Abstract
Upon activation, mammalian eggs release billions of zinc ions in an exocytotic event termed the “zinc spark.” The zinc spark is dependent on and occurs coordinately with intracellular calcium transients, which are tightly associated with embryonic development. Thus, we hypothesized that the zinc spark represents an early extracellular physicochemical marker of the developmental potential of the zygote. To test this hypothesis, we monitored zinc exocytosis in individual mouse eggs following parthenogenetic activation or in vitro fertilization (IVF) and tracked their development. Retrospective analysis of zinc spark profiles revealed that parthenotes and zygotes that developed into blastocysts released more zinc than those that failed to develop. Prospective selection of embryos based on their zinc spark profile significantly improved developmental outcomes and more than doubled the percentage of embryos that reached the blastocyst stage. Moreover, the zinc spark profile was also associated with embryo quality as the total cell number in the resulting morulae and blastocysts positively correlated with the zinc spark amplitude (R = 0.9209). Zinc sparks can thus serve as an early biomarker of zygote quality in mouse model.
Highlights
Ca-Iono delivers a bolus of exogenous calcium directly into the egg Iono is thought to provide a better readout of egg quality as it only triggers the release of endogenous calcium stores to mount the activation-associated calcium transients
This study examined the relationship between the activation-induced zinc spark profile and embryo development in mouse, an analysis that cannot be conducted with human eggs in a research setting using United States federal funds[14,15,16]
Using parthenogenetic activation and in vitro fertilization (IVF) we found that parthenotes and embryos with better developmental outcomes released zinc sparks with higher amplitude and integrated intensity compared to embryos of lower quality
Summary
Results Distinct ionomycin-induced zinc spark profiles are correlated with egg activation and blastocyst formation. Because the correlation between zinc spark profiles and embryo quality is more robust in Ca-Iono method compared to Iono method, we activated eggs with Ca-Iono and separated and cultured the parthenotes in two groups based on their zinc spark amplitude: those in the top 50th percentile and those in the bottom 50th percentile.
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