Dynamic changes of intracellular zinc ion level during maturation, fertilization, activation, and development in mouse oocytes

Although it is well known that calcium oscillations are required for fertilization in all mammalian species studied to date, recent studies also showed the ejection of zinc into the extracellular milieu in a series of coordinated events, called "zinc spark," during mammalian fertilization. These results led us to the hypothesis that a zinc ion-dependent signal is important for oocyte maturation, fertilization (activation), and further embryonic development. In this study, we evaluated the amounts and localization of intracellular zinc ions during maturation, fertilization, activation, and embryonic development in mouse oocytes. Our results show that abundant zinc ions are present in both immature and mature oocytes. After in vitro fertilization, the amounts of zinc ions were dramatically decreased at the pronuclear (PN) stage. Artificial activation by cycloheximide, SrCl2, and TPEN also reduced the amounts of zinc ions in the PN stage. On the other hand, PN embryos derived from sperm injection still showed high level of zinc ions. However, the amounts of zinc ions rapidly increased at the blastocysts regardless of activation method. We showed here that the amounts of zinc ions dramatically changed during maturation, fertilization, activation, and development in mouse oocytes.