Relationship between sperm concentration and polyspermy in intact and zona-free mouse eggs inseminated in vitro.

Intact and zona-free mouse eggs were cultured with preincubated (capacitated) spermatozoa for 1 or 4 hr. High proportions of eggs (84%--100%), examined either 1 or 4 hr after insemination, were undergoing fertilization in the intact and zona-free eggs in sperm concentration from 25--800 X 10(3) sperm/ml. The average number of spermatozoa attached to the zona pellucida and to the vitellus was only slightly increased as the sperm concentration increased. Polyspermy was increased from 25--200 X 10(3) sperm/ml but there was no clear correlation between the incidence of polyspermy and further increase of sperm concentration in both the intact and zona-free eggs. Besides a functional zona reaction, there was a definite vitelline block to further sperm entry. It seems that due to the chance collision of sperm and egg with the subsequent formation of a block mechanism in the zona pellucida and in the vitelline membrane within a short time, polyspermy cannot be increased by further increase of sperm concentrations.