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Researchers have discovered a molecule present on the outer surface of a human egg that binds sperm and eggs together before fertilisation. Understanding this mechanism may help people with previously unexplained fertility problems.

'The details we've discovered here fill in a huge gap in our knowledge of fertility and we hope they will ultimately help many of those people who currently cannot conceive', said Professor Anne Dell of Imperial College London, who was involved in the study.

Researchers from several institutions worldwide, including Imperial College London, the University of Hong Kong, Academica Sinica, Taiwan, and the University of Missouri in the USA, performed the study on 195 unfertilised eggs.

Using a highly sensitive technique called mass spectrometry, the researchers identified a molecule called sialyl-LewisX (SLeX) on the outer surface of the egg, which is known as the zona pellucida. This 'sugar coat' causes the outside of the egg to become 'sticky', allowing the sperm to bind to it.

'This research provides an enlightening answer to a basic important question and human fertilisation - how does a sperm bind to an egg? But this is only a first step that will lead to more discoveries', explained Professor William Yeung of the University of Hong Kong who was involved in the study.

Women who lack SLeX on their eggs may find incoming sperm could fail to fuse with the surface of their eggs, preventing fertilisation. If SLeX is identified as a causal factor, these women could be treated using ICSI to artificially achieve fertilisation.

Dr Poh-Choo Pang of Imperial College London, who was also involved in the study, said: 'We hope that our study will open up new possibilities for understanding and addressing the fertility problems that many couples face'.

'Although clinical treatments are still a way off, we are very excited about the new research into fertility that we hope will now be possible, building on our work'.