Pollutants mature sperm prematurely
Hormone-like chemicals in food and pesticides may stop adult sperm fertilizing eggs, suggests a new study. Some think that the findings may partly explain falling fertility rates.
Scientists have long speculated that chemicals similar to female hormone oestrogen in food and pesticides could cut sperm counts. Most of the debate has centred on whether such compounds stunt the growing testicles in babies.
Now comes some of the first evidence that environmental oestrogens can stop sperm from adult men fertilizing eggs.
Researchers at Kings College London have found that mouse sperm bathed in low levels of the chemicals mature too fast.
In the female reproductive tract, sperm that mature too early are useless. "If [sperm] peak too quickly and haven't bumped into an egg they won't be able to fertilise," explains lead researcher Lynn Fraser.
But it is unclear whether chemicals eaten or absorbed by the body are transferred to the testes. "In order to be concerned we need to know whether environmental oestrogens get into sperm," points out male reproductive clinician Harry Fisch of the Columbia-Presbyterian Medical Center in New York.
Fraser's team exposed mouse sperm in test tubes to oestrogens from female mice and from soya, legumes, hops, beer and industrial products such as synthetic paints, herbicides and pesticides. The levels were roughly comparable to those that a person might have in their blood.
After 30 minutes, the majority of sperm treated with environmental oestrogens had undergone fast-track development. Many prematurely released the enzymes that enable them to punch through the egg's jelly coat - normally this happens only on contact with an egg.
After an hour, three-quarters of the hormone-exposed sperm were mature enough to fertilize eggs - compared to just a third of untreated sperm. The environmental oestrogens were more potent than natural oestrogen.
Fraser suspects that different environmental oestrogens could combine into an even more potent cocktail. "My suspicion is that low levels of three or four will have an additive effect on sperm function," she says. She presented the results at a meeting of the European Society of Human Reproduction and Embryology in Vienna this week.
Ten years ago, a group of reproductive biologists suggested that sperm counts had plummeted by as much as 50 per cent worldwide - and that environmental oestrogens might be to blame. Fears have also been raised by studies finding reproductive problems in animals exposed to pollutants.
Today, researchers are still arguing about whether sperm counts are falling globally or regionally, as studies have produced conflicting results.
A baby born with two good testicles is well off for fertility in later life
Fertility rates - the number of children born for every 100 women - have declined dramatically over the past few decades in industrialized countries. But other changes, such as unhealthy lifestyles and women having children later in life, may explain this.
Testing whether environmental chemicals affect either sperm counts or fertility is very difficult. Niels Skakkebaek, who studies reproductive health at the University of Copenhagen in Denmark maintains that these oestrogens are a potential problem: "It's definitely of concern that we are exposing ourselves to these agents."
But encounters with damaging compounds during fetal development, which could stunt sperm production for life, may have a greater effect on fertility than exposure during adulthood, when the testes are relatively resilient to damage. "A baby born with two good testicles is well off for fertility in later life," says Skakkebaek.
© Nature News Service / Macmillan Magazines Ltd 2002
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