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#birth defects

3 public questions tagged with this topic.

Teratogens

Teratogenic exposure prior to week 3 results in:

During first two weeks post-fertilization, preimplantation and implantation phases, conceptus consists of small number of totipotent blastomeres with regulative capacity. Teratogenic exposure causing injury to few cells can be compensated by remaining cells which proliferate to replace lost lineage, resulting in complete recovery without malformation. Alternatively widespread cytotoxic damage kills all cells causing embryonic death and spontaneous abortion. This all-or-none principle distinguishes cleavage stage from organogenesis where injury produces specific defects. Therefore teratogenic exposure prior to week three results in embryonic death or recovery rather than major structural anomalies.

Ref: Sadler, Langman's Medical Embryology, 14th ed., Chapter 8: Pre-week 3 exposure death or recovery.

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Teratogens

Teratogen causing cyclopia in embryos:

Cyclopia single median eye results from failure of embryonic forebrain to divide into bilateral hemispheres due to deficient midline specification by Sonic hedgehog pathway. Steroidal alkaloid cyclopamine from Veratrum californicum binds Smoothened G-protein coupled receptor blocking hedgehog signal transduction, preventing expression of Pax2 and separation of eye field into two optic vesicles. Lambs whose mothers grazed plant and chick embryos treated experimentally display holoprosencephaly with cyclopia. Classic teratological outbreak traced to cyclopamine led to discovery of hedgehog pathway requirement. Thus teratogen causing cyclopia is cyclopamine potent sonic hedgehog antagonist producing midline facial defect.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Cyclopamine causes cyclopia Shh inhibition.

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Fertilization in mammals-iv

Teratogens exert maximum effects during:

Teratogenic susceptibility varies with developmental stage. Preimplantation period weeks 1-3 often leads to all-or-none embryonic loss due to totipotency. Organogenesis period spanning weeks 3-8 post-fertilization, when gastrulation, neurulation and formation of heart, limbs, eyes occur, represents peak sensitivity. Exposure to drugs like thalidomide, alcohol, retinoic acid or infections during this window disrupts morphogenetic signaling such as Shh, Wnt, Fgf causing major structural anomalies rather than lethality. After week 8 fetal growth and functional maturation predominate, teratogens tend to cause physiological defects or growth restriction not gross anatomical malformations, making early organogenesis most vulnerable phase.

Ref: Moore & Persaud, The Developing Human, 11th ed., Chapter 8: Teratogen sensitivity peak during organogenesis weeks 3-8.

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