“Zinc sparks” fly during human conception

Chuck Bednar for redOrbit.com – Your Universe Online

Conceiving children often involves, ahem, “fireworks.” (At least we hope so.)

But according to new research published in Nature Chemistry on Monday, the fertilization of a mammalian egg actually does produce “zinc sparks,” caused by the release of thousands of zinc atoms required to induce the egg-to-embryo transition.

(This movie shows zinc being released from the egg during a zinc spark. In this movie, hot spots of fluorescence at the cell surface demonstrate how groups of zinc-rich packages are released from the egg during fertilization, resulting in a zinc spark.)

The authors of the study were able to capture the first images of these molecular fireworks using newly-developed technology. In doing so, they discovered that the sparks originate from miniature, tiny zinc-rich packages just below the egg’s surface – a discovery which should help improve upon current in vitro fertilization methods.

“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,” Northwestern University professor Teresa K. Woodruff, an expert in ovarian biology and one of two corresponding authors of the paper, said in a statement. “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, a professor in obstetrics and gynecology and director of the Women’s Health Research Institute at Northwestern University Feinberg School of Medicine, fellow corresponding author and Northwestern University Chemistry of Life Processes Institute director Thomas V. O’Halloran, and their colleagues said that their research provides the first quantitative physical measurements of zinc localization in single cells in a mammal.

The investigative team used a combination of four different physical methods to determine the amount of zinc contained in an egg, as well as its location both during the time of fertilization and in the two hours period immediately afterwards. Using sensitive imaging techniques, they were able to observe and count individual zinc atoms in egg cells and visualize zinc spark waves in three dimensions.

They also developed a novel vital fluorescent sensor that they used for live-cell zinc tracking, and by using it, they were able to discover nearly 8,000 compartments in the egg. Each of those compartments contained approximately one million zinc atoms, and those packages released the chemical simultaneously in a concerted process, comparable to neurotransmitter release in a person’s brain or insulin release in the pancreas.

The findings were later confirmed using chemical methods that trapped cellular zinc stores and allowed the substance to be mapped on a nanometer scale using an electron microscope that was custom-made specifically for this project. Additional high-energy X-ray imaging experiments enabled Woodruff, O’Halloran and their fellow scientists to precisely map the location of zinc atoms in both 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. Then there is another burst of zinc release,” said O’Halloran, according to Daily Mail. “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.”

The newly-published paper is the culmination of six years of work, and establishes how eggs compartmentalize and distribute zinc to control the developmental processes which allow it to become a healthy embryo, the researchers said. They said that zinc played a key role in allowing an egg to grow and change into a new genetic organism, and the release of these zinc sparks is essential for embryo formation during the first two hours post-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.”

“We had to develop a slew of methods to be convinced we were seeing the right thing,” he added. “Science is about testing and retesting ideas. All of our complementary results point to the same conclusion: the zinc originates in packages called vesicles near the cell’s surface.”


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