Polyspermy-preventing mechanisms in mouse eggs fertilized in vitro

The time sequence of the mechanisms that prevent polyspermy in cumulus-free mouse eggs was determined by continuous micropscopic observations during in vitro fertilization. The zona reaction had begun, and the vitelline surface block to polyspermy was established, in less than one minute following sperm fusion with the vitellus. In less than five minutes, the zona reaction had advanced to the stage that no more spermatozoa could become firmly attached to the surface of the zona pellucida.