The excitement of newly sparked love is often compared, figuratively speaking, to seeing fireworks. That’s at the macroscopic level of human interaction. It turns out something no less amazing goes on at the cellular level. Scientists at Northwestern University have now revealed that the union of male and female gametes launches a fireworks show, quite literally. From "Stunning Zinc Fireworks When Egg Meets Sperm":

Sparks literally fly when a sperm and an egg hit it off. The fertilized mammalian egg releases from its surface billions of zinc atoms in "zinc sparks," one wave after another, a Northwestern University-led interdisciplinary research team has found. (Emphasis added.)

Man-made fireworks shows rely on metals for the bright colors that ignite in the air. Uncannily, fertilization also involves metallic particles. The scientists used fluorescent tags to track the movement of zinc atoms during fertilization (see here for a video clip showing a zinc spark in action). Just like artificial fireworks, the metals must be carefully packaged before ignition:

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.

This is not just for show. Zinc is "part of a master switch" that "controls the decision to grow and change into a completely new genetic organism." Even though it has a serious function, it’s a beautiful show to watch — all programmed to perfection:

"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 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."

Their paper is published in Nature Chemistry. The zinc atoms are not just ejected from the membranes, but are packaged in vesicles that are sent out by the well-known process of exocytosis:

We show that the zinc spark arises from a system of thousands of zinc-loaded vesicles, each of which contains, on average, 10⁶ zinc atoms. These vesicles undergo dynamic movement during oocyte maturation and exocytosis at the time of fertilization.

That’s another uncanny way this process is like fireworks: rockets are launched out into the environment.

Metal Containers

The zinc story is part of a bigger design: how cells handle metals. By themselves, metal atoms can be toxic to cells. That’s why living cells have multiple complex pathways for their care and handling. Lawrence Livermore Laboratory describes the containers as "cupboards" that provide a ready supply:

Nearly 40 percent of all proteins require metal ions such as zinc, copper, manganese or iron for activity.

"We don’t understand very well how cells maintain balance when the cell is stressed by metal excess or metal deficiency," said LLNL researcher Jennifer Pett-Ridge. "By storing the metal in a special intracellular compartment, the cell creates a bit of a pantry cupboard for itself and can better maintain its equilibrium."

The "cupboards" are vacuoles in the "pantry" of the cell: "A vacuole is an enclosed compartment, which is filled with water containing inorganic and organic molecules including enzymes." Safely enclosed, the toxic metals can’t get out to cause damage until the cell needs them to build proteins. Copper, for instance, goes into action when zinc is in short supply:

"Our research shows the great lengths cells will take to restrict adverse reactions with copper, even at the expense of limiting cell activity," said UCLA postdoc Marcus Miethke, co-first author of the paper appearing in the journal Nature Chemical Biology.

How this "matchmaking of metals and proteins occurs with such precision" has long been a puzzle in physiology, the article states. By studying how it works in Chlamydomonas, a single-cell green alga, the Lawrence Livermore researchers "shed new light" on the process. (Neither of these articles appealed to evolution to explain the origin of any of this.) The system allows cells to maintain constant levels in the "pantry," something like a just-in-time delivery system:

The copper quota in this cell is maintained at a relatively constant level over orders of magnitude of extracellular copper. However, copper will hyper-accumulate when Chlamydomonas is starved for zinc….

This imprisoned form of the metal is hidden from cellular sensors of copper. But this trapping of copper is not a dead end for the metal; when the zinc starvation stress is alleviated, copper is released and is used preferentially over extracellular sources of copper for biosynthesis.

Clearly there is elaborate signaling and control going on behind the scenes, even in the simplest microbial life. As the Northwestern University team found, the system can sometimes put on quite a show.

Image source: Michael B./Flickr.