Fig. 3. ERK inactivation in late two-cell-stage embryos induces developmental arrest in the G2 phase, just before M phase in the four-cell stage. (A) BrdU incorporation into the nucleus was investigated. Late two-cell-stage embryos were cultured in the presence of BrdU and U0126 or BrdU and DMSO (control), fixed, and stained with anti-BrdU antibody. Fluorescence was viewed with a confocal microscope at the early and late four-cell stages. (B) Cyclin B1 (cyc B1) accumulation in the nucleus and phosphorylation of histone H3 (p-Histone H3) were examined. Late two-cell-stage embryos were cultured in the presence of U0126 or DMSO (control). Embryos were fixed and stained with anti-cyclin B1, Hoechst or anti-phospho-histone H3 antibody. Fluorescence was viewed with a confocal microscope (cyc B1) or a DeltaVision Image Restoration Microscope (Hoechst and p-Histone H3). Scale bars: 20 µm.