Teratology

Teratology is the study of abnormalities of physiological development in organisms during their life span. It is a sub-discipline in medical genetics which focuses on the classification of congenital abnormalities in dysmorphology caused by teratogens and also in pharmacology and toxicology. Teratogens are substances that may cause non-heritable birth defects via a toxic effect on an embryo or fetus. Defects include malformations, disruptions, deformations, and dysplasia that may cause stunted growth, delayed mental development, or other congenital disorders that lack structural malformations. These defects can be recognized prior to or at birth as well as later during early childhood. The related term developmental toxicity includes all manifestations of abnormal development that are caused by environmental insult. The extent to which teratogens will impact an embryo is dependent on several factors, such as how long the embryo has been exposed, the stage of development the embryo was in when exposed (gestational timing), the genetic makeup of the embryo, and the transfer rate of the teratogen. The dose of the teratogen, the route of exposure to the teratogen, and the chemical nature of the teratogenic agent also contribute to the level of teratogenicity.

The term was borrowed in 1842 from the French tératologie, where it was formed in 1830 from the Greek τέρας teras (word stem τέρατ- terat-), meaning "sign sent by the gods, portent, marvel, monster", and -ologie (-ology), used to designate a discourse, treaty, science, theory, or study of some topic.

Old literature referred to abnormalities of all kinds under the Latin term Lusus naturae (lit. 'freak of nature'). As early as the 17th century, Teratology referred to a discourse on prodigies and marvels of anything so extraordinary as to seem abnormal. In the 19th century, it acquired a meaning more closely related to biological deformities, mostly in the field of botany. Currently, its most instrumental meaning is that of the medical study of teratogenesis, congenital malformations or individuals with significant malformations. Historically, people have used many pejorative terms to describe/label cases of significant physical malformations. In the 1960s, David W. Smith of the University of Washington Medical School (one of the researchers who became known in 1973 for the discovery of fetal alcohol syndrome), popularized the term teratology. With the growth of understanding of the origins of birth defects, the field of teratology as of 2015^[update] overlaps with other fields of science, including developmental biology, embryology, and genetics.

Until the 1940s, teratologists regarded birth defects as primarily hereditary. In 1941, the first well-documented cases of environmental agents being the cause of severe birth defects were reported.

Teratogenesis occurs when the development of an embryo is altered negatively due to the presence of teratogens. Teratogens are the causes of teratogenesis. Common examples of teratogens include genetic disorders, maternal nutrition and health, and chemical agents such as drugs and alcohol. Lesser known examples that will be covered include stress, caffeine, and deficiencies in diet and nutrition. Although teratogens can affect a fetus during any time in the pregnancy, one of the most sensitive time frames for them to be exposed to the developing embryo is during the embryonic period. This period is in effect from about the fourteenth day following when a female's egg is implanted into a specific place in the reproductive organs and sixty days after conception. Teratogens are able to cause abnormal defects through certain mechanisms that occur throughout the development of the embryo.

In 1959 and in his 1973 monograph Environment and Birth Defects, embryologist James Wilson put forth six principles of teratogenesis to guide the study and understanding of teratogenic agents and their effects on developing organisms. These principles were derived from and expanded on by those laid forth by zoologist Camille Dareste in the late 19th century:

The mechanisms of these teratogens lie in specific alterations to genes, cells, and tissues within the developing organism that cause deviation from normal development and can result in functional defects, growth stunts, malformation, and even death. Finally, susceptibility to teratogens is more elevated during specific, critical periods during development.

The natural metabolic processes of the human body produce highly reactive oxygen-containing molecules called reactive oxygen species. Being highly reactive, these molecules can oxidatively damage fats, proteins, and DNA, and alter signal transduction. Teratogens such as thalidomide, methamphetamine, and phenytoin are known to enhance ROS formation, potentially leading to teratogenesis