embryonic period

The embryonic period begins with gastrulation (week 3), which establishes the trilaminar embryonic disc consisting of ectoderm, mesoderm, and endoderm, and ends at week 8 when the embryo has developed recognizable human features and all major organ systems have begun to form (organogenesis). Neurulation occurs during week 3-4, forming the neural tube from ectoderm; failure of neural tube closure leads to defects like spina bifida or anencephaly. Cardiac development begins early, with a primitive heart tube forming and beginning to beat by day 22. Limb buds appear around week 4 and differentiate through week 8. By the end of week 8, the embryo is termed a 'fetus,' and subsequent growth/maturation defines the fetal period. Clinically, the embryonic period is the most vulnerable time for teratogen-induced malformations because organogenesis is occurring; exposures during this window (e.g., to alcohol, isotretinoin, valproate, ACE inhibitors, or infections like rubella) can cause structural anomalies. Before week 3, teratogen exposure typically follows an 'all-or-none' principle (either the embryo is unaffected or lost), whereas after week 8, teratogens more often cause functional impairment or growth restriction rather than gross structural defects. This concept is frequently tested on board exams in the context of critical periods of organ development and teratogenicity windows for specific organ systems (e.g., heart most sensitive weeks 3-6, CNS throughout but especially weeks 3-16, limbs weeks 4-8).