Zn2+ Sparks as a Polyspermy Block
Mammalian eggs have been shown to both sequester Zn2+ in their cortical granules during maturation, and also to release the cation after fertilization. The Zn2+ is released suring the cortical granule exocytosis of the slow block to polyspermy, and can be imaged as Zn2+ sparks. Although Zn2+ sparks have been shown to be conserved between mouse and humans, the role that this Zn2+ may be playing outside of a fertilized egg, was yet to be determined.
Experiments described in a recent manuscript from the O'Hallaran and Woodruff groups at Northwestern University (PMID: 28102396), indicated that the Zn2+ released by mammalian eggs at fertilization, alters the zona pellucida (ZP). The group specifically used transmission electron microscopy to document that Zn2+ exposure changes the ultrastructure of the SP that surrounds mouse eggs. Specifically, they propose that Zn2+ aids in the ZP polymerization, and this process aids in preventing sperm from entering an already-fertilized egg.