Sparks fly when egg meets sperm: study

Sparks fly when an egg meets sperm, according to a new study which found that the fertilised mammalian egg releases from its surface billions of zinc atoms that create tiny 'zinc sparks'.
A Northwestern University-led interdisciplinary research team used cutting-edge technology to capture images of these molecular fireworks for the first time and pinpoint the origin of the zinc sparks: tiny zinc-rich packages just below the egg's surface.
Zinc fluctuations play a central role in regulating the biochemical processes that ensure a healthy egg-to-embryo transition, and this new unprecedented quantitative information should be useful in improving in vitro fertilisation methods, researchers said.

"The amount of zinc released by an egg could be a great marker for identifying a high-quality fertilised egg, something we can't do now," said Teresa K Woodruff, an expert in ovarian biology and one of two corresponding 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," Woodruff said.
The study, published in the journal Nature Chemistry, provides the first quantitative physical measurements of zinc localisation in single cells in a mammal.

The research team, including experts from the US 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 fertilisation and in the two hours just after.

Sensitive imaging methods allowed the researchers to see and count individual zinc atoms in egg cells and visualise zinc spark waves in three dimensions.
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.
"On cue, at the time of fertilisation, we see the egg release thousands of packages, each dumping a million zinc atoms, and then it's quiet," said Thomas V 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 said.