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Participation of the female pronucleus derived from the second polar body in full embryonic development of mice

The second polar body (2PB), extruded from metaphase II oocytes after fertilization or oocyte activation, has a haploid set of female chromosomes like its sister, the fertilized (activated) oocyte. In the present study, the female pronucleus of fertilized mouse oocytes (zygotes) was replaced with the 2PB nucleus from the same or different oocytes to examine the developmental potential of the 2PB nucleus. When the female pronucleus (FPN) was synchronously (FPN and 2PB were same age) replaced with the 2PB nucleus, the rate of reconstructed zygotes developing to blastocysts decreased with the age of donors and recipients (from 70% at 20–21 h to 15% at 26–27 h after hCG injection). When nuclei were replaced asynchronously (FPN and 2PB were of different ages), a higher developmental rate to blastocysts was obtained with young recipient zygotes (20 h after hCG injection) than with aged recipient zygotes (24 h after hCG injection) (64% versus 20%, P < 0.01) irrespective of the age of the 2PB. In this second group of embryos, in which nuclei were replaced asynchronously, the 2PB nuclei were prematurely condensed at the time of first mitosis. These findings indicate that after being extruded from the oocytes the cell cycle of the 2PB progressed more slowly than did that of the zygote. After the transfer of reconstructed embryos into pseudopregnant females, normal pups with an expected coat colour were born, indicating the competence of the 2PB chromosomes for full embryo development.