You can forget about the birds and the bees. If you really want to learn how babies are made, you need to know about Juno and Izumo.
Fertilisation takes place when an egg cell and a sperm cell recognise one another and fuse to form an embryo. But how they recognise each other in order to hook up had remained a mystery.
Researchers said on Wednesday they have identified a protein on the egg cell's surface that interacts with another protein on the surface of a sperm cell, allowing the two cells to join.
This protein, dubbed Juno in honour of the ancient Roman goddess of fertility and marriage, and its counterpart in sperm, named Izumo after a Japanese marriage shrine, are essential for reproduction in mammals including people, they said.
This new understanding of the role of these two proteins could help improve the treatment of infertility and guide the development of new contraceptives, the researchers said.
"By identifying this interaction between Juno and Izumo, we now know the identity of the receptor proteins found on the surface of our father's sperm and our mother's egg that must interact at the moment at which we were conceived," said Gavin Wright of the Wellcome Trust Sanger Institute in Britain, one of the researchers in the study published in the journal Nature.
The researchers are now screening infertile women to investigate whether problems with the Juno receptor are to blame.
"It is remarkable that about 20 per cent of infertility cases have an unexplained cause," said Enrica Bianchi of the Sanger Institute, another of the researchers. "We are now asking whether Juno is involved in these cases of unexplained infertility."
Wright said that if defects in the Juno receptor were implicated in human infertility, a simple, non-invasive genetic screening test could be developed to identify affected women.
Japanese researchers identified the sperm cell's Izumo protein in 2005, but its egg-cell counterpart had been elusive.