Relationship between Sperm Concentration and Polyspermy in Intact and Zona-Free Mouse Eggs Inseminatedin Vitro

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Abstract
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 × 103 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 × 103 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.