At least half of capacitated, motile mouse sperm can fertilize zona-free mouse oocytes

The percentage of individual sperm capable of fertilizing zona pellucida-free mouse oocytes was investigated by placing motile sperm near zona-free oocytes with a micromanipulator. Incubation with one or two capacitated sperm per oocyte resulted in 50% and 70% fertilization, respectively, compared to 88% for cumulus intact (10(5) sperm/ml) and 87% for zona-free (2 x 10(3) sperm/ml) control oocytes. When sperm were treated with .1 microM calcium ionophore A23187 to facilitate the acrosome reaction, fertilization rates for single motile sperm were markedly lower than for capacitated, nontreated single sperm (4% and 35%, respectively). Similar fertilization rates resulted when one sperm was incubated per two ova (4% and 48% per sperm for A23187-treated and controls, respectively). When a lower dose of A23187 (.001 microM) was used to treat sperm, 7% of oocytes incubated with single sperm were fertilized. These experiments demonstrate that at least half of motile, capacitated mouse sperm are capable of fertilizing zona-free mouse oocytes in vitro, and that motile, A23187-treated mouse sperm resulted in poor fertilization rates.