Mouse spermatid nuclei can support full term development after premature chromosome condensation within mature oocytes.

The nucleus of round spermatids, the earliest haploid male germ cells, can participate in the formation of normal zygotes when incorporated into activated oocytes. In this study, we injected mouse round spermatids into homologous mature oocytes that were kept arrested at metaphase II to induce premature chromosome condensation (PCC) of the spermatid nuclei. After full condensation of the spermatid chromosomes, the oocytes were activated by Sr2+-containing medium, into which cytochalasin B was added to prevent extrusion of the segregated female and male chromosomes as polar bodies. Out of 142 oocytes examined, 104 (73%) formed two male (pseudo)pronuclei and two female pronuclei. To restore the diploid state of these zygotes, one of the female pronuclei was removed. When cultured in vitro for 72 hours, all (n = 37) of the constructed embryos developed to the morula/blastocyst stage. When 2-cell embryos and morulae/blastocysts were transferred into pseudopregnant females, 14 (13/96) and 24% (9/37), respectively, developed into term offspring. This study indicates that the spermatid chromosomes, which had undergone PCC, moved safely to opposite poles after oocyte activation. Since round spermatids contain no (in the mouse) or little (in patients with spermatogenic failure) oocyte-activating factor, this method may be used to rescue oocytes that fail to be activated at the time of spermatid injection.