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Why does it take 200 million sperm to fertilize a single egg?
One reason is that sperm, when they arrive in the uterus, face a bombardment by the immune system. Perhaps, says molecular anthropologist Pascal Gagneux, many are needed so that some will survive. On the other hand, there may be a benefit to culling so many sperm.
Zoologist - School - Gagneux - Elevator - Spiel
"I'm a lonely zoologist in a medical school," Gagneux said. "My elevator spiel is that all of life is one big compromise. [For an egg], being too easy to fertilize is bad; being too difficult to fertilize is also bad."
Gagneux's lab at the University of California, San Diego, has discovered the makings of a "secret handshake" between sperm and the cells lining the uterus. Uterine cells, they report in the Journal of Biological Chemistry, 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.
Reaction - Gagneux - Cervix - Millions - US
The leukocytic reaction is not well understood. What we do know, Gagneux explained, is that "after crossing the cervix, millions of sperm—a U.S. population worth 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 ****, 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...
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