Teratogenesis

Baby born with deformities 

The effect produced by any Teratogen depends upon the developmental stage in which the foetus is exposed to the agent. Several important phases in human development are recognized:

• The time from conception until implantation known as the "all or none" period, when insults to the embryo are likely to result in death of the conceptus and miscarriage (or resorption), or in intact survival. At this stage, the embryo is undifferentiated and repair and recovery are possible through multiplication of the still totipotential cells to replace those which have been lost. Exposure of embryos to teratogens during the preimplantation stage usually does not cause congenital malformations, unless the agent persists in the body beyond this period.

• The embryonic period, from 18 to 54-60 days after conception is the period when the basic steps in organogenesis occur. This is the period of maximum sensitivity to teratogenicity since not only are tissues differentiating rapidly but damage to them becomes irreparable. Exposure to teratogenic agents during this period has the greatest likelihood of causing a structural anomaly. Since teratogens are capable of affecting many organ systems, the pattern of anomalies produced depends upon which systems are differentiating at the time of teratogenic exposure.

• The foetal phase, from the end of the embryonic stage to term, is the period when growth and functional maturation of organs and systems already formed occurs. Teratogen exposure in this period will affect foetal growth (e.g., intrauterine growth retardation), the size of a specific organ, or the function of the organ, rather than cause gross structural anomalies. The term foetal toxicity is commonly used to describe such an effect. Of particular interest is the potential effect of psychoactive agents (e.g., antidepressants, antiepileptics, alcohol and other drugs of abuse) on the developing central nervous system, which has led to a new field of behavioural teratology. [2]

Teratogenesis is the phenomenon of development of defects during the embryonic stages of a foetus.

Teratology is the study of abnormal development in embryos and the causes of congenital malformations or birth defects. These anatomical or structural abnormalities are present at birth although they may not be diagnosed until later in life.[1]

 A teratogenic agent is a chemical, infectious agent, medication, environmental agent, a physical condition or deficiency that the mother might be suffering from which on fetal exposure, can alter fetal morphology or subsequent function and in severe cases result in loss of pregnancy. Such agents can cross the placental barrier and cause or increase the incidence of physical malformations and behavioural or cognitive deficits. Teratogenicity depends upon the ability of the agent to cross the placenta.[1]

 

There are no absolute teratogens, however, may agents can exhibit teratogenic effects under certain circumstances. The dose and time of exposure often determine the severity of the damage and the type of defect that occurs. Usually an increased prevalence of a particular birth defect leads to the discovery of a teratogenic agent.[2]

The embryo is most susceptible to teratogenic agents during periods of rapid differentiation. The stage of development of the embryo determines susceptibility to teratogens.[1]

The stages of dvelopement  of a foetus