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A team of researchers has discovered the making of a "secret handshake" between sperm and the cells lining the uterus that finally let one sperm out of nearly 200 million to make it through and fertilise a single egg, say researchers.
Uterine cells express a receptor that recognizes a glycan molecule on the surface of sperm cells.
It's possible that this interaction may adjust the female's immune response and help sperm make it through the leukocytic reaction, said the researchers from University of California, San Diego.
The leukocytic reaction is not well understood.
What we do know, explained molecular anthropologist Pascal Gagneux, is that "after crossing the cervix, millions of sperm that arrive in the uterus are faced by a barrage of macrophages and neutrophils".
This attack by the innate immune system kills a majority of the sperm cells in semen, winnowing hundreds of millions of sperm down to just a few hundred that enter the fallopian tubes.
The defensive response may be beneficial in preventing polyspermy, when an egg is fertilized by more than one sperm and cannot develop.
"It's somewhat embarrassing how little we can say about what this [interaction] means," said Gagneux in a paper published in the Journal of Biological Chemistry.
Reproduction, he said, "is a very, very delicate tug-of-war at many levels. The fact that there is (also) this immune game going on is completely fascinating."