Fertilization of Sheep Ova Following Mechanical Penetration of the Zona Pellucida

Many spermatozoa may become attached to the zona pellucida of the ovine ovum, but rarely does more than one penetrate the zona. Thus, polyspermy is uncommon, apart from when "aged" eggs (i.e. eggs more than 36 hr old after ovula-tion) are exposed to sperm (Killeen 1969). In most species polyspermy is fatal (Beatty 1957) and in the ewe polyspermic ova do not appear to develop beyond the formation of pronuclei (I. D. Killeen and N. W. Moore, unpublished data). The block to penetra-tion of the zona by more than one sperm-the zona reaction-must occur rapidly, but little is known of the mechanisms involved. In this experiment micromanipula-tion techniques were used to study the response of the zona to mechanical penetration and to examine the fertilizing capacity of spermatozoa placed directly within the perivitelline space.