Post meiosis I egg + preacrosomally reacted
Sperm undergoes acrosome reaction on binding
to zona pellucida or expanded cumulus glycoproteins.
Sperm penetrates zona via the lytic enzymes
of the acrosome and gets to perivitelline space.
Binds to specific receptors on the egg.
Egg responds by producing a Ca++
Egg then resumes meiosis II and extrudes
(loses) the 2nd polar body.
Coincidently, the egg engulfs the sperm.
Also coincidently, granules in the egg
cytoplasm cortex (near the egg surface) exocytose and release enzymes that
alter the zona pellucida and the egg surface so other sperm can't bind:
cortical reaction = block to polyspermy.