Possible Involvement of DNA Topoisomerase II Enzyme in the Process of Meiotic Arrest in Mouse Metaphase II Oocytes

The effects of etoposide supplementation to mouse gametes co-culture medium on fertilization rate, nuclear events and development to the blastocyst stage were investigated. In vivo matured mouse oocytes were co-cultured with sperms in TYH medium supplemented with 50 ?g/ml etoposide an specific DNA topoisomerase II (top II) blocker. Experiment 1 evaluated the effects of top II blocker on fertilization rate and nuclear events in fertilized oocytes whole-mounted 5 hr after gametes co-culture. Top II blocking decreased the fertilization rate significantly (p<0.01) as compared with the control. Furthermore, the fertilized oocytes were blocked at the metaphase II stage and the extrusion of the second polar body an the formation of the female and male pronuclei were completely inhibited. Experiment II examined the ability of the oocytes fertilized in the presence of top II blocker to cleave and develop to the blastocyst stage in vitro. The cleavage rate of oocytes fertilized in the presence of etoposide was significantly lower (p<0.001) than the control (11% vs 86.3%; respectively). Non of the eggs fertilized in the presence of etoposide (n=154) developed beyond the 2-cell stage, whereas (76.3%) of the control (n=145) developed to the blastocyst stage. It is concluded that the use of top II blocker (etoposide) in mouse gametes co-culture medium inhibits exit from meiotic arrest after sperm penetration. Thus, suggesting a possible involvement of top II in meiotic arrest.