Different populations of mouse egg cortical vesicles are responsible for post-fertilization zinc sparks and proteolysis of the zona pellucida

Fertilization of mouse eggs by sperm triggers exocytosis of cortical granules, releasing zinc sparks as well as ovastacin that cleaves ZP2 in the zona pellucida. The mechanism by which zinc accumulates in cortical granules prior to fertilization has yet to be determined. We microinjected cRNAs encoding eight zinc transporters (ZnTs) into mouse oocytes and observed that only ZnT2 and ZnT4 accumulate in structures located in the cortex. We genetically ablated the single-copy genes encoding each of the two transporters. The absence of both transporters in ovulated eggs impaired accumulation of zinc in cortical granules and release of zinc sparks. Ovastacin and zinc are present in mostly non-overlapping populations of cortical granules. Our results provide new insights into the biology of mammalian cortical granules, but the release of zinc sparks does not affect the post-fertilization number of sperm in the perivitelline space nor increases polyspermy.