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#fetal development

13 public questions tagged with this topic.

Teratogens

Fetal Alcohol Syndrome (FAS) primarily results from damage to:

Facial dysmorphology and microcephaly hallmark fetal alcohol syndrome originate primarily from injury to two populations. Cranial neural crest cells arising at dorsal neural tube edges migrate into frontonasal prominence contributing to facial skeleton highly sensitive to oxidative apoptosis, impaired L1 adhesion and reduced sonic hedgehog trophic support after ethanol. Depleted crest leads to midfacial hypoplasia. Simultaneously ventricular zone neuronal progenitors and postmitotic neurons undergo ROS-mediated apoptosis and cell cycle arrest, reducing brain volume and cortical complexity. Liver, lung less affected. Combined neural crest and neuronal cell loss explains neurocristopathy features central to fetal alcohol syndrome presentation.

Ref: Moore, The Developing Human, 11th ed., Chapter 20: FAS neural crest and neuronal damage.

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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

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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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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Gene and Genome

Which globin gene is expressed during fetal stage?

Human beta-globin cluster on chromosome 11 contains developmentally regulated genes arranged in order of expression: embryonic epsilon, fetal G-gamma and A-gamma, minor delta and adult beta. Gamma globin combines with alpha globin to form fetal hemoglobin HbF, alpha2gamma2, which exhibits higher oxygen affinity than adult HbA due to reduced binding of 2,3-bisphosphoglycerate, facilitating oxygen transfer across placenta. Expression peaks during fetal liver hematopoiesis, declines perinatally as BCL11A-mediated silencing induces hemoglobin switching toward beta globin, central to sickle cell therapeutic reactivation strategies.

Ref: Schechter AN, Blood 2008: Globin Gene Regulation, Fetal Gamma Globin and HbF Switching

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