Size of zinc spark reveals best egg for IVF

If we have the ability up front to see what is a good egg and what's not, it will help us know which embryo to transfer, avoid a lot of heartache, and achieve pregnancy much more quickly," says Eve Feinberg. (Credit: qrevolution/Flickr)

An explosion of zinc fireworks occurs when a sperm enzyme activates the human egg. New research suggests the size of the spark reveals the chances the egg has of developing into an embryo.

The discovery has potential as a way to help doctors choose the best eggs to transfer during in vitro fertilization (IVF).

“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),” says Teresa Woodruff, professor of obstetrics and gynecology at Northwestern University Feinberg School of Medicine. “It’s a way of sorting egg quality in a way we’ve never been able to assess before.”

For the study, 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.

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“It was remarkable,” Woodruff says. “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. ”

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,” says Tom O’Halloran, professor in chemistry and a co-senior author of the study in the journal Scientific Reports.

Eggs compartmentalize and distribute zinc to control the development of a healthy embryo. Over the last six years this team has shown that zinc controls the decision to grow and change into a completely new genetic organism.

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“This is an important discovery because it may give us a non-invasive and easily visible way to assess the health of an egg and eventually an embryo before implantation,” says coauthor Eve Feinberg, who took care of the patients who provided eggs for the basic science study and collaborated with the research team. She will become an assistant professor of obstetrics and gynecology at Feinberg and will be ambulatory medical director of Northwestern Medicine’s Fertility and Reproductive Medicine division in July.

“There are no tools currently available that tell us if it’s a good quality egg,” Feinberg says. “Often we don’t know whether the egg or embryo is truly viable until we see if a pregnancy ensues. That’s the reason this is so transformative. If we have the ability up front to see what is a good egg and what’s not, it will help us know which embryo to transfer, avoid a lot of heartache, and achieve pregnancy much more quickly.”

First author Francesca Duncan, who will become the executive director of Northwestern’s Center for Reproductive Science in August, made the human zinc spark discovery. “We now know that the release of zinc at the time of fertilization is a conserved phenomenon, which will help us address one of the largest unanswered questions in reproductive medicine—what makes a good egg?”

The Thomas J. Watkins Endowment and a research grant from Ferring Pharmaceuticals and the W.M. Keck Foundation supported the work.

 Source: Northwestern University