Fertilization of mammalian eggs by sperm injection : the effect of sperm placement on pronuclear formation and embryonic development

Abstract

Five hundred and two unfertilized hamster ova were assigned to 1 of 8 treatments. Four of the treatments involved the injection of sperm into the perivitelline space or the cytoplasm. The remaining 4 treatments were control treatments designed to monitor the incidence of parthenogenesis and assure the viability of sperm and ova. Approximately 14 hours after injection, 17% of ova with motile and 30% with non-motile sperm injected into the cytoplasm contained a decondensed sperm or male pronucleus. None of the ova with sperm injected into the perivitelline space showed signs of sperm incorporation. In a second experiment, 156 fertilized mouse ova, and 701 unfertilized mouse ova were assigned to 1 of 13 treatments. Five of the treatments involved injection of sperm into the cytoplasm or the perivitelline space of unfertilized ova. The remaining 8 were control treatments. Of the ova with sperm injected into the perivitelline space, 7%-11% cleaved. However, none of these exhibited a decondensed sperm or male pronucleus. All but 1 of the ova which cleaved following injection of sperm into the perivitelline space contained an unchanged sperm, suggesting they were parthenotes. The cleaved egg in which no sperm was found occurred following injection of a motile sperm into the perivitelline space. It is not known if this egg was fertilized, however, no sperm tail remanents were observed, indicating that it, too, was a parthenote. Three to 5% of the unfertilized mouse ova with sperm injected into the cytoplasm cleaved or exhibited a decondensed sperm. Sperm involvement in the fertilization process was confirmed in these ova. However, 89% of the ova with sperm injected into the cytoplasm lysed following treatment. The results of these experiments indicate sperm involvement in the fertilization process when injected into the cytoplasm, but not when injected into the perivitelline space. When 1-3 motile sperm were placed in contact with the vitelline membrane of zona-free eggs, 32% and 71% of hamster and mouse ova, respectively, became fertilized. Evidently the injection procedure rendered sperm and/or ova incapable of fusing when sperm were injected into the perivitelline space, thus inhibiting fertilization.