Fact check: When sperm fertilizes an egg a tiny flash appears?

When a sperm meets an egg, something stunning happens. Not metaphorically—literally. A tiny flash of light appears the moment an egg is activated. It's not visible to the naked eye, but under a special microscope, it looks like a spark. That spark is real. And it could change how we think about life, fertility, and the science behind both.

The footage and findings come from a study published in Scientific Reports in April 2016.

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.

“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.

At the time, researchers at Northwestern University had managed to capture the moment a human egg releases a flash of light when activated by a sperm enzyme. That video, originally released as part of their research, resurfaced in 2025 and quickly went viral online.

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Social media posts and science blogs picked it up again, amazed by the real footage of what many now call the “spark of life.”

What fascinated people then—and now—is the clear visual proof of something previously hidden inside the body. The idea that life may begin with a literal spark remains a powerful and moving image, even nearly a decade later.

What actually happens during spark?

Scientists discovered that the spark comes from a sudden burst of zinc.

Here’s how it works:

• The egg stores zinc in thousands of tiny compartments.

• When the egg is activated, zinc ions are released in a quick burst.

• These ions bind to special fluorescent molecules.

• The result is a visible flash captured under a microscope.

Because fertilizing a human egg with sperm for research is illegal in the U.S., the team used a sperm enzyme instead. This allowed them to simulate fertilization without violating federal law.

The flash can’t be seen by the naked eye. But with the right imaging tools, it appears as a brief, glowing burst.

“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.

He’s a co-senior author and the Charles E. and Emma H. Morrison Professor in Chemistry at Northwestern. He also directs the Chemistry of Life Processes Institute.

The zinc spark was first observed in mice in 2011. It took researchers several years to develop the tools needed to see it in human eggs. By 2014, they had captured the spark in mice on video. Two years later, they filmed it happening in humans.

The discovery marked a major step forward. Teresa Woodruff, one of the study’s authors, called the moment "breathtaking." “To see the zinc radiate out in a burst from each human egg—there’s nothing like it,” she said.

Why it matters for fertility?

Roughly half of fertilized human eggs fail to develop into embryos. That’s a major challenge in fertility treatments like IVF.

One of the biggest problems is identifying which eggs are healthy enough to become embryos. Doctors often don’t know until they’ve transferred the embryo and waited to see if a pregnancy occurs. The zinc spark offers a new, visible clue.

“The more zinc released, the brighter the spark,” said Dr. Eve Feinberg, a fertility specialist and study co-author. “That brightness may tell us how viable an egg is before we even attempt fertilization.”

“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,” Feinberg said.

“There are no tools currently available that tell us if it’s a good quality egg. 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.”

This could help doctors choose the best eggs before going through the full IVF process.

It could:

• Save time

• Reduce emotional stress

• Improve pregnancy rates

• Lower the number of failed cycles

So why zinc?

Researchers found that zinc plays a key role in egg development. Each egg contains around 8,000 compartments, each holding about 1 million zinc atoms.

The egg needs to store and then release zinc at the right moment to begin embryonic development.

“The egg has to stockpile zinc, and then let go of some to start developing,” said Thomas O’Halloran, a chemist on the research team. The more precisely that zinc is managed, the better the chances of a successful pregnancy.

The study may lead to new tools for fertility clinics.

Imagine a microscope that lets doctors quickly scan eggs for zinc sparks. Bright flashes would signal good eggs. Dim or no flashes would suggest a lower chance of success.

This method would be non-invasive. It wouldn’t harm the egg. And it could give doctors and patients real-time information about egg quality—something not currently available.

“No tools exist right now to tell us if an egg is good,” said Feinberg. “Often we don’t know until much later. This changes that.”

Viral video reignites interest

In 2025, the video of the zinc spark gained a second life. Clips circulated across social media, sparking fresh interest in the science. People were stunned to learn the footage was real—not a computer simulation or special effect. It served as a reminder that sometimes science needs time to catch on. Even when something is published in a journal, it may take years before the public truly connects with it. This rediscovery also highlights the gap between scientific research and public awareness.

The researchers hope to use these findings to improve fertility outcomes.

More studies are needed to confirm that spark brightness can reliably predict egg viability. If confirmed, clinics could begin adopting the method into practice.

The goal: faster, more successful IVF treatments with fewer failed cycles. The study also raises new questions. What triggers the zinc release? Can other minerals or signals be tracked the same way? Could similar techniques help in other areas of medicine?

The spark is small. It lasts just a moment. But its impact could be huge.

For couples going through IVF, it could mean fewer failed attempts and faster results. For scientists, it opens a new window into how life begins. And for everyone else, it’s a rare glimpse into a process that usually stays hidden.

A decade after the footage was first recorded, the world is finally catching up.

And this time, people are watching.

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