A stunning explosion of zinc fireworks occurs when a human egg is activated by a sperm enzyme, and the size of these “sparks” is a direct measure of the quality of the egg and its ability to develop into an embryo, according to new research from Northwestern Medicine.

 

The discovery has potential to help doctors choose the best eggs to transfer during in vitro fertilization (IVF), the scientists said.

This is the first time the zinc sparks have been documented in a human egg.


“This means if you can look at the zinc spark at the time of fertilization, you will know immediately which eggs are the good ones to transfer in in vitro fertilization (IVF),” said Teresa Woodruff, one of the study’s two senior authors and an expert in ovarian biology at Northwestern. “It’s a way of sorting egg quality in a way we’ve never been able to assess before.”


Woodruff is the Thomas J. Watkins Memorial Professor in Obstetrics and Gynecology at Northwestern University Feinberg School of Medicine and director of Northwestern’s Center for Reproductive Science.


Scientists activated the egg by injecting a sperm enzyme into the egg that triggers calcium to increase within the egg and zinc to be released from the egg. (The eggs in the study were not fertilized with actual sperm because that is not permitted in human research under federal law.)


“It was remarkable,” Woodruff said. “We discovered the zinc spark just five years ago in the mouse, and to see the zinc radiate out in a burst from each human egg was breathtaking.


“All of biology starts at the time of fertilization, yet we know next to nothing about the events that occur in the human. This discovery required a unique partnership between biologists and chemists and non-federal dollars to support the research,” she said.


The study was published April 26 in Scientific Reports.


As the zinc is released from the egg, it binds to small molecule probes, which emit light in fluorescence microscopy experiments. Thus the rapid zinc release can be followed as a flash of light that appears as a spark.


“These fluorescence microscopy studies establish that the zinc spark occurs in human egg biology, and that can be observed outside of the cell,” said Tom O’Halloran, a co-senior author.  O’Halloran is the Charles E. and Emma H. Morrison Professor in Chemistry in the Weinberg College of Arts and Sciences and director of Northwestern’s Chemistry of Life Processes Institute.


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