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In the beginning egg meets sperm. It is the critical, earliest moment in conception.

Now researchers at the Swedish medical university, Karolinska Institutet have produced a three-dimensional portrait of a complete egg receptor, the part of the egg that binds to sperm at the beginning of fertilization.

They describe the work in a new paper published in the journal Cell.

Conception begins when the sperm binds to proteins in the extracellular coat of the egg, known as the zona pellucida. Critical to this process is a receptor ZP3.

Using data from the European Synchrotron Radiation facility, scientists in Luca Jovine's research team produced detailed structural information on ZP3. The researchers say their findings may help us understand how mutations in the sperm receptor gene can cause infertility.

"The results give us a remarkable picture of the female side of fertilization," Jovine says. "But this is, of course, half of the story. The next step will be to tackle the corresponding molecules on the sperm that allow it to bind to the egg."

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