Cytochalasin D Inhibits Penetration of Hamster Eggs by Guinea Pig and Human Spermatozoa

Fertilization experiments using zona-free hamster eggs and spermatozoa from both guinea pig and human were conducted in the presence of cytochalasin D to evaluate the possible role of actin filaments in fertilization processes. When the actin filament inhibitor, cytochalasin D, was added to fertilization media at concentrations of 10 to 30 microM, penetration of eggs was significantly inhibited. Preincubation of the eggs with cytochalasin D and washing prior to addition of spermatozoa had no effect on penetration as quantitated by the number of swollen heads in the egg cytoplasm. However, spermatozoa preincubated with cytochalasin D and subsequently washed prior to egg addition showed reduced penetration of the same magnitude as when spermatozoa and eggs were coincubated with cytochalasin D. Both the percentage of zona-free eggs showing decondensed sperm heads and the penetration indices (total decondensed spermatozoa/total eggs) were significantly affected when spermatozoa were exposed to cytochalasin D. The DMSO vehicle used to dissolve cytochalasin D had little effect on the number of decondensed heads. When the concentration of cytochalasin D was increased (DMSO remaining constant) in human sperm experiments, percent penetration decreased and progressively fewer decondensed spermatozoa were recorded, indicating dose-responsiveness to cytochalasin D. Motility parameters of human spermatozoa were not altered at any of the concentrations of cytochalasin D tested. Neither guinea pig sperm motility nor acrosome reaction was altered significantly by cytochalasin D or the DMSO vehicle. These experiments suggest that cytochalasin D may be an inhibitor of some fertilization processes such as sperm penetration or sperm head decondensation.