Plastics chemical, Bisphenol A, harms eggs in unborn mice
Female mice exposed to a common chemical found in plastics while in the womb develop abnormal eggs, according to a new study. Based on this finding, researchers speculate that the chemical, bisphenol A, might increase the risk of spontaneous abortion and genetic disorders in humans, such as Down's syndrome.
Bisphenol A, or BPA, is commonly found in hard plastics and the lining of tin cans. The chemical has come under scrutiny before because it can mimic the hormone oestrogen.
Patricia Hunt at Washington State University in Pullman, US, and colleagues exposed pregnant mice to 20 micrograms of BPA per kilogram of their bodyweight each day over a one-week period. During this same period, the reproductive cells of female mice developing inside in the womb begin a process of division known as meiosis.
Meiosis ultimately yields mature eggs, which have half the number of chromosomes as other cells in the body. But the process is gradual. In humans, for example, cells only reach the end of this division process before ovulation.
Researchers followed the female mouse pups that had been exposed to BPA in the womb. When these mice reached adulthood, Hunt's team examined their eggs. More than one in every 20 cells taken from those mice had unpaired chromosomes – an abnormal trait. This trait was completely absent in cells taken from the BPA-free control mice.
An analysis of slightly more mature eggs taken from another group of females exposed to BPA in utero found that 10 eggs out of 56 sampled had one or more extra chromosomes than normal. In contrast, only one out of 57 such cells from a control group showed the same defect.
By staining the cells, the researchers found that the chromosomes in eggs from BPA-exposed mice connect in odd places during cell division. This might explain why they do not segregate properly during meiosis.
The embryos of mice exposed to BPA in utero also show abnormalities, suggesting that the chemical affects the grandchildren of exposed mice – a "grandmaternal" impact.
Hunt is concerned that BPA might disrupt egg development in humans as well. The types of chromosomal defects seen in her mice are known to increase the risk of spontaneous abortion and genetic disorders such as Down's syndrome.
"It's very concerning," agrees Dorothy Warburton at Columbia University in New York, who was not involved in the study. But she notes that measuring exposure to BPA in humans "is a not an easy thing to do".
The US Environmental Protection Agency has declared that exposure to 50 micrograms or less of BPA per kilogram of bodyweight is safe for humans.
Steve Hentges of the American Plastics Council in Arlington, Virginia, US, stresses that this guideline refers to eating or drinking BPA. He believes that the unborn mice were exposed to higher levels of BPA because they received it through implanted pellets rather than through their food.
For this reason, Hentges argues against drawing conclusions about human health from the study: "These results do not seem to be relevant for reproduction and in particular fertility." He adds that other mouse studies that involved BPA levels similar to those used in Hunt's experiments did not show an increased risk of genetic abnormalities across generations.
However, Hunt is convinced that the BPA amounts used in her study are equivalent to current human exposure. She notes that previous research has found that mice exposed to the chemical in utero have abnormal prostate and breast tissue development that could put them at greater risk of developing cancer in these tissues.
Journal reference: PLoS Genetics (DOI: 10.1371/journal.pgen.0030006)