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 potential to achieve fertilization and embryonic development when injected into eggs. However, injection of spermatids alone seldom activates eggs in the mouse, as spermatids by themselves cannot induce an increase in intracellular Ca2+, a prerequisite for egg activation. We injected a mouse round spermatid into an egg simultaneously with partially purified sperm factor from differentiated hamster spermatozoa. The combined injection produced repetitive Ca2+ increases (Ca2+ oscillations) lasting for at least 4 h as observed at fertilization, and induced activation in 92% of eggs. This method provided 75% fertilization success associated with male and female pronucleus formation 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% developed to blastocysts during 5 days of culture in vitro, while no blastocysts were obtained following injection of sperm factor alone. Furthermore, the 2-cell embryos, that were created by spermatids and sperm factor and transplanted into foster mothers, developed into normal offspring, although the percentage was only 22%. All infants grew into healthy adults carrying normal chromosomes. The sperm factor served as a complementary factor for successful fertilization by round spermatid injection.