania University, call the research 'compelling'. They add: 'These findings raise the exciting possibility that, because Izumo is sperm-specific and extracellular, this protein and its interacting partners could be new targets for non-hormonal contraception'. Dr Simon Fishel, a UK fertility specialist, said the study was 'fascinating and tantalising'. He added: 'They are identifying the actual sperm molecule involved in fusing with the egg - and that begins the whole process'.