Introduction |
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Introduction:
The exact cause for many growth abnormalities is unknown
in approximately 50% of cases.
The common causes of malformations can be divided into
3 groups:
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genetic
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environmental
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multifactorial
Effect of stage of development of
embryo/foetus on effect of teratogen:
The timing of the teratogenic insult influences the occurrence
and type of malformation produced. Developing tissues are most sensitive
to teratogens during the critical period for each organ. Critical time
period corresponds to establishment of each organ system. For most structures,
critical time period occurs during first few weeks of development. Agents
that operate before critical time period usually kill the embryo
Table 1: Sensitive periods during embryogenesis for which
certain tissues are susceptible to teratogens.
From Table 1, you can see that the central nervous system
is sensitive to teratogenic insults throughout prenatal and in some animals,
early postnatal development. Therefore a wide range of abnormalities are
possible within this system.
Teratology is the study of
developmental defects.
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All animal species are affected
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Incidence is usually 1% or less
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Many are dead at birth
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Defects usually result from exposure of foetus to agent
early on in development (day 21-28 of gestation)
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