Poisonous plants can be considered teratogens. For instance, the skunk cabbage Veratrum californicum, naturally found growing in meadows of the Rocky mountains, can cause severe birth defects in the offspring of sheep or cattle that have grazed on this plant. These birth defects include neurological damage and cyclopia, the fusion of two eyes into one. Humans and other mammals are also susceptible to this teratogen. The following picture depicts a cyclopic lamb that was born from a ewe who had eaten skunk cabbage during her pregnancy. As with many other teratogens, the mechanism underlying these defects remains to be elucidated.
from Gilbert, Developmental Biology, 4th ed.
Recently, a high incidence of deformed frogs has been reported in North America. Many of these frogs contain extra digits and limbs or are completely missing a limb. The scientific community is greatly alarmed by these findings and has been investigating possible causes of these developmental defects. Several hypotheses have been formed and among these is the thought that a chemical environmental pollutant may be responsible. Other potential causes for these deformities include ultraviolate radiation and parasites.
Recently, an article in Science (1999,Vol. 284;pp.802-804) has reported that a trematode parasite has the ability to induce frog limb deformities closely resembling those found in nature and is the most likely candidate for causing the limb abnormalities seen in the frogs in California. This is some of the first evidence giving scientists a possible cause for the limb anomalies that have been seen in frogs in recent years. This is indeed proof that a naturally occuring parasite has teratogenic potential.
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This metamorphosing tadpole owes its extra leg to Ribeiroia trematodes (inset), which are about 500 micrometers long. |
While researchers have shown that trematodes are the most likely cause of the limb deformities found in California, this may not be the cause of frog limb abnormalities found in other states. There are most likely more than one causative agent.
Everyone is exposed to a basal amount of radiation naturally. Only when radiation is used in excessive amounts will it be teratogenic. The majority of existing data in this area of teratology is from animal studies, as well as, studies conducted after the bombing of Hiroshima and Nagasaki, Japan.
Fetal exposure to radiation in adults may induce growth impairments, malignancy, and chromosome fragmentation. The same might happen to a fetus, as well as, impaired cell division, cell death, and mutations. A preimplantation embryo will most likely be killed by a small dose of radiation, as little as 10 rads. These newborns will present with a growth impairment (which will improve after birth), microcephaly, hydrocephaly, microphthalmia, optic atrophy, retinal displasia and cataracts. As you can see, the brain and eyes are severely affected. Less common effects are skeletal, visceral, and genital abnormalities. Small doses of radiation may alter developing germ cells, thereby postponing obvious defects to the next generation.
In the early 1900Ős, X-ray radiation was used to induce abortions. A single dose of 360 rads before the 14th week of gestation was enough to kill a fetus. It was finally published in 1929 that X-rays are teratogenic, linking X-ray exposure during pregnancy to small head size and mental retardation.
The bombing of the Japanese cities of Hiroshima and Nagasaki (in 1945) induced an increase in newborns with microcephaly and mental retardation. There was also a marked increase in fetal and neonatal death. Studies have since been able to link the incidence of microcephaly directly with the distance of the mother from the explosion of the bomb.
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References:
- Brent, R.L., Teratogen Update: Environmentally Induced Birth Defect Risks, ed. J.L. Sever and R.L. Brent. 1986, New York: Alan R. Liss, Inc.
- Miller, R.W. and M.J. J., Teratogen Update: Environmentally Induced Birth Defect Risks, ed. J.L. Sever and R.L. Brent . 1986, New York: Alan R. Liss, Inc.
- Stevenson, R.E., Human Malformations and Related Anomalies, ed. Stevenson, Hall, and Goodman. Vol. 1. 1993, New York: Oxford University Press.
- Warkany, J., Congenital Malformations. 1971, Chicago: Year Book Medical Publishers.
