Induction of DNA replication in the germinal vesicle of the growing mouse oocyte

Growing mouse oocytes are physiologically arrested in the G2 phase of prophase of the first meiotic division. Growing oocytes were isolated from ovaries of 9- to 12-day-old mice and fused with parthenogenetic one-cell eggs or two-cell embryos derived from fertilized eggs. Resulting hybrids were injected with Dig-11-dUTP and examined for DNA replication using immunofluorescence. Parthenogenetic one-cell eggs fused at telophase II, G1, and middle-to-late S phase, and also S-phase two-cell blastomeres, were able to trigger DNA synthesis in oocyte germinal vesicle (GV) in the majority of hybrids cultured to the end of the first cell cycle. Activation of replication in the GV occurred within 2-3 h after fusion of growing oocytes with S-phase eggs. We show indirectly that the reactivation of replication in GVs was not dependent on the breakdown of the GV envelope. Although GVs had the ability to renew DNA replication after fusion, the G2 blastomere nuclei were incapable of reinitiating DNA replication under the influence of S-phase one-cell eggs. We hypothesize that the nuclei of growing oocytes arrested in meiotic prophase are in a physiological state that is equivalent to replication-competent G1, and not G2, nuclei.