Birth of normal mice after removal of the supernumerary male pronucleus from polyspermic zygotes.

Each year, world wide, tens of thousands of zygotes derived from the in-vitro insemination of human oocytes undergo polyspermic fertilization. These embryos must presently be discarded because it has never been demonstrated in any mammal that polyspermic zygotes can develop normally to term after removal of the supernumerary male pronucleus. Our study was undertaken to test the developmental potential of polyspermic zygotes corrected by micromanipulation. Mouse oocytes were inseminated zona-free, and polyspermic zygotes were manipulated so as to remove one of the two male pronuclei. Surviving embryos were then observed for further development in vitro and after transfer into pseudopregnant females. Of 58 zygotes manipulated, 18 developed to the blastocyst stage and were transferred. Five animals (two male and three females) were born. The agouti coat colour marker confirmed the genotypes of the gametes. All five animals developed to normal-appearing adults, and all five produced at least 10 normal offspring. One adult founder animal was karyotyped and found to have a normal chromosome complement. These results demonstrate for the first time that a mammalian egg that has undergone polyspermic fertilization can develop normally after restoration of the diploid state by micromanipulation. Accordingly, the results provide impetus for attempting to rescue polyspermic human embryos.