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Why do you need 200 million sperm to fertilize a single egg?
One of the reasons is that sperm, when they arrive in the uterus, are facing a bombardment of the immune system. Perhaps, explains the molecular anthropologist Pascal Gagneux, it takes a lot for some to survive. On the other hand, it might be advantageous to eliminate as many sperms.
"I am a solitary zoologist in a medical school," said Gagneux. "What I think is that all life is a big compromise. [For an egg]being too easy to fertilize is bad; being too difficult to fertilize is also bad. "
Gagneux's laboratory at the University of California at San Diego has
discovered the formation of a "secret handshake" between sperm and
cells lining the uterus. Uterine cells, they report in the Journal of biological chemistry,
express a receptor that recognizes a molecule of glycan on the surface of
spermatozoa. It is possible that this interaction may adjust the woman's behavior.
immune response and help the sperm to cross the leukocyte reaction.
The leukocyte reaction is not well understood. What we know,
Gagneux explained, is that "after crossing the cervix, millions of sperm
– an American population of spermatozoa – that arrive in the uterus are
facing a barrage of macrophages and neutrophils. "
This attack of the innate immune system kills the majority of the sperm contained in the sperm, bringing back hundreds of millions of sperm to a few hundred sperm entering the fallopian tubes. The defensive response may be beneficial in preventing polyspermia, when an egg is fertilized by more than one sperm and can not grow.
As sperm are covered with sialic acid-rich glycans and the innate immune system uses sialic acid to differentiate human cells from invaders, Gagneux and his laboratory initially expected glycan to be involved in interactions with innate immune cells called neutrophils. But the neutrophils that they tested did not seem to see much difference between sperm with and without sialic acid.
At the same time, the team observed sialic acid binding receptors called
siglecs on the cells of the endometrium. In solution, these receptors of the endometrium
can bind to the whole sperm. According to Gagneux, the binding interaction
can help the sperm to operate this gantlet – for example, by cushioning the
immune response. Alternatively, this can be a way for the uterine cells to
eliminate defective sperm. In the immune system, this clbad of receptors helps
cells to recognize sialic acid molecules as "self," and in this context
they can increase or reduce inflammation.
"It's a little embarrbading to see the little we can say about what that means. [interaction] means, he says. The first step in understanding its physiological significance will be to look for a direct interaction between sperm and intact uterine tissue – this article only examined sperm interacting with purified proteins.
In some ways, added Gagneux, it's humbling to work in such a poorly understood field. Reproduction, he said, "is a very delicate struggle on many levels. The fact that there is (also) this immune game is completely fascinating. "
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