PARTICIPATION OF THE FEMALE PRONUCLEUS DERIVED FROM THE 2ND 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 chromo somes like its sister, the fertilized (activated) oocyte. In the prese nt study, the female pronucleus of fertilized mouse oocytes (zygotes) was replaced with the 2PB nucleus from the same or different oocytes t o examine the developmental potential of the 2PB nucleus. When the fem ale pronucleus (FPN) was synchronously (FPN and 2PB were same age) rep laced with the 2PB nucleus, the rate of reconstructed zygotes developi ng to blastocysts decreased with the age of donors and recipients (fro m 70% at 20-21 h to 15% at 26-27 h after hCG injection). When nuclei w ere replaced asynchronously (FPN and 2PB were of different ages), a hi gher developmental rate to blastocysts was obtained with young recipie nt zygotes (20 h after hCG injection) than with aged recipient zygotes (24 h after hCG injection) (64% versus 20%, P<0.01) irrespective of t he age of the 2PB. In this second group of embryos, in which nuclei we re 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 s lowly than did that of the zygote. After the transfer of reconstructed embryos into pseudopregnant females, normal pups with an expected coa t colour were born, indicating the competence of the 2PB chromosomes f or full embryo development.