This note defines a teratogen and explains the principles governing the capacity of a teratogen to produce teratogenic effect with emphasis on the effect of a teratogen on different developmental stages.
A teratogen is any agent/factor that can produce a
congenital anomaly or raise the incidence of an anomaly in the population.
The teratogens have potential to affect the normal development of human
embryo following maternal exposure to them.
These agents are usually environmental factors such as drugs, chemicals,
radiation, maternal diseases and viruses.The effect varies from death of
embryo to any structural, functional, metabolic and behavioral abnormalities. Developmental disorders present at birth are termed either
as birth defects, congenital anomalies or congenital malformation.
Teratology is the branch of science that studies
the cause, mechanisms, and patterns of abnormal development. A fundamental concept in teratology is that certain
stages of embryonic development are more vulnerable to disruption than others.
Teratogenicity, the capacity of an agent to produce
birth defects, is governed by certain factors described as Principles of
Teratology.
1. Developmental
stage at the time of exposure
2. Dose
and duration of exposure
3. Genotype
of the embryo
4. Mechanisms of action is specific and
may involve inhibition of specific biochemical or molecular process; cell
death; decreased cell proliferation or other cellular phenomena.
5. Manifestation of abnormal development
are death, malformation, growth retardation and functional disorders.
Stages of Developmental and Effects of Teratogens
1) Pre-embryonic period
This is the first two weeks after fertilization.
During this period, the zygote divides; implantation occurs; amnion, chorionic
sac and yolk sac are formed and the embryo becomes bilaminar.
Effect of teratogens:
Teratogens acting during this
period either kill the embryo (leading to spontaneous abortion which goes
unnoticed as the woman may not realize that she is pregnant) or their
disruptive effects are compensated for by powerful regulatory properties of
early embryo (so unlikely to produce congenital anomalies).
2) Embryonic period or period of embryogenesis or
organogenesis
This is the period from third to eighth weeks of
gestation, when organ systems are being established.
Effect of teratogens:
This is the most sensitive
period for inducing major birth defects.
3) Fetal period
This is the period from 9th week till birth. This
period is characterized by rapid growth and differentiation of the organ
system.
Effect of teratogens:
Fetal damage is unlikely to
produce malformations but can cause death, growth retardation, disruptions or
functional deficits.
Which stage of development is most vulnerable to
teratogens?
The period of embryogenesis is considered to be the
most critical stage of development in terms of teratogenic effect.
However, no stage of development is completely safe.
Each part, tissue and organ of an embryo has a critical period for which its
development may be disrupted. The type of congenital malformations produced
depends on which parts, tissues, and organs are most susceptible at the time of
exposure to teratogens.
REFERENCES:
Developing Human by Moore and Persaud
Langman's Medical Embryology by T W Sadler