Fertilization, embryonic development, and offspring from mouse eggs injected with round spermatids combined with Ca2+ oscillation-inducing sperm factor

Round spermatids, precursor male gametes, are known to possess the potentia l to achieve fertilization and embryonic development when injected into egg s. However, injection of spermatids alone seldom activates eggs in the mous e, as spermatids by themselves cannot induce an increase in intracellular C a2+, a prerequisite for egg activation. We injected a mouse round spermatid into an egg simultaneously with partially purified sperm factor from diffe rentiated hamster spermatozoa. The combined injection produced repetitive C a2+ increases (Ca2+ oscillations) lasting for at least 4 h as observed at f ertilization, and induced activation in 92% of eggs. This method provided 7 5% fertilization success associated with male and female pronucleus formati on and development to 2-cell embryos, while only 7% of eggs were fertilized by injection of a spermatid alone. Of the 2-cell embryos, similar to 50% d eveloped to blastocysts during 5 days of culture in vitro, while no blastoc ysts were obtained following injection of sperm factor alone. Furthermore, the 2-cell embryos, that were created by spermatids and sperm factor and tr ansplanted into foster mothers, developed into normal offspring, although t he percentage was only 22%. All infants grew into healthy adults carrying n ormal chromosomes. The sperm factor served as a complementary factor for su ccessful fertilization by round spermatid injection.