#pregnancy risks

Teratogens

Alcohol exposure during pregnancy causes defects similar to human FAS in:

Developmental toxicology testing for fetal alcohol spectrum relies on mammalian placental models recapitulating ethanol metabolism via alcohol dehydrogenase, generation of acetaldehyde, reactive oxygen species and pharmacokinetics comparable to human exposure. Mice exhibit craniofacial features resembling human fetal alcohol syndrome including midfacial hypoplasia, micrognathia, growth retardation and neurobehavioral deficits following gestational ethanol dosing during gastrulation or organogenesis. Genetic tractability allows investigation of L1CAM adhesion, Sonic hedgehog interaction. While rats show similar effects, mice provide best characterized mammalian system demonstrating human-like FAS defects following prenatal alcohol exposure.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Alcohol teratogenesis mouse FAS model.

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Teratogens

Zika virus causes microcephaly by infecting:

Zika virus microcephaly pathogenesis involves selective infection of embryonic neuroepithelium. Flavivirus crosses placenta via trophoblast and infects radial glial progenitors expressing AXL receptor in ventricular zone. Viral nonstructural proteins NS4A and NS4B inhibit Akt-mTOR signaling, induce endoplasmic reticulum stress, cell cycle arrest and apoptosis, depleting stem pool. Additionally virus dysregulates host microRNAs and centrosomal proteins, causing premature differentiation. Adult neurons possess interferon responses limiting replication. Preferential targeting of cycling neural progenitor cells explains cortex-specific thinning and severe brain size reduction observed after gestational infection.

Ref: Nature Reviews Neuroscience: Zika virus infection of neural progenitors microcephaly mechanism.

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Teratogens

Which system remains susceptible to teratogens throughout pregnancy?

Organ sensitivity windows vary; heart susceptible weeks three to six, limbs weeks four to eight, palate weeks six to nine. Central nervous system differs because neurogenesis, gliogenesis, neuronal migration, synaptogenesis, myelination extend beyond embryonic period throughout fetal life into adulthood. Proliferative ventricular zones continue generating neurons, vulnerability to agents causing oxidative stress, altering microRNAs or interfering with adhesion remains. Alcohol, Zika, antiepileptics, retinoids can affect cortical expansion even in third trimester leading to functional deficits. Therefore nervous system remains susceptible to teratogens throughout entire pregnancy unlike systems completing morphogenesis early.

Ref: Moore, The Developing Human, 11th ed., Chapter 18: Nervous system susceptibility throughout pregnancy.

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Teratogens

Maximum fetal susceptibility to teratogens occurs between:

Maximum susceptibility window corresponds to active organogenesis when organ anlagen originate and differentiate. Before week three, totipotency allows compensation or embryonic loss without specific defects. After week eight, organ templates established and growth predominates. Between weeks three and eight neural tube closes, pharyngeal arches form face, limb buds outgrow, heart septates, all highly dependent on morphogen gradients. Interruption causes severe structural anomalies like spina bifida, cleft lip, phocomelia. Hence greatest risk for major anatomical birth defects occurs during weeks three to eight, defining critical period for most classic teratogens affecting structural development.

Ref: Sadler, Langman's Medical Embryology, 14th ed., Chapter 8: Critical period weeks 3-8.

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Teratogens

Teratogens primarily exert their effects during:

Teratogen sensitivity correlates with developmental events. Pre-fertilization damage affects gametes; implantation period weeks one to two exhibits all-or-none death or recovery due to regulative ability. Embryonic period spanning weeks three through eight encompasses organogenesis when organ primordia form via induction, proliferation and morphogenesis requiring precise signaling by Sonic hedgehog, FGF, Wnt, retinoic acid. Disruption at this stage produces major structural malformations of neural tube, heart, face, limbs. Fetal period mainly growth and histogenesis with lower structural defect risk. Therefore teratogens exert principal structural effects during embryonic period weeks three to eight.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: Teratogenesis embryonic period sensitivity.

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