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When sperm meets egg, sparks fly … in a petri dish. It’s the first time scientists have witnessed the electrifying reaction during the fertilization process, and it’s not just explosive – it may help predict a patient’s success when undergoing in vitro.

Dr Teresa Woodruff, Northwestern Medicine Ovarian Biology Researcher: “When fertilization happens, what you see is this enormous spark.”

You can actually see it! The blast of light that marks the moment a sperm enters an egg.

Dr Teresa Woodruff: “It’s a really strong indication that this egg is competent to go through the remaining stages of embryogenesis, to make a live, healthy offspring.”

The experiment started in a petri dish – egg and sperm meet. Then, the spark … the bigger, the brighter, the better, according to Northwestern Medicine researcher Dr Teresa Woodruff.

Dr Teresa Woodruff: “We didn’t have a really good way of knowing whether or not that egg was of good quality at the time of fertilization. In in vitro, you put the sperm and eggs together and just wait. The next day you come back and see if there has been two cells or four cells or eight cells, and all you can do is hope that division tells you if the egg is of high quality.”

But the sparks tell a story – offering valuable information that may help fertility doctors take the guesswork out of selecting viable eggs during the in vitro process.

Dr Teresa Woodruff: “What IVF labs really need is better, earlier signals to know if that egg and that sperm is going to make a productive, healthy live offspring, and so that’s what we hope these zinc sparks begin to give us — a new level of insight about how high a quality that egg is. The importance of this is now we can transfer a single egg to a recipient mother rather than transferring two or three, because we want to reduce the number of twins and triplets, particularly in IVF.”
And that would reduce the number of high-risk pregnancies. Dr Woodruff and her team had to invent four different methods to even see the molecular light show.

Dr Teresa Woodruff: “The key for this signal is that it’s outside the egg. There are other things we could do, if we could take the egg and take a section or if we could look inside it, but that would disrupt the normal human egg developmental process.”
The discovery came after Dr Woodruff’s husband – a chemist – posed the question, what do you think zinc does in sperm?
Dr Teresa Woodruff: “I told him I couldn’t care less. Tell me what it’s doing in the egg, and that led to this entire new area of reproductive biology. There was nothing before this work to suggest zinc would have this important a role in the development of a mature egg and then in the transition of that egg into an embryo, so this is a phenomenal new discovery.”
The reaction is clear in mouse models – the next step for Dr Woodruff and her team will be to see the process in human eggs then inside the fallopian tube.