The Passage of Spermatozoa through and into the Zona Pellucida of the Rabbit Egg.

Polyspermy is pathological in mammals (Austin & Walton, i960), but is uncommon because mechanisms exist which prevent its occurrence. In the sheep, cow, pig and other species, after penetration by the first spermatozoon, the zona pellucida undergoes a change, the 'zona reaction', which bars further spermatozoa from passing through the zona, but still allows them to enter it. Unlike the eggs of the above species, the rabbit egg does not undergo a zona reaction; the barrier to polyspermy is in the vitelline membrane (Braden, Austin & David, 1954). Consequently, spermatozoa continue to penetrate the eggs. A large proportion of the spermatozoa that enter an egg penetrate the zona and reach the perivitelline space; however, a small proportion enter but do not pass through the zona. It remains to be explained why the latter fail to traverse the entire thickness of the zona. 6—9 hr. after ovulation, spermatozoa no longer have access to the zonae, because the latter become coated with a mucin layer which is impenetrable to spermatozoa (Pincus, 1930; Hammond, 1934). The aim of the present study was to determine the sperm pathway through the zona of the rabbit egg, in the hope that this would shed some light on the mechanism of sperm penetration through the zona.