Cleavage pace predicts blastocyst formation of cloned embryos.

Embryos were scored fast or slow according to the time spent until the three-cell stage was reached. Then blastocyst formation was recorded, and ESCs were derived (data pooled from 4 experiments), or fetal rates were determined at E10.5 after transfer in utero 12 hours after scoring (data pooled from 2 experiments). The p-value of Fisher's exact test shows that the difference in blastocyst formation of fast and slow is significant, difference in ESC formation efficiency is of borderline significance, and difference in fetal formation is not significant. Data were pooled from five independent NT experiments. Development data from fertilized control embryos (ICSI) are shown in bottom row. Note that fertilized embryos were not sorted into fast and slow, and therefore frequencies relate to the two-cell stage. Derived ESCs were pluripotent regardless of their origin (fast, slow) as demonstrated by in vitro differentiation into derivatives of the three germ layers (figure S5, movies S6, S7, S8) and by teratoma formation (data not shown).