Distribution and role of microfilaments during early events of sheep fertilization.

The distribution of actin was studied during early events of sheep fertilization by fluorescence microscopy after staining with 7-nitrobenz-2-oxal-1.3 diazole (NBD)-phallacidin and anti-actin antibody and by electron microscopy after heavy meromyosin labelling. unfertilized and fertilized eggs exhibited a continuous band of fluorescence with both NBD-phallacidin and anti-actin antibody. Unlike in mice, no high concentration of actin overlying the spindle was detected in ovulated sheep oocytes. At the site of sperm head incorporation, the fertilization cone developed above the decondensing male chromatin and was underlined by a submembranous area rich in microfilaments. A similar actin network was observed in the cortex of the second polar body. Cytochalasin D was used to investigate the role of actin during the fertilization process. This drug did not prevent sperm fusion and incorporation but inhibited polar body abstriction and fertilization cone development and retarded sperm tail incorporation. Moreover, in the presence of the drug, the anchorage of the metaphase II spindle at the surface of the egg was destroyed. The role of microfilaments in these early events is discussed.