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Full Version: Teratogen Effects and Evaluation - Are they Toxic?
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Summary: All teratogens are toxins and not all toxins are teratogens

The drug or chemical with a potential to affect the fetus resulting in any malformation of the structure is called as teratogen. Teratogen is different from a toxicant as the former brings about any undesirable effect at the cellular level of the unborn fetus and during the organ formation and the later is known to affect at any stage of development. The effect of teratogen is not noticed once the organ formation of the fetus is completed. Hence Teratogen can be called as toxins of the fetus in the womb affecting the generative stage. The specialized subject to study the congenital malformation is called as Teratology and the substance responsible for causing malformation is called as teratogen.

There are various factors responsible for the congenital malformation. Heredity, presence of any particular infection in the pregnant women in early terms of pregnancy (Rubella infection is a good example), Nutritive deficit carrying mother, physical parameters like temperature and radiation and other environmental factors like pollutants present in air and water and chemicals are the factors to be considered while finding out the reason for either child born with defect or death of the fetus in the womb.

The teratogen acts in several ways. It affects the zygote resulting in the inhibition of further growth or cause mutation in the zygote. Teratogen is known to block any one of the steps in fertilization thus preventing the formation of the embryo. Also it has potential effect on the embryo, interferes in the cell division. The effect of teratogen is observed even at the time of implantation. Implantation is the process of embryo embedding in the endometrial lining of the uterus and exposure to teratogen at this stage may prevent the implantation resulting in the embryo leaving the uterus. Also teratogenic chemicals interfere in the organ development of the fetus causing malformation. Any undesirable effect on the fetus by a chemical after organ formation is completed is explained in the terms of toxicity and teratogen does not come into picture for the effects caused at this stage.

To identify and understand the teratogens affecting humans, the substance is initially experimented on laboratory animals. The selection of animal for this procedure is quite complicated as any animal tested should be closer to humans in the way that chemical is metabolized in humans, should have placenta similar to humans and the similar pregnancy term as that of humans and should be able to produce many offspring in order to understand the effects of the teratogen in subsequent generation. Also the size of the animal and the testing cost are other factors to be considered while selecting a test animal to study the effect of teratogens.

Also the route of administration of the drug under evaluation is important. The most preferred route of administration in the test animal is through peritoneal injection. As to evaluate the effect of different dosage of drug at different stages of gestation the time of injection of the drug to the test animal and the dosage selection are very crucial factors. After the full term the pups of the test animal are observed for three types of malformation like malformation seen visually, malformation in soft tissues and anomaly in the bone. The effects of teratogen are observed only in the stages of development whereas the effect of a toxicant is prenatal as well as postnatal.

Some of the drugs classified to be teratogenic to humans are Thalidomide, Vitamin D, androgen, estrogen, chemical agents used in chemotherapy and some of the antimetabolites and alkylating compounds. Few examples of compounds with teratogenic threats to humans are quinine, meclizine, phenmetrazine, tetracycline and cyclophosphamide. Examples of the beneficial teratogenic drug detected in animals are salicylates, Vitamin A, Vitamin D, adrenocorticoids, insulin and sulfonylureas.

It is not necessary that all the teratogens are chemicals/drugs, even radiation, some metals, minerals and pesticides are teratogens. X-ray is identified as a potential teratogen. Teratogenic effects of molybdenum, lead, zinc and manganese were identified in different animals. There is no evident of Teratogenic effect of pesticides in humans. The study by Green and team in 1967 reported the teratogenic effect of DDT, heptachlor, aldrin and endrin in eggs of chicken.

In simpler terms it can be explained as 'All teratogens are toxins and not all toxins are teratogens'.
You have shared very interesting information about the teratogens, i hope this content will become more useful for the people, those who are in pregnant. Keep sharing this type of useful things for us.