The endocrine system is important to the functions of almost every other system in an organism; it produces and distributes hormones, which in turn guide and regulate many bodily functions as well as controlling physical growth and maturation. Unhappily, the endocrine system is also vulnerable to external, chemical disruptors -- and because it is crucial, either directly or indirectly, to the proper workings of nearly everything in a living creature, these disruptors can and do have very, very serious effects.
The most major endocrine glands are the pituitary gland, pancreas, adrenal gland, and the testes and ovaries, the gonads for males and females, respectively. Each hormone secreted into the bloodstream has a specific purpose, and is specifically designed for use by certain cells, called target cells; though it might come into contact with other types of cells it will not affect them, because only the target cells are equipped with receptors to activate it.
An endocrine disruptor is any chemical substance originating outside the organism which in some way interferes with the endocrine system, impeding the flow of hormones throughout the body. This might happen through direct interference with the production of hormones, with their passage from endocrine glands into the bloodstream, or through impeding the natural breakdown of hormones once they have run their course and achieved their purpose in their target cells. A number of chemicals have been found to have adverse effects on endocrinal functions, specifically pesticides such as DDT, and substances that have been rendered illegal or are under heavy regulation for industrial usage such as PCBs and heavy metals.
As the endocrine system is crucial to the production of hormones that control the reproductive system, disruptors could have a devastating impact on wildlife in areas that are affected by them. Alligators in Lake Apopka, Florida, have proven to be a useful group for study, as the lake is known to be heavily polluted; up to 90% of the alligator population in that lake shows physical abnormalities that can be traced to the pollutants, compared to 5% showing abnormalities in a relatively unpolluted lake. Most female alligators lay between thirty-five and fifty eggs each year; on average, about 90% of these eggs hatch successfully. By contrast, only half the eggs laid in Lake Apopka hatch.
Hormone circulation is critical to the development of any embryo; two of the most important hormones during this phase are testosterone and estradiol. Where male alligators studied in Lake Apopka were found to have normal levels of estradiol, females were found to have only half the amount found in healthy alligators, as well as inordinately high levels of oestrogen.
This hormonal oddity resulted in abnormalities in the ovaries of the affected females: rather than only a single oocyte in each follicle of the ovary, having a single nucleus -- as is normal -- every single female alligator studied had more than one oocyte per follicle, and often even several nuclei per oocyte. This condition is associated with exposure to oestrogen during development; however, the lake itself contained no oestrogen, which leads scientists to believe that it was endocrine disruptors present in the pollutants which caused an excess of oestrogen to be produced and secreted.
Alarmingly, another effect of these pollutants is the loss of gender differences in the alligators. The elevated oestrogen levels have caused feminisation of males; during embryonic development, the levels of oestrogen exceed those of testosterone, impeding proper development of the penis and testes in the male. In some cases the resulting reproductive organs are inefficient and the alligator is sterile. Similar effects have been observed in freshwater fish.
Endocrinal effects on wildlife due to contamination are in no way limited to alligators in central Florida. The Great Lakes region of Ontario and the north-eastern United States, because of industrial activity, is heavily polluted with polychlorinated biphenyls, known endocrine disruptors; reproductive problems stemming from hormonal imbalances as well as other endocrine system-related dysfunctions such as enlarged thyroids are seen in fish and shorebirds in the area.
Humans are also vulnerable to endocrine disruption. During embryonic development, all vertebrates are similar; this kingdom comprises fish, birds, and amphibians as well as humans, and thus scientists are apply knowledge gleaned from research related to alligators, freshwater fish, and shorebirds to humans, at least in the embryonic stages.
The most immediately striking example of endocrine disruptors having profound and lasting effects on humans is that of the drug diethylstilbestrol, or DES, which was a synthesised oestrogen prescribed to pregnant women during the 1950s and 1960s, purported to prevent miscarriages. It failed to do so -- and it also was a direct cause of abnormalities in uterine and ovarian development of girls born to mothers who had used the drug.
Exposure to endocrine disruptors can occur through direct contact with the chemical or through breathing in contaminated air, drinking contaminated water, or eating contaminated food. Known disruptors include a number of pesticides, herbicides, fungicides, and fumigants, used in manufacturing, agriculture, or even in the home in various cleaning products and garden supplies.
Less direct contact occurs when endocrine disruptors are released as waste products from factories, or when some plastics are burned. Furthermore, they can leach out of certain soft plastics, including the type used in the fabrication of intravenous bags used in hospitals. The chemicals tend to accumulate and linger in fat tissues; thus, the greatest risk of exposure comes in eating fatty products and fish from contaminated areas.
The full extent of the effects of endocrine disruptors on the environment and wildlife, as well as on humans, is unknown. However, studies have linked them to declining populations in alligators as well as some species of marine invertebrates, which leads them to believe that this is a serious problem that will likely only get worse.
Endocrinology, specifically research on things that cause the endocrine system to malfunction, is a burgeoning field. There haven't yet been enough studies conducted to determine the full range of effects caused by endocrine disruptors, nor is there even an exhaustive list of substances that fall into this category.
Sources:
Crisp, Thomas M., et al. "Special Report on Environmental Endocrine Disruption: An Effects Assessment and Analysis". Washington: United States Environmental Protection Agency, 1997.
United States Office of Development and Research. "EPA Fact Sheet". Washington: United States Environmental Protection Agency, 1997.
Schmitt, C.J. and Bunck, C.M. "Persistent Environmental Contaminants in Fish and Wildlife". National Biological Service. 3 June 2004. http://biology.usgs.gov/s+t/noframe/u208.htm
Crain, D. Andrew, et al. "Alterations in Steroidogenesis in Alligators (Alligator mississippiensis) Exposed Naturally and Experimentally to Environmental Contaminants". Department of Zoology, University of Florida. 3 June 2004. http://ehp.niehs.nih.gov/members/1997/105-5/crain-full.html
Giullette, Louise J. "Impact of endocrine disruptors on wildlife". Center for Health Effects of Environmental Contamination. 3 June 2004. http://www.cheec.uiowa.edu/conferences/edc_2000/guillette.html
Sedlak, David. "Challenges associated with quantification of trace concentrations of pharmaceuticals in a complex matrix". Center for Health Effects of Environmental Contamination. 3 June 2004. http://www.cheec.uiowa.edu/conferences/edc_2000/sedlak.html
"Endocrine Disruptors FAQ". National Resources Defence Council (NRDC). 3 June 2004. http://www.nrdc.org/health/effects/qendoc.asp
"Pathophysiology of the Endocrine System". Colorado State University. 3 June 2004. http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/
Props to rootbeer227 for a factual correction.
Node your homework.