Sparks literally fly when a sperm and an egg hit it off, researchers have found.
Researchers developed a special technique to capture the moment of fertilisation, and found billions of zinc atoms are released.
They say the discovery could have major implications for fertility treatments.
The Northwestern University-led interdisciplinary research team are the first to capture images of these molecular fireworks and pinpoint the origin of the zinc sparks: tiny zinc-rich packages just below the egg's surface.
'The amount of zinc released by an egg could be a great marker for identifying a high-quality fertilized egg, something we can't do now,' said Teresa Woodruff, an expert in ovarian biology and one of two authors of the study.
'If we can identify the best eggs, fewer embryos would need to be transferred during fertility treatments.
'Our findings will help move us toward this goal.'
Zinc fluctuations play a central role in regulating the biochemical processes that ensure a healthy egg-to-embryo transition.
This new unprecedented quantitative information should be useful in improving in vitro fertilization methods.
The study in Nature Chemistry provides the first quantitative physical measurements of zinc localization in single cells in a mammal.
After inventing a novel vital fluorescent sensor for live-cell zinc tracking, scientists discovered close to 8,000 compartments in the egg, each containing approximately one million zinc atoms.
These packages release their zinc cargo simultaneously in a concerted process, akin to neurotransmitter release in the brain or insulin release in the pancreas.
These findings were further confirmed with chemical methods that trap cellular zinc stores and enable zinc mapping on the nanometer scale in a custom-designed electron microscope developed for this project with funding from the W.M. Keck Foundation.
HOW THEY DID IT
The research team, including experts from the U.S. Department of Energy's Advanced Photon Source (APS), developed a suite of four physical methods to determine how much zinc there is in an egg and where it is located at the time of fertilization and in the two hours just after.
Sensitive imaging methods allowed the researchers to see and count individual zinc atoms in egg cells and visualize zinc spark waves in three dimensions.
Additional high-energy X-ray imaging experiments at the APS synchrotron at Argonne National Laboratory enabled the scientists to precisely map the location of zinc atoms in two and three dimensions.
'On cue, at the time of fertilization, we see the egg release thousands of packages, each dumping a million zinc atoms, and then it's quiet,' said Thomas O'Halloran, the other corresponding author.
'Then there is another burst of zinc release.
'Each egg has four or five of these periodic sparks. It is beautiful to see, orchestrated much like a symphony. We knew zinc was released by the egg in huge amounts, but we had no idea how the egg did this.'
O'Halloran is a 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.
The study establishes how eggs compartmentalize and distribute zinc to control the developmental processes that allow the egg to become a healthy embryo.
Zinc is part of a master switch that controls the decision to grow and change into a completely new genetic organism.
The studies reported in Nature Chemistry are the culmination of six years of work and build on prior discoveries made by the Woodruff and O'Halloran labs using data from work performed at Northwestern and the APS.
In previous studies in mouse eggs, this research team discovered the egg's tremendous zinc requirement for reaching maturity. In addition, the researchers determined that an egg loses 10 billion of its 60 billion zinc atoms upon fertilization in a series of four or five waves called 'zinc sparks.'
Release of zinc sparks from the egg is essential for embryo formation in the two hours following fertilization.
'The egg first has to stockpile zinc and then must release some of the zinc to successfully navigate maturation, fertilization and the start of embryogenesis,' O'Halloran said.
'But exactly how much zinc is involved in this remarkable process and where is it in the cell? We needed data to better understand the molecular mechanisms at work as an egg becomes a new organism.'
The researchers currently are working to see if they can correlate zinc sparks with egg quality, information that would be key to improving fertility treatments.
