Scientists have uncovered exactly how a human egg captures an incoming sperm to begin the process of fertilization and say their discovery could in future help couples who suffer from infertility.

In a study in the journal Science, an international team of researchers found that a specific kind of sugar molecule makes the outer coat of the egg "sticky," helping the egg and sperm bind together.

"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 Anne Dell of Imperial College London, who worked on the study with scientists from the universities of Missouri and Hong Kong, and the Academia Sinica in Taiwan.

The World Health Organization estimates that infertility affects up to 15 percent of reproductive-aged couples around the world and almost one in every seven couples in Britain has problems conceiving a child for various reasons, many of which remain unexplained by medical science.

Scientists already know that a sperm "recognizes" an egg when proteins on the head of the sperm meet and match a series of specific sugars in the egg's outer coating. Once a successful match has been made, the outside surfaces of the sperm and egg bind together before they merge and the sperm delivers its DNA to the inside, fertilizing the egg.

In this new study, the researchers used ultra-sensitive mass-spectrometric imaging technology to assess which molecules were most likely to be key in the binding process.

They found that a sugar chain known as the sialyl-lewis-x sequence (SLeX) is abundant on the surface of the human egg, and after experimenting with a range of synthesized sugars in the laboratory they found that it is SLeX that specifically binds sperm to an egg.

To make sure, they then tested their findings using the outer coats of unfertilized "non-living" human eggs.

Dell said the research was enormously difficult "because human eggs are very tiny -- about the size of a full stop -- so we didn't have much material to work with."

Poh-Choo Pang, also from Imperial and who worked on the study, said that although clinical treatments derived from this discovery are a long way off, it could open up new possibilities for understanding fertility problems faced by many couples.

The researchers said they now want to use the findings of this work to further investigate the proteins on the head of a sperm that enable it to recognize an egg.