Age-dependent development and surface ultrastructural changes following electrical activation of bovine oocytes

Electrical activation of oocytes is a key component of the current mammalian cloning regimen. The present study examined the surface ultrastructure alterations of bovine oocytes at different ages following electrical pulse (EP) activation. In Experiment 1, bovine oocytes 22, 30 and 40 h after in vitro maturation were activated by EP. Significantly higher pronuclear formation (57 or 59%) was obtained for the 30- or 40-h oocytes than for the 22-h oocytes (10%). More oocytes (58%) in the 40-h group activated spontaneously than in the 22- or 30-h groups. In Experiment 2, oocytes were cultured for 1, 2, 4 and 8 h after activation and then evaluated by scanning electron microscopy (SEM). The zona pelucida (ZP) meshwork was completely abolished within 1 h after EP in the majority of the oocytes in the 22- and 30-h groups, but no obvious changes were observed in the 40-h group. From 2 h onward, the ZP meshwork was gradually restored in most treated oocytes. It was also noted that the microvilli pattern on the vitelline surface was altered dramatically after EP treatment, with the proportion affected increasing with oocyte age. The present study extends the previous findings that older oocytes are more readily activated and that the age-dependent surface ultrastructural alterations correlate very well with the age dependent activation of oocytes.