Replicating DNA does not block germinal vesicle breakdown in mouse oocytes.

Mouse germinal vesicle GV-G2 stage oocytes were fused to very early S-phase blastomeres from four cell stage embryos to test whether oocytes have a mechanism which detects replicating DNA and thereafter arrests meiosis at the GV stage. Our results show that oocytes are unable to recognize replicating DNA in transplanted nuclei and undergo germinal vesicle breakdown (GVBD). GVBD and blastomere nucleus breakdown are only blocked by inhibitors of both oocyte maturation and DNA synthesis. These results show that mouse immature oocytes do not possess a feedback control capable of detecting replicating DNA.