Fears that man-made chemicals which mimic female hormones are causing a loss in male fertility may be misplaced, following new research which shows that sperm actively need the female hormone, oestrogens, to thrive.
Since sex hormones were discovered earlier this century, oestrogens have been regarded as an intrinsically female hormone, and androgens (such as testosterone) the essence of masculinity. Oestrogens regulate the menstrual cycle in women, and cause the changes in female body shape at puberty. Androgens cause hair growth and the deepening of the voice in boys. Limited versions of the same change can be induced in the opposite sex by administering those hormones.
However, men and women naturally both produce small quantities of both hormones, which scientists now think are "sides of the same coin" in regulating many body functions. They now know, for example, that it is oestrogen, not testosterone, which determines when the bones stop growing. Men who do not produce any oestrogen keep growing.
The essential role of oestrogen in male fertility was discovered by a team at the University of Illinois, which found that mice genetically engineered to have no cells which are sensitive to oestrogen are sterile.
This finding, reported today in the science journal Nature, has important implications for worries about "environmental hormones" such as phthlates, found in some plastics packaging, and other pollutants.
Earlier this week a survey suggested that global male fertility - measured in sperm counts - has halved over the past 50 years. This has been ascribed to increasing amounts of "oestrogen mimics" which were blamed for affecting males.
Richard Sharpe, of the Medical Research Council's Reproductive Biology Unit in Edinburgh, commenting on the latest work, said that this would at last give scientists some basis for future work to see how solidly based these claims are.
"The huge, but exciting, task that we now face is to discover what oestrogens are doing at their many different sites of action in the male," Mr Sharpe said.
Rex Heff, who led the Illinois scientists, wrote in Nature: "This finding is important given the recent concerns over reported declines in human sperm counts and speculation that exposure to environmental oestrogens may be the cause of this."