Methods
Octamer-binding transcription factor 4 (Oct4)-MerCreMer transgenic mice were crossed with R26R-enhanced yellow fluorescent protein (EYFP) mice to establish a tamoxifen-inducible tracing system so that Oct4-expressing potential ovarian germ stem cells in young adult mice (5-6 weeks old) can be labeled with EYFP. The germ cell activities of DNA replication, mitotic division, entry into meiosis and progression to primordial follicle stage were investigated by means of immunofluorescent staining of ovarian tissues collected at different time points post-tamoxifen injection (1 day, 3 days, 2 months and 4 months). Meiosis entry and primordial follicle formation were also measured by EYFP-labeled single-cell RT-PCR. Germ cell proliferation and mitotic division were examined through 5-bromodeoxyuridine triphosphate incorporation assay. At each time point, ovaries from two to three animals were used for each set of experiment. Main