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Evironmental Sentinels

DDT Exposure
  On the left, a healthy frog; on the right, the same species after exposure to DDT. [Click for a larger image.] Photo courtesy of Dr. Tyrone Hayes.
  RealVideoDr. Hayes explains why we should worry about the presence of artificial hormones in the environment. [Need help?]

Frogs and other amphibians live most of their lives in water, making them very sensitive to waterborne chemicals, which are absorbed through amphibians' permeable skin. For this reason, frogs often give the first warning of toxic pollutants in the environment. Some of these pollutants, such as DDT and PCBs, have long been recognized as dangerous. But recently, synthetic hormones or hormone-like substances have started to alarm some scientists and environmentalists. Of particular concern are androgens, male hormones which increase muscle mass and deepen the voice in men, and estrogens, female hormones which stimulate breast development and regulate menstrual cycles in women.

Dr. Hayes discusses other pollutants in the evironment that can affect development and reproduction in frogs and humans. [Need help?]

"Estradiol and other synthetic hormones are very stable and make it through sewage treatment," Hayes says. "Estradiol, the hormone in birth control pills, acts just like estrogen. It's an interesting compound; you can't be a female and reproduce without it. On the other hand, estrogen and estradiol can have dangerous side effects, such as breast and uterine cancer."

Some surprisingly common substances, such as the plastic in baby bottles and the pesticides in crop sprays, contain chemicals that mimic the action of hormones. Women exposed to androgens, the male hormones, can develop facial hair, for instance.

On the bottom, a normal tadpole. The one on top has been exposed to the human hormone androgen, and will never develop into a frog. [Click for a larger image.] Photo courtesy of Dr. Tyrone Hayes.

The sex hormones that control reproduction and development in humans control many of the same functions in frogs. The effects are most critical to both species during early development. For example, some tadpoles exposed to androgens never develop into adults. "If you saw them in the field," says Hayes, "they look like big healthy tadpoles, but they never develop into frogs. The hormone inhibits their thyroid gland so they're stuck in the tadpole stage." [See image above left.]

RealVideo Dr. Hayes explains what frogs can tell us about determining safe levels of pollutants in the environment. [Need help?]

In the same way that frogs are exposed to pesticides and hormones in the environment, people are also exposed. Studies have shown that women who eat fish from the Great Lakes, for example, are exposed to significant levels of PCBs and pesticides. "These compounds are toxic in themselves," says Hayes, "but they can go past the placenta and expose the neonate to compounds that can affect fetal development."

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