Practice question

Gastrulation stage corresponds to formation of primitive streak and ingression of mesoderm, around third week in humans. Ethanol exposure at this period preferentially kills anterior prechordal mesoderm and disrupts Sonic hedgehog signaling essential for midline facial prominence growth. Progenitors of frontonasal process, mediating midface formation, and forebrain neuroectoderm are depleted leading to characteristic fetal alcohol face: short palpebral fissures, smooth philtrum, thin upper lip and microcephaly with holoprosencephaly-like brain midline defects. Heart and limb less affected at this very early stage. Hence gastrulation alcohol insult leads to face and brain predominant anomalies reflecting midline pattern failure.