[Introduction] Recently, Kaushik Deb et al. (Science, 2006) reported that trophectoderm lineage specification occurs early in development, certainly by the two-cell stage. In which the lagging blastomere at the two-cell stage contributes mainly to trophectoderm (TE), whereas the leading blastomere is the precursor of the inner cell max (ICM). This study was designed to examine whether the difference between lagging and leading blastomeres at the two-cell stage on the first differentiation, and full-term development. [Methods] Mouse embryos were produced by ICSI using oocytes and sperm from B6D2F1 mice. First, lagging and leading blastomeres at the three-cell stage (30-36 h after ICSI; one cell from lagging blastomere and two cells from leading blastomere) were collected, and subsequently culture for preimplantation development. The appearance of TE and ICM cells of lagging and leading blastomeres during preimplantation development were examined using CDX2 and OCT4 markers, respectively. Second, full-term development of lagging and leading blastomeres were examined after transfer of blastocysts derived from those blastomeres into surrogate mother (ICR) on day 3 of pseudopregnant female. [Results] Both lagging and leading blastomeres at the second cleavage were identically contributed to TE and ICM cells. Expression of CDX2 and OCT4 were observed in both lagging and leading blastomeres at the expanded blastocysts stage, however the total numbers of cells and the numbers of blastomeres expressing OCT4 were lowest in lagging blastomere (33 and 5.7, vs. 44 and 7.2, in leading blastomere). Finally, offspring was obtained in both lagging and leading blastomeres (26% and 24%, respectively).
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