CDK1 was active during GVBD but remained inactivated after PBE in the GDF9-Ccnb1^−/− oocytes. (A) Selected timeframes of a representative multichannel video of the GDF9-Ccnb1^−/− (n = 93) and control oocytes (n = 68) progressing through meiosis. Chromosomes and spindle microtubules were visualized with H2B-mCherry and MAP7-EGFP, respectively. Bars, 20 µm. (B) Histone H1 kinase assay of the GDF9-Ccnb1^−/− and control oocytes during meiosis. Oocytes were collected at certain time points (2, 6, 10, and 18 h), and 10 oocytes were used in each lane. CDK1 activity in the GDF9-Ccnb1^−/− oocytes at 2, 6, or 10 h was comparable with that of control oocytes, but after PBE, the CDK1 activity in the GDF9-Ccnb1^−/− oocytes remained at a low level. CDK1 activity in control oocytes was reactivated and peaked at 18 h. The mean kinase activity from three separate experiments is shown. Data are presented as mean ± SEM. **, P = 0.0079. (C) Phosphorylation detection of lamin A/C in the GDF9-Ccnb1^−/− oocytes during GVBD. 150 oocytes were used in each lane.