The fertilization-induced Ca2+ oscillation in mouse oocytes is cytoplasmic maturation dependent

Abstract

Mature eggs (at metaphase II stage) produce a series of Ca 2+ oscillation at fertilization. To define whether the fertilization-induced Ca 2+ oscillation is restrict to the metaphase II eggs and cell cycle dependent, mouse oocytes at prophase I (arrested at germinal vesicle stage), metaphase I, metaphase II, as well as the pronuclear embryos at interphase of the first mitotic division derived from fertilization or parthenogenetic activation were inseminated after removal of zona pellucida. The results show that the fertilization-induced Ca 2+ oscillation is not specific to metaphase II eggs. This is supported by the fact that immature oocytes generated the Ca 2+ oscillations at fertilization regardless of their nuclear progression from prophase I to metaphase I (in vitro matured) stage. More interestingly, it was first found that pronuclear embryos at interphase derived from parthenogenetic activation showed Ca 2+ oscillations in response to fertilization while the zygotes at interphase did not after reinsemination or intracytoplasmic injection of sperm extracts which induce Ca 2+ oscillations in MII eggs. This suggests that the ability of oocytes to generate Ca 2+ oscillation in response to sperm penetration is not regulated in a cell cycle dependent manner but dependent on the cytoplasmic maturation.