Key Points Summary — Fertilization and Early Embryogenesis — Summary

Fertilization occurs in the ampulla of the fallopian tube (NOT the uterus).
Acrosomal reaction: sperm contacts zona pellucida → acrosomal enzymes (hyaluronidase, acrosin) released → zona pellucida digested → sperm membrane fuses with oocyte.
Cortical reaction: sperm entry → $Ca^{2+}$ wave → cortical granules exocytose → zona pellucida hardens (zona reaction) → polyspermy prevented.
Sperm entry also triggers completion of meiosis II → polar body II → mature ovum.
Male pronucleus + female pronucleus fuse → zygote (2n).
Cleavage: Zygote → 2-cell → 4-cell → 8-cell → Morula (16-cell solid ball) → Blastocyst (fluid cavity + trophoblast + ICM).
Blastocyst implants in the endometrium approximately 6–7 days after fertilization (~day 20–21 of cycle).
Trophoblast (outer): forms placenta; secretes hCG.
Inner cell mass (ICM): forms the embryo proper.
hCG maintains corpus luteum → continued progesterone → no menstruation → pregnancy continues.