Cyclin B1^−/− embryos arrest at the four-cell stage after two normal divisions. (A) Zygotes obtained from crosses of viable Cyclin B1^+/− mice were cultured up to 3.5 d in vitro (left). Out of 83 Cyclin B1 homozygous–null embryos, 92% arrest at the four-cell stage or earlier (23 independent experiments). Distribution of the number of blastomeres found in Cyclin B1^−/− embryos (right); 75% arrest at the four-cell stage; in 6% of embryos, one (or in 2%, two) blastomeres can divide once more. (B) Differential interference contrast time-lapse imaging of a wild-type (top) and a Cyclin B1^−/− embryo (bottom) from the same litter (bar, 20 µm; time in hours:minutes). Representative frames from Video 1 shown from the late two-cell stage till the beginning of the 32-cell stage and blastocoel formation. No difference in the temporal dynamics of the first two divisions was observed between wild-type and knockout embryos, but the Cyclin B1^−/− embryo arrests after the second division. (C) H2B-RFP mRNA was injected into zygotes to visualize DNA in wild-type (top) and Cyclin B1^−/− embryos (bottom). Representative frames from Video 2, shown from the late two-cell stage until the blastocyst stage. Chromosome condensation, metaphase plate formation, and chromosome segregation were indistinguishable between wild-type and Cyclin B1^−/− embryos. In 80% of cases (five of six embryos in three independent experiments), arrested Cyclin B1^−/− embryos showed variable and transient rounds of incomplete chromatin condensation in one or more blastomeres (see time point 27 h:20 min, bottom, and Video 2). Bar, 20 µm. Time in hours:minutes.