Practice question

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.