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

Abstract

Electrical activation of oocytes is a key component of the current mammaliancloning regimen. The present study examined the surface ultrastructurealterations of bovine oocytes at different ages fol-lowing electrical pulse(EP) activation. In Experiment 1, bovine oocytes 22, 30 and 40 h afterin vitro mat-uration were activated by EP. Significantlyhigher pronuclear formation (57 or 59%) was obtained for the 30- or40-h oocytes than for the 22-h oocytes (10%). More oocytes (58%)in the 40-h group activated spon-taneously than in the 22- or 30-h groups. InExperiment 2, oocytes were cultured for 1, 2, 4 and 8 h after activation andthen evaluated by scanning electron microscopy (SEM). The zona pelucida (ZP)meshwork was completely abolished within 1 h after EP in the majority of theoocytes in the 22- and 30-h groups, but no obvious changes were observed inthe 40-h group. From 2 h onward, the ZP meshwork was gradually restored inmost treated oocytes. It was also noted that the microvilli pattern on thevitelline surface was altered dramatically after EP treatment, with theproportion affected increasing with oocyte age. The present study extends theprevious findings that older oocytes are more readily activated and that theage-dependent surface ultrastructural alterations correlate very well with theage dependent activation of oocytes.