ESTABLISHMENT OF THE BLOCK AGAINST SPERM PENETRATION IN PARTHENOGENETICALLY ACTIVATED BOVINE OOCYTES MATURED IN-VITRO

The ability of a single electric pulse to mimic a block against sperm penetration in bovine oocytes matured in vitro was investigated. Confo cal laser scanning microscopy detected a global loss of spots, presume d to be cortical granules, stained with Lens culinaris agglutinin, in pulsed oocytes. Transmission electron microscopy revealed that cortica l granule exocytosis occurred within 1 min of stimulation and the numb er of remaining cortical granules was significantly reduced in all pul sed oocytes. The ability of pulsed oocytes to undergo fertilization in vitro was also affected, as only 31% of the pulsed oocytes were penet rated compared with 87% in the control group. Since incidences of pene tration in pulsed oocytes (31%), and of polyspermy in control oocytes (18%) did not differ and were highly correlated (P = 0.009) among tria ls (n = 15), the induced block is considered to be comparable with the natural block triggered by a spermatozoon. The increased resistance o f the zona pellucida to pronase E observed in pulsed oocytes suggests that the induced block depends, at least partly, on modifications of z ona pellucida glycoproteins. Finally, the majority (66%) of pulsed, pe netrated oocytes did not form male pronuclei, probably as a consequenc e of asynchrony between the formation of female pronucleus and sperm p enetration. The reduced ability of the cytoplasm to induce the formati on of a male pronucleus was accompanied by a fall in histone HI kinase activity to basal values by 3 h after stimulation. These results demo nstrate that a single electric pulse can induce a block against sperm penetration similar to that of the spermatozoon itself.