Teratology

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.[8] These principles were derived from and expanded on by those laid forth by zoologist Camille Dareste in the late 1800s:[8][9]

1. Susceptibility to teratogenesis depends on the genotype of the conceptus and the manner in which this interacts with adverse environmental factors.
2. Susceptibility to teratogenesis varies with the developmental stage at the time of exposure to an adverse influence. There are critical periods of susceptibility to agents and organ systems affected by these agents.
3. Teratogenic agents act in specific ways on developing cells and tissues to initiate sequences of abnormal developmental events.
4. The access of adverse influences to developing tissues depends on the nature of the influence. Several factors affect the ability of a teratogen to contact a developing conceptus, such as the nature of the agent itself, route and degree of maternal exposure, rate of placental transfer and systemic absorption, and composition of the maternal and embryonic/fetal genotypes.
5. There are four manifestations of deviant development (death, malformation, growth retardation and functional defect).
6. Manifestations of deviant development increase in frequency and degree as dosage increases from the No Observable Adverse Effect Level (NOAEL) to a dose producing 100% lethality (LD₁₀₀).

Studies designed to test the teratogenic potential of environmental agents use animal model systems (e.g., rat, mouse, rabbit, dog, and monkey). Early teratologists exposed pregnant animals to environmental agents and observed the fetuses for gross visceral and skeletal abnormalities. While this is still part of the teratological evaluation procedures today, the field of Teratology is moving to a more molecular level, seeking the mechanism(s) of action by which these agents act. One example of this is the use of mammalian animal models to evaluate the molecular role of teratogens in the development of embryonic populations, such as the neural crest,[10] which can lead to the development of neurocristopathies. Genetically modified mice are commonly used for this purpose. In addition, pregnancy registries are large, prospective studies that monitor exposures women receive during their pregnancies and record the outcome of their births. These studies provide information about possible risks of medications or other exposures in human pregnancies. Prenatal alcohol exposure (PAE) can produce craniofacial malformations, a phenotype that is visible in Fetal Alcohol Syndrome. Current evidence suggests that craniofacial malformations occur via: apoptosis of neural crest cells,[11] interference with neural crest cell migration,[12][13] as well as the disruption of sonic hedgehog (shh) signaling.[14]

Understanding how a teratogen causes its effect is not only important in preventing congenital abnormalities but also has the potential for developing new therapeutic drugs safe for use with pregnant women.

Common causes of teratogenesis include:[15][16]

• Genetic disorders and chromosomal abnormalities
• Maternal health factors
  • Nutrition during pregnancy (e.g., spina bifida resulting from folate deficiency[16])
  • Metabolic disorders such as diabetes and thyroid disease
  • Stress

• Chemical agents
  • Prescription and recreational drugs (e.g., alcohol,[17] thalidomide[16][18])
  • Environmental toxins and contaminants (e.g., heavy metals such as mercury[19] and lead,[20] polychlorinated biphenyls (PCBs)[21])
• Vertically transmitted infections such as rubella and syphilis
• Ionizing radiation such as X-rays and that emitted from nuclear fallout

Alcohol is known to act as a teratogen.[28] Prenatal alcohol exposure (PAE) remains the leading cause of birth defects and neurodevelopmental abnormalities in the United States, affecting 9.1 to 50 per 1000 live births in the U.S. and 68.0 to 89.2 per 1000 in populations with high levels of alcohol use.[29]

In humans, vaccination has become readily available, and is important to the prevention of various diseases such as polio and rubella among others. There has been no association between congenital malformations and vaccination, as shown in Finland in which expecting mothers received the oral polio vaccine and saw no difference in infant outcomes than mothers who had not received the vaccine.[30] However, it is still not recommended to vaccinate for polio while pregnant unless there is risk of infection.[31] Another important implication of this includes the ability to get the influenza vaccine while pregnant. During the 1918 and 1957 influenza pandemics, mortality from influenza in pregnant women was 45%. Munoz et al. demonstrated that there was no adverse outcome observed in the new infants or mothers.[32]

Evidence for congenital deformities found in the fossil record is studied by paleopathologists, specialists in ancient disease and injury. Fossils bearing evidence of congenital deformity are scientifically significant because they can help scientists infer the evolutionary history of life's developmental processes. For instance, because a Tyrannosaurus rex specimen has been discovered with a block vertebra, it means that vertebrae have been developing the same basic way since at least the most recent common ancestor of dinosaurs and mammals. Other notable fossil deformities include a hatchling specimen of the bird-like dinosaur, Troodon, the tip of whose jaw was twisted.[33] Another notably deformed fossil was a specimen of the choristodere Hyphalosaurus, which had two heads- the oldest known example of polycephaly.[34]

Thalidomide is a teratogen known to be significantly detrimental to organ and limb development during embryogenesis.[35] It has been observed in chick embryos that exposure to thalidomide can induce limb outgrowth deformities, due to increased oxidative stress interfering with the Wnt signaling pathway, increasing apoptosis, and damaging immature blood vessels in developing limb buds.[36][37]

Retinoic acid (RA) is significant in embryonic development. It induces the function of limb patterning of a developing embryo in species such as mice and other vertebrate limbs.[38] For example, during the process of regenerating a newt limb an increased amount of RA moves the limb more proximal to the distal blastoma and the extent of the proximalization of the limb increases with the amount of RA present during the regeneration process.[38] A study looked at the RA activity intracellularly in mice in relation to human regulating CYP26 enzymes which play a critical role in metabolizing RA.[38] This study also helps to reveal that RA is significant in various aspects of limb development in an embryo, however irregular control or excess amounts of RA can have teratogenic impacts causing malformations of limb development. They looked specifically at CYP26B1 which is highly expressed in regions of limb development in mice.[38] The lack of CYP26B1 was shown to cause a spread of RA signal towards the distal section of the limb causing proximo-distal patterning irregularities of the limb.[38] Not only did it show spreading of RA but a deficiency in the CYP26B1 also showed an induced apoptosis effect in the developing mouse limb but delayed chondrocyte maturation, which are cells that secrete a cartilage matrix which is significant for limb structure.[38] They also looked at what happened to development of the limbs in wild type mice, that are mice with no CYP26B1 deficiencies, but which had an excess amount of RA present in the embryo. The results showed a similar impact to limb patterning if the mice did have the CYP26B1 deficiency meaning that there was still a proximal distal patterning deficiency observed when excess RA was present.[38] This then concludes that RA plays the role of a morphogen to identify proximal distal patterning of limb development in mice embryos and that CYP26B1 is significant to prevent apoptosis of those limb tissues to further proper development of mice limbs in vivo.

Plants can have mutations that leads to different types of deformations such as:

• Fasciation: Development of the apex (growing tip) in a flat plane perpendicular to the axis of elongation
• Variegation: Degeneration of genes, manifesting itself among other things by anomalous pigmentation
• Virescence: Anomalous development of a green pigmentation in unexpected parts of the plant
• Phyllody: Floral organs or fruits transformed into leaves
• Witch's broom: Unusually high multiplication of branches in the upper part of the plant, mainly in a tree
• Pelorism: Zygomorphic flower regress to their ancestral actinomorphic symmetry
• Proliferation: Repetitive growth of an entire organ, such as a flower